Chemical Reactions

Word Equations

Write word equations that describe chemical reactions.

Cookbooks are similar to chemical word equations

What’s for dinner?

Various ways of recording recipes have developed over the centuries. The cookbook shown above was written by a woman who probably collected all her own recipes. Later, printed cookbooks became available (even guys had no excuse for not being able to cook). Today we can find recipes on a number of internet sites and can quickly search for information on how to cook anything we want. Reading a recipe sometimes requires we understand a few codes and symbols (what’s the difference between a tsp and a Tsp?), but the information on what we start with and what we end up with is there.

Writing Chemical Equations

Chemical reactions are occurring all around you. Plants use sunlight to drive their photosynthetic process and produce energy. Cars and other vehicles burn gasoline in order to power their engines. Batteries use electrochemical reactions to produce energy and power many everyday devices. Many chemical reactions are going on inside you as well, especially during the digestion of food.

In math class, you have written and solved many mathematical equations. Chemists keep track of chemical reactions by writing equations as well. In any chemical reaction one or more substances, called reactants , are converted into one or more new substances, called products . The general form of the equation for such a process looks like this.

$text{Reactants} rightarrow text{Products}$

Unlike in a math equation, a chemical equation does not use an equal sign. Instead the arrow is called a yield sign and so the equation is described as “reactants yield products”.

Word Equations

You can describe a chemical reaction by writing a word equation . When silver metal is exposed to sulfur it reacts to form silver sulfide. Silver sulfide is commonly known as tarnish and turns the surface of silver objects dark and streaky black (see Figure below ). The sulfur that contributes to tarnish can come from traces of sulfur in the air or from food such as eggs. The word equation for the process is:

$text{Silver} + text{sulfur} rightarrow text{Silver sulfide}$

The silver and the sulfur are the reactants in the equation, while the silver sulfide is the product.

Figure 11.1

The coffee percolator on the left has been tarnished from exposure to sulfur. Tarnish is the chemical compound silver sulfide. The same percolator on the right has been polished with a tarnish removal product in order to restore its silver finish.

Another common chemical reaction is the burning of methane gas. Methane is the major component of natural gas and is commonly burned on a gas stove or in a Bunsen burner ( Figure below ). Burning is a chemical reaction in which some type of fuel is reacted with oxygen gas. The products of the reaction in the burning of methane as well as other fuels are carbon dioxide and water. The word equation for this reaction is:

$text{Methane}+text{oxygen} rightarrow text{carbon dioxide}+text{water}$

Figure 11.2

A Bunsen burner is commonly used to heat substances in a chemistry lab. Methane is reacted with oxygen to form carbon dioxide and water.

Word equations can be very useful, but do have one major drawback. They cannot be used for any quantitative work. A word equation does not tell how many moles of each material are needed or how many moles of product are formed.

Summary

Word equations are used to describe the conversion of reactants to products.

Practice

Questions

Read the material at the link below and do the practice problems:

Practice Problems

Review

Questions

Write the generic form of a chemical reaction.
What are reactants?
What are products?

chemical reaction : Conversion of reactants to products
product: The result of chemical reaction
reactant: The starting material for a chemical reaction
word equation: A description of a chemical reaction using the names of the compounds.

Chemical Equations

Describe the symbols used in a chemical equation.

Recipes specify the amount of ingredients, like a chemical equation

How do you make Shrimp gumbo?

Shrimp gumbo is one of many enjoyable dishes that are part of the Cajun culture in Louisiana. It’s a spicy dish that needs careful control of all the ingredients so that it has a “kick”, but is not overwhelming. Recipes tell not only what is in the preparation, but describes how much of each ingredient and details of how to cook the meal. Similarly, We need this type of information in order to carry out chemical reactions successfully and safely.

Chemical Equations

Word equations are time-consuming to write and do not prove to be convenient for many of the things that chemists need to do with equations. A chemical equation is a representation of a chemical reaction that displays the reactants and products with chemical formulas. The chemical equation for the reaction of methane with oxygen is shown:

$text{CH}_4+text{O}_2 rightarrow text{CO}_2+text{H}_2text{O}$

The equation above, called a skeleton equation, is an equation that shows only the formulas of the reactants and products with nothing to indicate the relative amounts. The first step in writing an accurate chemical equation is to write the skeleton equation, making sure that the formulas of all substances involved are written correctly. All reactants are written to the left of the yield arrow, separated from one another by a plus sign. Likewise, products are written to the right of the yield arrow, also separated with a plus sign.

