NAME: _______________________________

UNIT #10: Reaction Rates

Heat/Energy in Chemical Reactions

Le Chatlier’s Principle

Potential Energy Diagrams

1. REACTION RATES

a) The speed of a chemical reaction determined by the change in concentration of a

reactant or product per unit time, expressed as mol/(L∙s).

b) Reaction rates are determined experimentally by measuring the concentrations of

reactants and/or products in an actual chemical reaction.

c) Factors Effecting Reaction Rates

1. Nature of Reactants: The more reactive a substance, the faster the reaction rate.

2. Concentrations: Increased concentration of reactants increases reaction rate by

increasing the chance of particle collisions; likewise, decreasing concentration of

reactants decreases reaction rate.

3. Surface Area: Increased surface area increases reaction rate due to more exposed

particles to react.

Ex. Pulverizing (grinding) a solid reactant will increase its reaction rate;

granulated sugar will dissolve faster in water than a sugar cube.

4. Temperature: Generally, increasing temperature increases reaction rate by

increasing the average kinetic energy of reactants; thus, collisions between

particles are more frequent and occur with more energy, increasing the chance

that product will be formed.

5. Catalysts: Catalysts are compounds introduced to a reaction that are not

consumed in the reaction. The activation energy of a reaction is the minimum

amount of energy required to start a reaction. Catalysts increase reaction rates

by decreasing activation energy and therefore, more particle collisions have

sufficient energy to initiate product formation.

2. HEAT/ENERGY IN CHEMICAL REACTION

a) The Law of Conservation of Energy states that in any chemical or physical process,

energy is neither created or destroyed but instead, is conserved.

b) In a chemical reaction, the bonds between atoms of the reactants are broken.

Breaking bonds requires an input of energy. Energy contained by reactants is

expressed as the Hreactants.

c) Atoms rearrange and form new bonds, resulting in the product. When bonds are

formed, energy is released. Energy contained by products is expressed as the Hproducts.

d) The heat of reaction is an expression of the net energy involved in a chemical reaction

and is expressed as ΔHreaction = Hproducts - Hreactants

e) An endothermic reaction is one that requires a net input of energy, meaning it takes

more energy to break bonds of reactants than is released in the formation of product.

Since energy is transferred and conserved, the resulting products contain more energy.

Since the ΔHreaction = Hproducts - Hreactants, the ΔHreaction for an endothermic reaction is

always positive.

f) An exothermic reaction is one that has a net release of energy, meaning more energy

is released in the formation of products than is required to break the bonds of the

reactants. Since energy is transferred and conserved, the resulting products contain

less energy. Since the ΔHreaction = Hproducts - Hreactants, the ΔHreaction for an exothermic

reaction is always negative.

g) Summary:

Endothermic reaction Reactants + Energy → Products

Hproducts is greater than Hreactants and energy appears on the reactant side of the

equation. ΔHreaction is positive.

Exothermic reaction Reactants → Products + Energy

Hproducts is less than Hreactants and energy appears on the product side of the equation.

ΔHreaction is negative.

3. LE CHATELIER’S PRINCIPLE

a) Many chemical reactions occur in both a forward and a reverse direction. This is

depicted as Reactants ↔ Products.

b) Chemical equilibrium is a state in which the rate of the forward reaction is equal to

the rate of the reverse reaction.

c) Le Chatelier’s principle states that if a stress is applied to a system at equilibrium, the

system shifts in the direction that relieves the stress.

1. Stress can be a change in concentration, volume and pressure or temperature.

2. Change in Concentration

a. Addition of reactants: system compensates by using up additional reactants

to form product. System shifts right towards products.

b. Removal of reactants: system compensates by reversing reaction to replace

reactants. System shifts left towards reactants.

c. Addition of products: system compensates by reversing reaction to use up

additional products. System shifts left towards reactants.

d. Removal of products: system compensates by producing more product.

System shifts right towards products.

3. Change in Volume and Pressure

a. Changes in volume and pressure effect only a system that contains reactants

and products in a gaseous state.

b. Changes in volume and pressure affect a system in equilibrium only if the

total number of moles of gaseous reactants is different than the total

number of moles of gaseous products. Coefficients of the balanced equation

are used to determine moles of reactants and products.

c. Changes in volume and pressure are interrelated because decreasing the

volume of a reaction vessel at constant temperature increases the pressure

inside. Conversely, increasing the volume decreases the pressure.

d. Decreasing volume/increasing pressure will shift a gaseous reaction towards

the side of the equation with fewer moles.

e. Increasing volume/decreasing pressure will shift a gaseous reaction toward

the side of the equation with more moles.

4. Changes in Temperature

a. In an endothermic reaction, energy is on the reactant side of the equation

and predictions of equilibrium shift are equivalent to changes in

concentration of reactants.

1. Increase in temperature: system compensates by using up additional

energy. System shifts right towards products.

2. Decrease in temperature: system compensates by producing more

energy. System shifts left towards reactants.

b. In an exothermic reaction, energy is on the product side of the equation and

predictions of equilibrium shift are equivalent to changes in concentration of

products.

1. Increase in temperature: system compensates by using up additional

energy. System shifts left towards reactants.

2. Decrease in temperature: system compensates by producing more

energy. System shifts right towards products.

4. POTENTIAL ENERGY DIAGRAMS

a) A potential-energy diagram is used to depict the shift in energy as a chemical reaction

progresses. Specifically, it shows the changing potential energy between the reactants

and products.

b) A potential energy diagram shows:

1. The potential energy of the reactant.

2. The potential energy of the activated complex. The activated complex is an

intermediate structure of atoms that exists as bonds are breaking and new bonds

are forming. It occurs temporarily before the final product is formed.

3. The potential energy of the product.

4. The activation energy of the forward reaction, which is the minimum energy

required to form an activated complex. On the diagram, activation energy is the

difference between the PE of the reactants and the PE of the activated

complex.

5. The heat of reaction, which is the difference between the PE of the product and

the PE of the reactant.

c) In an endothermic reaction, the Hproduct is greater than the Hreactants due to an input of

energy into the reaction. Therefore, the potential energy of the products will be

higher than the reactants on the diagram.

d) In an exothermic reaction, the Hproduct is less than the Hreactant due to a release of

energy. Therefore, the potential energy of the products will be lower than the

reactants on the diagram.

e) Potential energy diagrams can also depict the energy shift of a catalyzed reaction in

which the activated energy is reduced and consequently, the reaction is accelerated.

UnitINoteQuizQuestions

Unit 10.1: Collision Theory

1. a

2. a

3. a

4. a

5. a

6. Which shows the activation energy of the

reaction?

7. Which letter is the enthalpy of the reaction?

8. Which letter shows the potential energy of CH4

and O2?

9. Is the reaction endo/exothermic?

10. a

Unit 10.2: Le Châtelier's principle

1. a

2. A

3. A

4. A

5. A

6. A

7. A

8. A

9. A

10. A