Chapter NineChapter Nine

Chemical Reactions

Chapter Nine | Slide 2 of 37

Chemical Reactions

There are five main types of Chemical Reactions– Combination or Synthesis– Decomposition– Single Replacement– Double Replacement– Combustion

Chapter Nine | Slide 3 of 37

Types of Reactions

1. Combination/Synthesis Reactions2 or more substances react to form 1 product

A + B AB

Example: 2 Al (s) + 3 Br2 (l) 2 AlBr3 (s)

4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)

N2 (g) + 3 H2 (g) 2 NH3 (g)

Chapter Nine | Slide 4 of 37

When a hot nail is stuck into a pile of zinc and sulfur, a fiery combination reaction occurs and zinc sulfide forms.

Chemical Reactions cont’d

Chapter Nine | Slide 5 of 37

Types of Reactions

2. Decomposition Reactions1 substance undergoes a reaction to produce 2 or more

substances

XY X + Y

2 H2O (l) 2 H2 (g) + O2 (g)

CaCO3 (s) CaO (s) + CO2 ()

)( O (l) O2H(l) OH 2 2222MnO

22

Chapter Nine | Slide 6 of 37

Types of Reactions

3. Single Replacement Reactions

Element + Compound Element + Compound

X + YZ Y + XZ

Fe (s) + CuSO4 (aq) Cu (s) + FeSO4 (aq)

Zn (s) + CuSO4 (aq) Cu (s) + ZnSO4 (aq)

Chapter Nine | Slide 7 of 37

Types of Reactions

4. Double Replacement Reactions Compound + Compound Compound + Compound

AX + BY AY + BX

2 KI (aq) + Pb(NO3) 2 (aq) 2KNO3 (aq) + PbI2 (s)

K2SO4 (aq) + BaCl2 (aq) 2 KCl (aq) + BaSO4 (s)

One of the products is a _____________ or a __________

CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2(g)

Chapter Nine | Slide 8 of 37

A double-replacement reaction involving solutions of potassium iodide and lead nitrate produces yellow, insoluble lead iodide as one of the products.

Chemical Reactions cont’d

James Scherer/Houghton Mifflin Company

Chapter Nine | Slide 9 of 37

Types of Reactions

5. Combustion ReactionBurning/reacting a substance in ____________ gas

Examples:

2C2H2 + 5O2 4CO2 + 2H2O

C3H8 + 5O2 3CO2 + 4H2O

C4H8 + 6O2 4CO2 + 4H2O

Chapter Nine | Slide 10 of 37

→ CC 9.1

Combustion reaction and global warming

Chemical Reactions cont’d

Chapter Nine | Slide 11 of 37

Chemical Reactions cont’d

→ Aluminum reacting with iodine (purple smoke)

→ Formation of copper and zinc sulfate

← Mercury (II) oxide decomposing (orange solid)

← Formation of silver chloride and sodium nitrate

Chapter Nine | Slide 12 of 37

Collision Theory

• In order for a reaction to occur, three things must happen– Reacting particles must _________– Reacting particles must interact with proper

______________– Reacting particles must have enough ________ to react

• Activation energy: the minimum combined kinetic energy that colliding reactant particles must possess in order for their collision to result in a reaction

Chapter Nine | Slide 13 of 37

→ Fig. 9.5 Rubbing a match head against a rough surface provides the activation energy needed for the match to ignite.

Chemical Reactions cont’d

Chapter Nine | Slide 14 of 37

NO2 + CO NO + CO2

The most favorable collision orientation is one that puts an O atom from NO2 in close proximity to the C atom of CO.

Chemical Reactions cont’d

Chapter Nine | Slide 15 of 37

Reactions and the Transfer of Energy

• Exothermic reactions:– Processes in which a system releases energy into its surroundings– Exothermic reactions FEEL ________ (we, in the surroundings,

feel the ___________ given off by the reaction)

• Endothermic reactions:– Processes in which energy is added to a system from its

surroundings in order for the reaction to occur– Endothermic reactions FEEL ________ (we are the surroundings,

and the system “steals” heat from us, leaving us __________)

Chapter Nine | Slide 16 of 37

Energy diagram graphs showing the difference between an exothermic and an endothermic reaction.

Chapter Nine | Slide 17 of 37

BLANK

Chapter Nine | Slide 18 of 37

Figs. 9.8a-d A fire (a) is a much faster reaction than the ripening of fruit (b), which is much faster than the process of rusting (c), which is much faster than the process of aging (d).

Chemical Reactions Rates

Vince Streano/Getty Images

Myrleen Ferguson Cate/PhotoEdit

Sam Fried/Photo Researchers

© Cecile Brunswick/ Peter Arnold, Inc.

Chapter Nine | Slide 19 of 37

Chemical Reactions Rates

Chapter Nine | Slide 20 of 37

Catalysts lowers the activation energy for chemical reactions. Reactions proceed more rapidly with the lowered activation energy.