It is often important to know the physical states of the reactants and products taking part in a reaction. To do this, put the appropriate symbol in parentheses after each formula: ( s ) for solid, ( l ) for liquid, ( g ) for gas, and ( aq ) for an aqueous (water-based) solution. The previous reaction becomes:

$text{CH}_4(g) + text{O}_2(g) rightarrow text{CO}_2(g) + text{H}_2text{O}(l)$

The Table below shows a listing of symbols used in chemical equations. Some, such as the double arrow which represents equilibrium, and the use of a catalyst in a reaction, will be treated in detail in other concepts.

Symbols Used in Chemical Equations
Symbol	Description
+	Used to separate multiple reactants or products
$rightarrow$	yield sign; separates reactants from products
$rightleftarrows$	replaces the yield sign for reversible reactions that reach equilibrium
( s )	reactant or product in the solid state
( l )	reactant or product in the liquid state
( g )	reactant or product in the gas state
( aq )	reactant or product in an aqueous solution (dissolved in water)
$overset{Pt}{rightarrow}$	formula written above the arrow is used as a catalyst in the reaction
$overset{Delta}{rightarrow}$	triangle indicates that the reaction is being heated

Summary

Click on the image above for more content

http://www.youtube.com/watch?v=lSoRj_iBwYc

Review

Questions

What does a skeleton equation tell you?
Why would you want to know the physical state of materials?
What does the symbol $rightarrow$ mean?
If I see $Delta$ over the arrow, what will I do?

chemical equation: A representation of a chemical reaction that displays the reactants and products with chemical formulas
skeleton equation: An equation that shows only the formulas of the reactants and products with nothing to indicate the relative amounts.

Balancing Equations

Balance chemical equations when given the skeleton information.

Portrait of John Dalton

Any Leftovers?

When you cook a meal, quite often there are leftovers because you prepared more than people would eat at one sitting. Sometimes when you repair a piece of equipment, you end up with what are called “pocket parts”, small pieces you put in your pocket because you’re not sure where they belong. Chemistry tries to avoid leftovers and pocket parts. In normal chemical processes, we cannot create or destroy matter (law of conservation of mass). If we start out with ten carbon atoms, we need to end up with ten carbon atoms. John Dalton’ atomic theory said that chemical reactions basically involve the rearrangement of atoms. Chemical equations need to follow these principles in order to be correct.

Balancing Chemical Equations

A balanced equation is a chemical equation in which mass is conserved and there are equal numbers of atoms of each element on both sides of the equation. We can write a chemical equation for the reaction of carbon with hydrogen gas to form methane (CH ₄ ).

$& quad text{C}(s) quad quad + quad text{H}_2 (g) quad quad rightarrow quad quad quad quad text{CH}_4(g)\& 2 text{C atoms} quad quad 2 text{H atoms} quad quad quad 1 text{C atom}, 4 text{H atoms}$

In order to write a correct equation, you must first write the correct skeleton equation with the correct chemical formulas. Recall that hydrogen is a diatomic molecule and so is written as H ₂ .

When we count the number of atoms of both elements, shown under the equation, we see that the equation is not balanced. There are only 2 atoms of hydrogen on the reactant side of the equation, while there are 4 atoms of hydrogen on the product side. We can balance the above equation by adding a coefficient of 2 in front of the formula for hydrogen.

$text{C}(s)+2text{H}_2(g) rightarrow text{CH}_4 (g)$

A coefficient is a small whole number placed in front of a formula in an equation in order to balance it. The 2 in front of the H ₂ means that there are a total of $2 times 2 = 4$ atoms of hydrogen as reactants. Visually, the reaction looks like the Figure below .

Figure 11.3

Reaction between carbon and hydrogen to form methane.

In the balanced equation, there is one atom of carbon and four atoms of hydrogen on both sides of the arrow. Below are guidelines for writing and balancing chemical equations.

Determine the correct chemical formulas for each reactant and product.
Write the skeleton equation.
Count the number of atoms of each element that appears as a reactant and as a product. If a polyatomic ion is unchanged on both sides of the equation, count it as a unit.
Balance each element on at a time by placing coefficients in front of the formulas. No coefficient is written for a 1. It is best to begin by balancing elements that only appear in one formula on each side of the equation. NEVER change the subscripts in a chemical formula – you can only balance equations by using coefficients.
Check each atom or polyatomic ion to be sure that they are equal on both sides of the equation.
Make sure that all coefficients are in the lowest possible ratio. If necessary, reduce to the lowest ratio.

