Chemical ReactionsChapter 7

Describing Reactions 7.1

What type of change is happening in the picture?

When charcoal burns, it changes into other substances while producing heat and light.

Burning is a chemical change

Chemical Equations

One way to describe a change of state is to describe what is present before and after the change.

In a chemical reaction, the substances that undergo change are called reactants.

The new substances formed as a result of that change are called products. ◦In the picture, the reactants are the carbon in

the charcoal, and the oxygen in the air.◦The product is CO2 gas.

Using Equations to Represent Reactions

Reactants ProductsTo describe burning of charcoal:

Carbon + Oxygen Carbon Dioxide

C + O2 CO2

A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas

Conservation of Mass

What happens to the products in a chemical reaction?

During a chemical reaction, the mass of the products is always equal to the mass of the reactants.

This is called the Law of Conservation of Mass, that mass is neither created nor destroyed.

This law was established by Antoine Lavoisier

Conservation of Mass

Equation reads “one carbon atom reacts with one molecule of oxygen and forms one molecule of carbon dioxide”.

If you have 6 C atoms, they will react with 6 O2 molecules to form 6 CO2 molecules

The equation has the same number of atoms on each side of the equation

Balancing Equations

Some chemical reactions are powerful enough to propel a space craft. ◦Rocket fuels contain a compound called

hydrazine, N2H4 ◦When hydrazine burns in the presence of

oxygen, the reaction produces nitrogen, water vapor, and heat.

◦You can describe this reaction by writing a chemical equation:

◦N2H4 + O2 N2 + H2O

Balancing Equations

N2H4 + O2 N2 + H2OIf we count the atoms on both sides, we will see

that the # of atoms on the left are not equal to the # of atoms on the ride side of the equation.

This equation is NOT balancedIn order to show that mass is conserved during a

reaction, a chemical equation must be balanced.You balance a chemical equation by changing the

coefficients (# in front of the formula)

Steps to Balancing Equations

No coefficient- is an understood “1”

First, count the number of atoms of each element on each side of equation

Starting with metals, change coefficients in front of formulas until balanced.

Try◦ Na + H2O NaOH + H2

HCl + CaCO3 CaCl2 + CO2 + H2O

MATH PRACTICE

Write a balanced equation for the reaction between copper and oxygen to produce copper (II) oxide, CuO.◦Steps:

Write equation with reactants on the left and products on the right

◦Cu + O2 CuO Balance atoms

◦2Cu + O2 2CuO

Math Practice

Balance the following chemical equations:

◦H2O2 H2O + O2

◦Mg + HCl H2 + MgCl2

Counting Chemicals

How many shoes do you have? Shoes are counted in pairs. How many eggs are in a dozen? Bottle

rockets in a gross? Pair, dozen, gross are all UNITs we use to

count.How do chemists count particles?DEMO

Counting Moles

Chemists need to be able to count atoms or molecules.

These units are too small to be counted, so chemists have another way to count them.

Chemists use a unit called the mole (No, not that kind of mole!)

A mole is an amount that contains 6.02 x 1023 particles of that substance ◦Aka “Avogadro’s #’◦Particles: atoms, molecules or ions◦Ex: 1 mole of Fe (Iron) contains 6.02 x 1023 atoms of

iron.

Molar Mass

Does a dozen eggs weigh the same as a dozen oranges?

Demo◦A mole of carbon has a different mass than a mole

of sulfurThe mass of one mole of substance is called a

molar massThe molar mass for chemicals is the same as

the atomic mass.◦Ex: Carbon’s mass is 12 amu or 12 grams per one

mole of carbon 12 grams C or 1 mole C

1 mole C 12 grams C

Mole Mass Conversions

For a compound, add the atomic masses of its components atoms◦EX: CO2

◦ 1 atom of Carbon x 12 g◦+ 2 atoms of Oxygen x 16 g

44 grams of CO2 per one mole of CO2

You can use this molar mass to convert moles of a substance to mass and vice versa.

44.0 g CO2 or 1 mol CO21 mol CO2 44.0 g CO2

“Molar mass” is the same as

“formula mass”

Mole-Mass Conversions

Suppose you have 55 g of CO2. You can use the g/mol conversion factor to calculate how many moles:◦55.0 g CO2 1 mol CO2 1.25 mol CO2

44g CO2

◦You can also convert from moles back to grams

◦2 mol CO2 44g CO2 88.0 g CO2

1 mol CO2

Types of Reactions7.2

Classifying Reactions

How do you classify matter?◦(solid, liquid, gas – remember?)