Chemical Reactions cont’d

Chapter Nine | Slide 21 of 37

Chemical Equilibrium

• Most of the reactions that we have looked at so far go to completion:– NaCl (aq) + AgNO3 (ag) AgCl (s) + NaNO3 (aq)

• But some reactions are reversible:– H2 (g) + I2 (g) ↔ 2HI (g)

• Reversible reactions reach a state called equilibrium.

Chapter Nine | Slide 22 of 37

An Analogy for Equilibrium

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Equilibrium

• Equilibrium:– The state in which forward and reverse chemical

reactions occur ______________ at the same _______

• What is true at equilibrium?– Rates of reactions are __________

• What is not true at equilibrium? – There are not ___________ amounts of products and

reactants at equilibrium

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Fig. 9.10 Graphs showing how reaction rates and reactant concentration vary with time.

Chemical Reactions cont’d

Chapter Nine | Slide 25 of 37

Equilibrium Constants

Equilibrium Constants A numerical value that characterizes the relationship between the

concentrations of reactants and products in a system at chemical equilibrium.

• For the balanced chemical equation:– wA + xB ↔ yC + zD

– Square brackets [ ] indicate molar concentrations– Only gases and substances in solution are included in the equilibrium expression

xw

zy

eq BA

DCK

][][

][][

Chapter Nine | Slide 26 of 37

Practice Writing Equilibrium Constants

• Write the equilibrium constants for the following chemical equations:– I2 (g) + Cl2 (g) ↔ 2ICl (g)

– 2Cl2 (g) + 2H2O (g) ↔ 4HCl (g) + O2 (g)

– 2NO (g) ↔ N2 (g) + O2 (g)

Chapter Nine | Slide 27 of 37

Temperature Dependence of Equilibrium Constants

• Equilibrium constants are defined (have certain values) at a given temperature– Their values change when the temperature changes

• For exothermic reactions– Equilibrium constant decreases with increasing temperature

wA + xB ↔ yC + zD + heat

• For endothermic reactions– Equilibrium constant increases with increasing temperature

heat + wA + xB ↔ yC + zD

xw

zy

eq BA

DCK

][][

][][

Chapter Nine | Slide 28 of 37

Position of Reactions

Equilibria can favor either the reactants or the products in a chemical reaction. “The equilibrium lies to the left” or toward the reactants or “the equilibrium lies to the right” or toward the products

• Equilibrium position– A qualitative indication of the relative amounts of reactants and

products present at equilibrium for a chemical reaction• “Reaction lies to the right” = more products than reactants (Keq > 1)

• “Reaction lies to the left” = more reactants than products (Keq < 1)

Chapter Nine | Slide 29 of 37

Position of Reactions

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→ Fig. 9.11 Henri Louis Chatelier was amazingly diverse in his interests.

Chemical Reactions cont’d

Edgar Fahs Smith Collection, University of Pennsylvania

Chapter Nine | Slide 31 of 37

Le Chatelier’s Principle

• Even though an equilibrium can favor one side of the reaction, __________ can alter the position of an equilibrium (whether the equilibrium lies to the left or to the right).

• Le Chatelier’s Principle:– If a _________ (change in conditions) is applied to a

system in equilibrium, the system will _____________ (change the equilibrium position) in the direction that best reduced the ___________ imposed on the system

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Potential Stresses

• Adding or removing reactants• Adding or removing products• Raising or lowering the temperature• Raising or lowering the pressure• Adding a catalyst

Chapter Nine | Slide 33 of 37

← Fig. 9.12 Concentration changes that result when H2 is added to an equilibrium mixture.

Chemical Reactions cont’d

Chapter Nine | Slide 34 of 37

Let’s look at the effects of certain stresses on the following reaction:2A (g) + 3B (g) 1C (l) + 4D (g) + energy

Stress What the System Wants to Do to Relieve the Stress

How the System Relieves the Stress

1. Add more of a reactant

2. Add more of a product

3. Remove some of the reactants

4. Remove some of the products

5. Increase the temperature

6. Decrease the temperature

7. Increase the pressure

8. Decrease the pressure

Chapter Nine | Slide 35 of 37

→ Fig. 9.13 Equilibrium mixtures changing color with difference in temperatures.

Chemical Reactions cont’d

Chapter Nine | Slide 36 of 37

One Last Comment

• CATALYSTS DO NOT AFFECT THE POSITION OF AN EQUILIBRIUM OR Keq

• CATALYSTS CAN MAKE A REACTION GO FASTER, BUT THEY CANNOT MAKE A REACTION HAPPEN THAT WOULDN’T HAVE HAPPENED WITHOUT THE CATALYST

Chapter Nine | Slide 37 of 37

Practice with Le Chatelier’s Principle

• How will the gas-phase equilibrium– CO (g) + 3H2 (g) CH4 (g) + H2O (g) + heat

Be affected by each of the following?– A. The removal of CH4 (g)?

– B. The addition of H2O (g)?

– C. A decrease in the temperature?

– D. A decrease in the volume of the container (an increase in pressure).