Sample Problem: Balancing Chemical Equations

Aqueous solutions of lead(II) nitrate and sodium chloride are mixed. The products of the reaction are an aqueous solution of sodium nitrate and a solid precipitate of lead(II) chloride.

Step 1: Plan the problem.

Follow the steps for writing and balancing a chemical equation.

Step 2: Solve.

Write the skeleton equation with the correct formulas.

$text{Pb(NO}_3)_2(aq)+text{NaCl}(aq) rightarrow text{NaNO}_3(aq)+text{PbCl}_2(s)$

Count the number of each atom or polyatomic ion on both sides of the equation.

reactants	products
1 Pb atom	1 Pb atom
2 NO ₃^– ions	1 NO ₃^– ions
1 Na atom	1 Na atom
1 Cl atom	2 Cl atoms

The nitrate ions and the chlorine atoms are unbalanced. Start by placing a 2 in front of the NaCl. This increases the reactant counts to 2 Na atoms and 2 Cl atoms. Then place a 2 in front of the NaNO ₃ . The result is:

$text{Pb(NO}_3)_2 (aq)+2text{NaCl}(aq) rightarrow 2text{NaNO}_3 (aq)+text{PbCl}_2(s)$

The new count for each atom and polyatomic ion becomes:

reactants	products
1 Pb atom	1 Pb atom
2 NO ₃^– ions	2 NO ₃^– ions
2 Na atom	2 Na atom
2 Cl atoms	2 Cl atoms

Step 3: Think about your result.

The equation is now balanced since there are equal numbers of atoms of each element on both sides of the equation.

Summary

The process of balancing chemical equations is described.

Practice

Questions

Get some experience in balancing chemical equations at the following web site:

http://www.sciencegeek.net/APchemistry/APtaters/EquationBalancing.htm

Review

Questions

What is the law of conservation of mass?
How did Dalton describe he profess of a chemical reaction?
Why don’t we change the subscripts in order to balance an equation?

balanced equation: A chemical equation in which mass is conserved and there are equal numbers of atoms of each element on both sides of the equation.
coefficient: A small whole number placed in front of a formula in an equation in order to balance it.

Combination Reactions

Define combination reaction.
Write products of combination reactions when given the reactants.

A wheel contains a wheel rim and a tire, which is an example of a combination reaction

How useful is a wheel rim?

A wheel rim is not very useful by itself. Driving on the rim can damage it and make for a very rough ride. When the rim is combined with a tire, the product can be put on a car and used for a safe and comfortable ride. The two separate items have combined to make something that improves the car ride.

Combination Reactions

A combination reaction is a reaction in which two or more substances combine to form a single new substance. Combination reactions can also be called synthesis reactions. The general form of a combination reaction is:

$A+B rightarrow AB$

One combination reaction is two elements combining to form a compound. Solid sodium metal reacts with chlorine gas to produce solid sodium chloride.

$2text{Na}(s)+text{Cl}_2 (g) rightarrow 2text{NaCl}(s)$

Notice that in order to write and balance the equation correctly, it is important to remember the seven elements that exist in nature as diatomic molecules (H ₂ , N ₂ , O ₂ , F ₂ , Cl ₂ , Br ₂ , and I ₂ ).

One sort of combination reaction that occurs frequently is the reaction of an element with oxygen to form an oxide. Metals and nonmetals both react readily with oxygen under most conditions. Magnesium reacts rapidly and dramatically when ignited, combining with oxygen from the air to produce a fine powder of magnesium oxide.

$2text{Mg}(s)+text{O}_2(g) rightarrow 2text{MgO}(s)$

This reaction can be seen in the following video: http://www.youtube.com/watch?v=NnFzHt6l4z8 (0:37).

Click on the image above for more content

Sulfur reacts with oxygen to form sulfur dioxide.

$text{S}(s)+text{O}_2(g) rightarrow text{SO}_2 (g)$

When nonmetals react with one another, the product is a molecular compound. Often, the nonmetal reactants can combine in different ratios and produce different products. Sulfur can also combine with oxygen to produce sulfur trioxide.

