Chapter 11Chemical Reactions

Word Equations

Reactants Products

Law of conservation of mass

Iron + oxygen iron(III) oxide

Hydrogen peroxide water + oxygen

Methane + oxygen carbon dioxide + water

Chemical Equations

Fe + O2 Fe2O3

Skeleton equations show reactants and products but do not give relative amounts

Fe(s) + O2(g) Fe2O3(s)

Physical state can be indicated by (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution

Chemical Equations

What is a catalyst? A catalyst is a species

that speeds up the rate of a reaction, but is not consumed.

Balancing Chemical Equations

In a balanced equation, each side must have the same number of atoms of each element.

C(s) + O2(g) CO2

Already balanced! H2(g) + O2(g) H2O(l)

Not balanced. What can we do to balance it?

Creating a bicycle: Frame + wheel + handlebar + pedal bicycle F + W + H + P FW2HP2

However, this does not tell us the quantities of the reactants.

The balanced equation would be: F + 2W + H + 2P FW2HP2

Balancing chemical equations

Rules for Balancing Equations

1. Determine the correct formulas for all reactants and products

2. Write reactants on the left and products on the right (skeleton equation)

3. Count the number of atoms of each element in the reactants and products

Rules for Balancing Equations

4. Balance the elements 1 at a time using coefficients. It is best to begin with elements that appear only once on each side of the equation. Do not change subscripts!

5. Check each atom or ion to be sure that it is balanced

6. Make sure that the coefficients are in the lowest possible ratio that balances

Examples

Problem:

AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + Ag(s)

Answer:

2AgNO3(aq) + Cu(s) Cu(NO)3(aq) + 2Ag(s)

TYPES OF

Chapter 11

Reaction TypesThe five general types

of reaction are

• Combination• Decomposition• Single-replacement• Double-replacement• Combustion

Combination Reactions

• Two or more substances form a single new substance

• An example is 2Mg + O2 2MgO• We can represent combination reactions as A + B AB • Complete and balance the following

combination reaction: Be + O2 • Answer: 2Be + O2 2BeO

Decomposition Reactions• A single compound breaks down into two

or more simpler products• An example is 2HgO 2Hg + O2

• We can represent combination reactions as AB A + B• Complete and balance the following

decomposition reaction: HI • Answer: 2HI H2 + I2

Single Replacement• One element replaces another element from a compound

• An example is 2K + 2H2O 2KOH + H2

• We can represent single replacement reactions as

A + BC AC + B• Complete and balance the following single replacement

reaction:

Zn + Ag(NO3)

• Answer: Zn + 2Ag(NO3) 2Ag + Zn(NO3) 2

Double Replacement How would we show the general

equation for a double replacement? (Use A,B,C, and D)

Double replacements consist of ionic compounds (charges must equal zero)

Therefore: AB + CD AD + CB

Double Replacement An example of a double replacement reaction:

K2CO3 + BaCl2 2KCl + BaCO3

Notice that the cations and anions exchange places

Combustion Finally, a combustion reaction is easy

to identify because it usually involves a hydrocarbon reacting with oxygen to form water and carbon dioxide.

Example: CH4 + 2O2 CO2 + 2H2O What is CH4? Why does oxygen exist as a diatomic

molecule?

Predicting Products

What are some clues that might help you to predict the products of reactions?

PRACTICE

Complete and balance the reaction:

Al + Cl2 What type of reaction is it? Do the same for the following

Mg + Cl2

C2H2 + O2

Reactions in Aqueous Solution

70 % Earth’s surface is water

66 % Human body is water

Chemistry in water is very important.

Net Ionic Equations

When the same ionic compounds dissolve in water, they dissociate into ions.

For example, NaCl becomes NA+ and CL-

AgNO3 becomes Ag+ and NO3-

Complete Ionic Equation

A complete ionic equation shows dissolved ionic compounds as dissociated free ions.

Ag+ (aq) + NO-3(aq) + Na+

(aq) + Cl-(aq) AgCl(5) +

Na +(aq) + Cl-

(aq)

…continued Ions that appear unchanged on both side of the

equation may be cancelled out. So, Ag+

(aq) + Cl-(aq) AgCl(5)

The new equation is the net ionic equation Na+

(aq) + NO-3 (aq) are known as spectator ions.

Examples

Pb(5) + AgNO3(aq) Ag (5)+Pb(NO3)2

NO-3(aq) is the spectator ion

Net ionic equation: Pb(5) + Ag(aq) Ag(5)

+ Pb2+(aq)

Balance the equation with respect to mass & charge.

Pb(5) + 2Ag+(aq) 2Ag(5)+Pb2+

(aq)

Predicting Precipitates

You can predict the formation of a precipitate by using solubility rules.