Chemical reactions are also classified into different types:◦Synthesis◦Decomposition◦Single-replacement◦Double-replacement◦Combustion

Synthesis & Decomposition

Synthesis reactions are reactions in which 2 or more substances react to form a single substance◦A + B AB (like a MARRIAGE)◦Ex: 2 Na + Cl2 2 NaCl◦Video

Decomposition reactions are reaction in which one substance is broken down into two or more simpler substances (opposite of synthesis)◦AB A + B (like a DIVORCE)◦Ex: 2H2O 2 H2 + O2

◦ Video

Here is another example of a synthesis reaction

Another view of a decomposition reaction

Single and Double Replacment

Single Replacement is a reaction where 1 element takes the place of another element in a compound◦A + BC B + AC (like a LOVE TRIANGLE)◦Ex: Cu +2Ag(NO3) 2Ag + Cu(NO3)2

◦ Video

Double Replacement two different compounds exchange positive ions and form 2 new compounds◦AB + CD AD + CB (like DO-Si-DO and change

partners)◦Ex: Pb(NO3) + 2KI PbI2 + 2 KNO3

◦ Video

◦ With precipitation

Single Replacement Reactions

Combustion

Combustion reaction one in which a substance reacts rapidly with oxygen, producing heat and light◦Ex: CH4 + 2O2 CO2 + 2 H2O

◦Always react with oxygen and usually produces CO2 , gas and water Products in combustion are ALWAYS carbon dioxide and

water. (although incomplete burning does cause some by-products like carbon monoxide)

Many reactions can be classified by more than one type.

Oxidation-Reduction (Redox)

Oxidation-reduction reaction: a reaction where electrons are transferred from one reactant to another, aka redox

Ex: Calcium reacts with oxygen to produce calcium oxide

2Ca + O2 2CaO◦ The product (CaO is composed of ions where the reactants were

neutral atoms)◦ When calcium reacts with oxygen, each neutral atom loses

electrons to form Ca+2 ionsCa Ca2+ + 2e-

When an element loses electrons during a chemical reaction it is called oxidation

The calcium lost electrons so it was oxidized

Oxidation Can occur without

oxygen

Oxidation-Reduction (Redox)

Reduction is the process where an element gains electrons

Each neutral oxygen atom gains two electrons becoming O2- ion

O + 2e- O2- The oxygen gained electron and has been

reduced◦Oxidation and reduction always occur together◦If one element loses electrons another element

HAS to gain them

OIL RIGOxidati

onIs

LossReducti

onIs

Gain

Mixed Practice

• State the type, classify and balance the following reactions:

1. _Pb(NO3)2 + _HCl _PbCl2 + _HNO3

2. _C2H6 + _O2 _CO2 + _HOH

3. _Ca + _HCl _CaCl2 + _H2

4. _Hg + O2 HgO

5. _SO2 + _O2 _SO3

Do you see a Redox reaction?

DR

Combustion

SR

Synthesis

Synthesis

Redox

Energy Changes in Reactions

7.3

Chemical Bonds and Energy

Where does the heat come from when you light a propane grill?◦C3H8 + 5O2 3CO2 +4H2O + Heat◦This equation shows that the heat released in the

reaction came from the reactants.◦Chemical energy is the energy stored in the

chemical bonds of a substance◦energy changes in chemical reactions are

determined by changes that occur in the chemical bonding

◦Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products

Breaking & Forming Bonds

Breaking chemical bonds requires energy.◦Where could this energy come from when using

a propane grill? Grills have a lighter which produces a spark,

giving enough energy to break the bonds and start the reaction

The formation of chemical bonds releases energy. (resulting in heat and light that you see)

Endothermic and Exothermic Reactions

Physical changes can release or absorb energy◦Exothermic: releases◦Endothermic: absorbing◦During a chemical reaction energy is either

released or absorbed◦A chemical reaction that releases energy to its

surroundings is called an exothermic reaction

◦A chemical reaction that absorbs energy from its surroundings is an endothermic reaction

•As you go from left to right in each graph, what happens to the reactants?• They react to form the products

•What point on each graph represents the highest energy?• The energy is highest at each curve’s peak

•What do the double-headed arrows represent?• The difference in chemical energy between the reactants

and products•Which type of reaction has products with a greater amount of energy that the reactants?• endothermic

Reaction rates7.4

Reaction Rates

A reaction rate is the rate at which reactants change into products over time◦Rate just means a change over time, like distance over

time= speedReaction rates tell you how fast a reaction is

going◦How fast reactants are consumed, products are formed

or energy released/absorbed. Factors that affect reaction rates are:

◦Temperature◦Surface Area◦Concentration◦Stirring ◦Catalysts

Temperature and Surface Area

How does temperature affect reaction rates?◦ Increasing temp causes particles to move faster and

collide, # of collisions increases then rate increases◦Decreasing the temperature will decrease the

reaction rateSurface area is the amount of area exposed

◦An increase in the surface area increases the exposure of reactants to one another allowing more collisions And therefore allowing an increase in the reaction rate (newspapers)

Stirring and Concentration

Stirring increases the reaction rate by increasing the number of collisions between the particles of the reactants.◦ (washing machine vs. soaking)

Concentration is the number of particles in a given volume. (ex sugar in tea)◦The more particles of reactant, the higher the

reaction rateGas concentration changes with pressure

(less room)◦The greater the pressure of the gas, the greater

it’s concentration and the faster it’s reaction rate

Catalysts

A catalyst is a substance that affects the reaction rate without being used up in the reaction.

They can be used to speed up or slow down reactions

Graph shows how a catalyst can lower the amount of energy needed to cause a reaction