$2text{S}(s)+3text{O}_2 (g) rightarrow 2text{SO}_3 (g)$

Transition metals are capable of adopting multiple positive charges within their ionic compounds. Therefore, most transition metals are capable of forming different products in a combination reaction. Iron reacts with oxygen to form both iron(II) oxide and iron(III) oxide.

$& 2text{Fe}(s)+text{O}_2 (g) rightarrow 2text{FeO}(s)\& 4 text{Fe}(s)+3text{O}_2 (g) rightarrow 2text{Fe}_2text{O}_3(s)$

Sample Problem: Combination Reactions

Potassium is a very reactive alkali metal that must be stored under oil in order to prevent it from reacting with air. Write the balanced chemical equation for the combination reaction of potassium with oxygen.

Step 1: Plan the problem

Make sure formulas of all reactants and products are correct before balancing the equation. Oxygen gas is a diatomic molecule. Potassium oxide is an ionic compound and so its formula is constructed by the crisscross method. Potassium as an ion becomes K ⁺ , while the oxide ion is O ²⁻ .

Step 2: Solve

The skeleton (unbalanced) equation:

$K(s)+O_2(g) rightarrow K_2O(s)$

The equation is then easily balanced with coefficients.

$4text{K}(s)+text{O}_2(g) rightarrow 2text{K}_2text{O}(s)$

Step 3: Think about your result

Formulas are correct and the resulting combination reaction is balanced.

Combination reactions can also take place when an element reacts with a compound to form a new compound composed of a larger number of atoms. Carbon monoxide reacts with oxygen to form carbon dioxide according to the equation:

$2text{CO}(g)+text{O}_2(g) rightarrow 2text{CO}_2(g)$

Two compounds may also react to from a more complex compound. A very common example is the reactions of oxides with water. Calcium oxide reacts readily with water to produce an aqueous solution of calcium hydroxide.

$text{CaO}(s)+text{H}_2text{O}(l) rightarrow text{Ca(OH)}_2(aq)$

Sulfur trioxide gas reacts with water to form sulfuric acid. This is an unfortunately common reaction that occurs in the atmosphere in some places where oxides of sulfur are present as pollutants. The acid formed in the reaction falls to the ground as acid rain.

$text{SO}_3(g)+text{H}_2text{O}(l) rightarrow text{H}_2text{SO}_4(aq)$

Acid rain degrades marble statues and kills trees

Figure 11.4

Acid rain has severe consequences on both natural and man-made objects. Acid rain degrades marble statues like the one on the left (A). The trees in the forest on the right (B) have been killed by acid rain.

Summary

Combination reactions occur when two or more substances combine to form a new substance.

Practice

Questions

Complete the reactions and balance the equations on the worksheet at the link below:

http://www.sciencegeek.net/Chemistry/chempdfs/EquationsWorksheet2.pdf

Review

Questions

What are combination reactions?
Write the product of the following reaction: $text{Mg} + text{H}_2text{O} rightarrow$
Is $text{CH}_4 + 2text{O}_2 rightarrow text{CO}_2 + 2text{H}_2text{O}$ a combination reaction? Explain your answer.

combination reaction: A reaction in which two or more substances combine to form a single new substance.

Decomposition Reactions

Define decomposition reaction.
Write the products of decomposition reactions when given the reactant.
Write the reactant of a decomposition reaction when given the products.

Apparatus used by Lavoisier to study the decomposition of mercuric oxide

How does a decomposition reaction work?

Antoine Lavoisier is widely known as the “father of modern chemistry”. He was one of the first to study chemical reactions in detail. Lavoisier reacted mercury with oxygen to form mercuric oxide as part of his studies on the composition of the atmosphere. He was then able to show that the decomposition of mercuric oxide produced mercury and oxygen. The diagram above shows the apparatus used by Lavoisier to study the formation and decomposition of mercuric oxide.

Decomposition Reactions

A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general form of a decomposition reaction is:

$AB rightarrow A+B$

Most decomposition reactions require an input of energy in the form of heat, light, or electricity.

Binary compounds are compounds composed of just two elements. The simplest kind of decomposition reaction is when a binary compound decomposes into its elements. Mercury(II) oxide, a red solid, decomposes when heated to produce mercury and oxygen gas.

$2text{HgO}(s) rightarrow 2text{Hg}(l)+text{O}_2(g)$

Figure 11.5

Mercury(II) oxide is a red solid. When it is heated, it decomposes into mercury metal and oxygen gas.

A reaction is also considered to be a decomposition reaction even when one or more of the products are still compounds. A metal carbonate decomposes into a metal oxide and carbon dioxide gas. For example, calcium carbonate decomposes into calcium oxide and carbon dioxide.

$text{CaCO}_3(s) rightarrow text{CaO}(s)+text{CO}_2(g)$

Metal hydroxides decompose on heating to yield metal oxides and water. Sodium hydroxide decomposes to produce sodium oxide and water.

$2text{NaOH}(s) rightarrow text{Na}_2text{O}(s)+text{H}_2text{O}(g)$

Some unstable acids decompose to produce nonmetal oxides and water. Carbonic acid decomposes easily at room temperature into carbon dioxide and water.

$text{H}_2text{CO}_3(aq) rightarrow text{CO}_2(g)+text{H}_2text{O}(l)$

Sample Problem: Decomposition Reactions

When an electric current is passed through pure water, it decomposes into its elements. Write a balanced equation for the decomposition of water.

Step 1: Plan the problem

Water is a binary compound composed of hydrogen and oxygen. The hydrogen and oxygen gases produced in the reaction are both diatomic molecules.

Step 2: Solve

The skeleton (unbalanced) equation:

$text{H}_2text{O}(l) overset{text{elec}}{rightarrow} text{H}_2 (g)+text{O}_2 (g)$

Note the abbreviation “elec” above the arrow to indicate the passage of an electric current to initiate the reaction. Balance the equation.

$2text{H}_2text{O}(l) overset{text{elec}}{rightarrow} 2text{H}_2(g)+text{O}_2(g)$

Step 3: Think about your result

The products are elements and the equation is balanced.

Summary

A definition of decomposition reaction and example reactions are given.

Practice

Questions

Write the reactions (including names and balanced equations) as requested on the following web site:

http://www.sciencegeek.net/Chemistry/chempdfs/EquationsWorksheet3.pdf

Review

Questions

What is a decomposition reaction?
What is usually needed for a decomposition reaction to take place?
Are elements always the product of a decomposition reaction?

decomposition reaction: A reaction in which a compound breaks down into two or more simpler substances.

Combustion Reaction

Define combustion reaction.
Write the products of combustion reactions when given the starting materials.

A marshmallow burning on a stick is a combustion reaction

How do you cook the perfect marshmallow?

Roasting marshmallows over an open fire is a favorite past-time for campers, outdoor cook-outs, and just gathering around a fire in the back yard. The trick is to get the marshmallow a nice golden brown without catching it on fire. Too often we are not successful and we see the marshmallow burning on the stick – a combustion reaction taking place right in front of us.

Combustion Reactions

A combustion reaction is a reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat. Combustion reactions must involve O ₂ as one reactant. The combustion of hydrogen gas produces water vapor.

$2text{H}_2 (g)+text{O}_2 (g) rightarrow 2text{H}_2text{O}(g)$

Notice that this reaction also qualifies as a combination reaction.

The explosion of the Hindenburg was a combustion reaction

Figure 11.6

Explosion of the Hindenberg.

The Hindenburg was a hydrogen-filled airship that suffered an accident upon its attempted landing in New Jersey in 1937. The hydrogen immediately combusted in a huge fireball, destroying the airship and killing 36 people. The chemical reaction was a simple one: hydrogen combining with oxygen to produce water.

Many combustion reactions occur with a hydrocarbon, a compound made up solely of carbon and hydrogen. The products of the combustion of hydrocarbons are carbon dioxide and water. Many hydrocarbons are used as fuel because their combustion releases very large amounts of heat energy. Propane (C ₃ H ₈ ) is a gaseous hydrocarbon that is commonly used as the fuel source in gas grills.

$text{C}_3text{H}_8(g)+5text{O}_2(g) rightarrow 3text{CO}_2(g)+4text{H}_2text{O}(g)$

Practice Problem: Combustion Reactions

Ethanol can be used as a fuel source in an alcohol lamp. The formula for ethanol is C ₂ H ₅ OH. Write the balanced equation for the combustion of ethanol.

Step 1: Plan the problem

Ethanol and oxygen are the reactants. As with a hydrocarbon, the products of the combustion of an alcohol are carbon dioxide and water.

Step 2: Solve

Write the skeleton equation:

$text{C}_2text{H}_5 text{OH}(l)+text{O}_2(g) rightarrow text{CO}_2 (g)+text{H}_2text{O}(g)$

Balance the equation.

$text{C}_2text{H}_5text{OH}(l)+3text{O}_2(g) rightarrow 2text{CO}_2(g)+3text{H}_2text{O}(g)$

Step 3: Think about your result

Combustion reactions must have oxygen as a reactant. Note that the water that is produced is in the gas rather than the liquid state because of the high temperatures that accompany a combustion reaction.

Summary

Combustion reaction is defined and examples are given.

Practice

Questions

Write the reactions and balance the equations for the questions on the sheet found on this web site:

http://www.sciencegeek.net/Chemistry/chempdfs/EquationsWorksheet6.pdf

Review

Questions

What is needed for a combustion reaction to take place?
What is formed in any combustion reaction?
Mercury reacts with oxygen to form mercuric oxide. Is this a combustion reaction?
What are the products of any combustion reaction involving a hydrocarbon?

combustion reaction: A reaction in which a substance reactants with oxygen gas, releasing energy in the form of light and heat.

Single-Replacement Reactions

Define single-replacement reaction.
Give examples of single-displacement reactions.

Tarnish is an example of a single-replacement reaction

Why is the silver dark?

The cup shown above provides an example of tarnish, a chemical reaction caused when silver metal reacts with hydrogen sulfide gas produced by some industrial processes or as a result of decaying animal or plant materials:

$2text{Ag} + text{H}_2text{S} rightarrow text{Ag}_2text{S} + text{H}_2$

The tarnish can be removed using a number of polishes, but the process also removes a small amount of silver along with the tarnish.

Single-Replacement Reactions

A single-replacement reaction is a reaction in which one element replaces a similar element in a compound. The general form of a single-replacement (also called single-displacement) reaction is:

$A+BC rightarrow AC+B$

In this general reaction, element $A$ is a metal and replaces element $B$ , also a metal, in the compound. When the element that is doing the replacing is a nonmetal, it must replace another nonmetal in a compound, and the general equation becomes:

$Y+XZ rightarrow XY+Z$

$Y$ is a nonmetal and replaces the nonmetal $Z$ in the compound with $X$ .

Metal Replacement

Magnesium is a more reactive metal than copper. When a strip of magnesium metal is placed in an aqueous solution of copper(II) nitrate, it replaces the copper. The products of the reaction are aqueous magnesium nitrate and solid copper metal.

$text{Mg}(s)+text{Cu(NO}_3)_2(aq) rightarrow text{Mg(NO}_3)_2 (aq) + text{Cu}(s)$

This subcategory of single-replacement reactions is called a metal replacement reaction because it is a metal that is being replaced (zinc).

Hydrogen Replacement

Many metals react easily with acids and when they do so, one of the products of the reaction is hydrogen gas. Zinc reacts with hydrochloric acid to produce aqueous zinc chloride and hydrogen ( Figure below ).

$text{Zn}(s)+2text{HCl}(aq) rightarrow text{ZnCl}_2 (aq)+text{H}_2(g)$

In a hydrogen replacement reaction, the hydrogen in the acid is replaced by an active metal.

Zinc reacts with hydrochloric acid in a single-replacement reaction

Figure 11.7

Zinc metal reacts with hydrochloric acid to give off hydrogen gas in a single-displacement reaction.

Some metals are so reactive that they are capable of replacing the hydrogen in water. The products of such a reaction are the metal hydroxide and hydrogen gas. All group 1 metals undergo this type of reaction. Sodium reacts vigorously with water to produce aqueous sodium hydroxide and hydrogen (see Figure below ).

$2text{Na}(s)+2text{H}_2text{O}(l) rightarrow 2text{NaOH}(aq)+text{H}_2 (g)$

Figure 11.8

Sodium metal reacts vigorously with water, giving off hydrogen gas. A large piece of sodium will often generate so much heat that the hydrogen will ignite.

Halogen Replacement

The element chlorine reacts with an aqueous solution of sodium bromide to produce aqueous sodium chloride and elemental bromine.

$text{Cl}_2 (g) + 2text{NaBr}(aq) rightarrow 2text{NaCl}(aq)+text{Br}_2(l)$

The reactivity of the halogen group (group 17) decreases from top to bottom within the group. Fluorine is the most reactive halogen, while iodine is the least. Since chlorine is above bromine, it is more reactive than bromine and can replace it in a halogen replacement reaction.

Summary

The activity series describes the relative reactivities of metals and halogens.

Practice

Questions

Read the material at the link below and do the practice problems:

http://www.chemteam.info/Equations/SingleReplacement.html

Review

Questions

What is a metal replacement reaction?
Will a non-metal replace a metal?
What is the most reactive halogen?
What products will I get if I add potassium metal to water?

single-replacement reaction: A reaction in which one element replaces a similar element in a compound.

Activity Series

Define activity series.
Use the activity series to predict the outcome of reactions.

Sodium and silver have very different reactivities with water

What’s the difference between the two pictures above?

We see above two metals that can be exposed to water. The picture on the left is of sodium, which gives a violent reaction when it comes in contact with water. The picture on the right is of silver, a metal so unreactive with water that it can be made into drinking vessels. Both metals have a single s electron in their outer shell, so you would predict similar reactivities. However, we have a better tool that allows us to make better prediction about what will react with what.

The Activity Series

Single-replacement reactions only occur when the element that is doing the replacing is more reactive than the element that is being replaced. Therefore, it is useful to have a list of elements in order of their relative reactivities. The activity series is a list of elements in decreasing order of their reactivity. Since metals replace other metals, while nonmetals replace other nonmetals, they each have a separate activity series. The Table below is an activity series of most common metals and of the halogens.

Activity Series
Activity of Metals	Activity of Halogens
Li K React with cold water, replacing Ba hydrogen. Sr Ca Na	F ₂ Cl ₂ Br ₂ I ₂
Mg Al React with steam, but not cold Zn water, replacing hydrogen. Cr Fe Cd
Co Ni Do not react with water. React Sn with acids, replacing hydrogen. Pb
H ₂
Cu Hg Unreactive with water or acids. Ag Pt Au

For a single-replacement reaction, a given element is capable of replacing an element that is below it in the activity series. This can be used to predict if a reaction will occur. Suppose that small pieces of the metal nickel were placed into two separate aqueous solutions: one of iron(III) nitrate and one of lead(II) nitrate. Looking at the activity series, we see that nickel is below iron, but above lead. Therefore, the nickel metal will be capable of replacing the lead in a reaction, but will not be capable of replacing iron.

$& text{Ni}(s)+text{Pb(NO}_3)_2(aq) rightarrow text{Ni(NO}_3)_2(aq)+text{Pb}(s)\& text{Ni}(s)+text{Fe(NO}_3)_3(aq) rightarrow text{NR (no reaction)}$

In the descriptions that accompany the activity series of metals, a given metal is also capable of undergoing the reactions described below that section. For example, lithium will react with cold water, replacing hydrogen. It will also react with steam and with acids, since that requires a lower degree of reactivity.

Sample Problem: Single-Replacement Reactions

Use the activity series to predict if the following reactions will occur. If not, write NR. If the reaction does occur, write the products of the reaction and balance the equation.

A. $text{Al}(s)+text{Zn(NO}_3)_2 (aq) rightarrow$

B. $text{Ag}(s)+text{HCl}(aq) rightarrow$

Step 1: Plan the problem

For A, compare the placements of aluminum and zinc on the activity series. For B, compare the placements of silver and hydrogen.

Step 2: Solve

Since aluminum is above zinc, it is capable of replacing it and a reaction will occur. The products of the reaction will be aqueous aluminum nitrate and solid zinc. Take care to write the correct formulas for the products before balancing the equation. Aluminum adopts a 3+ charge in an ionic compound, so the formula for aluminum nitrate is Al(NO ₃ ) ₃ . The balanced equation is:

$2text{Al}(s)+3text{Zn(NO}_3)_2(aq) rightarrow 2text{Al(NO}_3)_3(aq)+3text{Zn}(s)$

Since silver is below hydrogen, it is not capable of replacing hydrogen in a reaction with an acid.

$text{Ag}(s)+text{HCl}(aq) rightarrow text{NR}$

Summary

Metals and halogens are ranked according to their ability to displacement other metals or halogens below them in the series.

Practice

Questions

Take the quiz on the web site below:

http://www.sophia.org/chemical-reactions-activity-series-concept

Review

Questions

What does the activity series tell us?
Can a metal undergo any of the reactions listed below it in the series?
List two metals that cobalt will displace and two that will displace it.

activity series: A list of elements in decreasing order of their reactivity.

Double-Replacement Reactions

Define double-replacement reaction.
Predict products of double-replacement reactions when given the reactants.

Bartering is like a double-replacement reaction

Wanna trade?

The practice of barter (trading one thing for another) has been in existence from the beginning of time. In the illustration above, Items like chickens were bartered for newspapers. You have something I want, and I have something you want. So we trade and we each have something new. Some chemical reactions are like that. Compounds swap parts and you have new materials.

Double-Replacement Reactions

A double-replacement reaction is a reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds. The general form of a double-replacement (also called double-displacement) reaction is:

$AB+CD rightarrow AD+CB$

In this reaction, $A$ and $C$ are positively-charged cations, while $B$ and $D$ are negatively-charged anions. Double-replacement reactions generally occur between substances in aqueous solution. In order for a reaction to occur, one of the products is usually a solid precipitate, a gas, or a molecular compound such as water.

Formation of a Precipitate

A precipitate forms in a double-replacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound. When aqueous solutions of potassium iodide and lead(II) nitrate are mixed, the following reaction occurs.

$2text{KI}(aq)+text{Pb(NO}_3)_2(aq) rightarrow 2text{KNO}_3(aq)+text{PbI}_2 (s)$

There are very strong attractive forces that occur between Pb ²⁺ and I ⁻ ions and the result is a brilliant yellow precipitate (see Figure below ). The other product of the reaction, potassium nitrate, remains soluble.

Figure 11.9

Formation of lead iodide precipitate.

Formation of a Gas

Some double-replacement reactions produce a gaseous product which then bubbles out of the solution and escapes into the air. When solutions of sodium sulfide and hydrochloric acid are mixed, the products of the reaction are aqueous sodium chloride and hydrogen sulfide gas.

$text{Na}_2text{S}(aq)+2text{HCl}(aq) rightarrow 2text{NaCl}(aq)+text{H}_2text{S}(g)$

Formation of a Molecular Compound

Another kind of double-replacement reaction is one that produces a molecular compound as one of its products. Many examples in this category are reactions that produce water. When aqueous hydrochloric acid is reacted with aqueous sodium hydroxide, the products are aqueous sodium chloride and water.

$text{HCl}(aq)+text{NaOH}(aq) rightarrow text{NaCl}(aq)+text{H}_2text{O}(l)$

Sample Problem: Double-Replacement Reactions

Write a complete and balanced chemical equation for the following double-replacement reactions. One product is indicated as a guide.

A. $text{NaCN}(aq)+text{HBr}(aq) rightarrow$ (hydrogen cyanide gas is formed)

B. $(text{NH}_4)_2text{SO}_4(aq)+text{Ba(NO}_3)_2(aq) rightarrow$ (a precipitate of barium sulfate forms)

Step 1: Plan the problem

In A, the production of a gas drives the reaction. In B, the production of a precipitate drives the reaction. In both cases, use the ionic charges of both reactants to construct the correct formulas of the products.

Step 2: Solve

A. The cations of both reactants are +1 charged ions, while the anions are -1 charged ions. After exchanging partners, the balanced equation is:

$text{NaCN}(aq)+text{HBr}(aq) rightarrow text{NaBr}(aq)+text{HCN}(g)$

B. Ammonium ion and nitrate ion are 1+ and 1− respectively, while barium and sulfate are 2+ and 2−. This must be taken into account when exchanging partners and writing the new formulas. Then, the equation is balanced.

$(text{NH}_4)_2text{SO}_4 (aq)+text{Ba(NO}_3)_2(aq) rightarrow 2text{NH}_4text{NO}_3(aq)+text{BaSO}_4(s)$

Step 3: Think about your result

Both are double-replacement reactions. All formulas are correct and the equations are balanced. Occasionally, a reaction will produce both a gas and a molecular compound. The reaction of a sodium carbonate solution with hydrochloric acid produces aqueous sodium chloride, carbon dioxide gas, and water.

$text{Na}_2 text{CO}_3 (aq)+2text{HCl}(aq) rightarrow 2text{NaCl}(aq)+text{CO}_2(g)+text{H}_2text{O}(l)$

Summary

The double-replacement reaction is described.
Examples of the double-replacement reaction are shown.

Practice

Questions

Read the material at the web site below and do the practice problems:

http://www.chemteam.info/Equations/DoubleReplacement.html

Review

Questions

What are the usual reactants in a double-replacement reaction?
List the three possible types of products.
Why would you not expect two ionic products?

double-replacement reaction: A reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds.

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