Chapter 10: Reaction Rates and Chemical Equilibrium

Do all things have the same reaction rate?

No, they are varied

Activation Energy

The minimum amount of energy needed to convert the colliding molecules into products

What are the conditions for a reaction to occur?

1) Collision: Reactants must collide

2) Orientation: The reactants must align properly (side-by-side) to break and form bonds

3) Energy: The collision provides sufficient energy for activation

Is proper orientation all that is needed to form products?

No, even when a collision has the proper orientation there still must be sufficient energy to break the bonds between the atom of the reactants

Rate of Reaction

The rate or speed of a reaction

What is the Rate of Reaction calculated by?

* Reactant used up in a certain period of time

OR
* Product formed in a certain period of time

What is the formula for Rate of Reaction?

Change in concentration of reactant or product/change in time

What factors affect the Rate of Reaction?

* Any reaction

* Temperature changes
* Changes in reactant concentration
* Adding a catalyst
* Surface area *(needed for a lab we’ll do)*

Rate of Reaction: Temperature

At higher temperatures, the increase in kinetic energy causes reactant molecules to

* Move faster
* Collide more
* Collide with more energy

What is the rate of change in Rate of Reaction: Temperature?

For every 10° increase, most reaction rates approximately double

Rate of Reaction: Reactant Concentration

When there are more reacting molecules, more collisions that form products can occur and the reaction goes faster

* More opportunities for collisions

Rate of Reaction: Catalysts

Adding a catalyst speeds up the rate of reaction by providing an alternate pathway that has a lower activation energy

* When activation energy is lowered, more collisions provide sufficient energy for reactants to form products

During reactions, do catalysts get changed or consumed?

No, they do not

Reversible Reactions

When a reaction proceeds in both a forward and reverse direction

* Both the forward and reverse directions occur at the same time

Do reversible reactions have the same rate at equilibrium?

Yes

* Before equilibrium, however, they have two rates

Do reversible reactions have the same concentrations at equilibrium?

No, not necessarily

Rate of reversible reactions

As the reaction progresses, the rate of the forward reaction decreases and the rate of the reverse reaction increases.

* At equilibrium, the rates of the forward and reverse reactions are equal

Concentration of reversible reactions

* Equillibrium

Equilibrium is reached when there are no further changes in the concentrations of the reactant and products

When is a reversible reaction at equilibrium?

* The rate of the forward reaction is equal to the rate of the reverse reaction
* The forward and reverse reactions continue at the same rate

Equilibrium Expression

Multiplies the products together and divides by the concentration of the reactants. Also raises the concentration (moles/L) of each species to a power that is equal to its coefficient in the balanced chemical equation

Equilibrium Constant

Kc, a numerical value obtained by substituting experimentally measured molar concentrations at equilibrium into the expression

What is equilibrium expression

Kc = \[C\]^c x \[D\]^d/\[A\]^a x \[B\]^b

What phases work with equilibrium expressions and which do not?

Gas and Aqueous WORK

Liquid and Solid DO NOT WORK

What do the brackets mean in an equilibrium expression?

Moles per liter

When will Kc stay the same?

* For the same chemical reaction

When the temperature stays constant

What is the unit for Kc

There are none

What can the values of Kc be? What causes them?

They can be large or small depending on whether equilibrium is reached with

* More product than reactant

OR
* More reactant than product

Small Kc Value

Little reaction occurred

* Low concentration of product, higher concentration of reactants
* Produced from the reverse reaction

Large Kc Value

Reaction essentially complete

* Low concentration of reactants, higher concentration of products
* Produced from the forward reaction

Kc Value ≈ 1

Reactants and products are in a similar concentration

What happens when conditions of a reaction at equilibrium are changed?

The forward and reverse reactions will no longer be equal

Le Châtelier’s Principle

When stress is placed on a reaction at equilibrium, the system responds by changing the rate of the forward or reverse reaction in the direction that relieves the stress.

* Tries to become equal again

What items are considered to be stress on equilibrium? What isn’t?

1) Concentration

2) Volume/Pressure (They’re related = Boyle’s Law)

3) Temperature

\
NOT catalysts

Stress on Equilibrium: Adding Reactant

The rate of the forward reaction increase to form more product until the system is again at equilibrium

* Shifts toward the products

Stress on Equilibrium: Removing Reactant

The rate of the reverse reaction increase to form more reactant until the system is again at equilibrium

* Shifts toward the reactants

In A + B ⇌ C + D, what would these shift to?

* ↑D
* ↓C
* ↑A
* ↓B

↑D = reactants

↓C = products

↑A = products

↓B = reactants

When determining what will happen when changing volume changes on equilibrium, what must you always do?

Count the moles

What will occur if you change the volume at equilibrium?

The concentration of gasses in the mixture will change

Stress on Equilibrium: Decrease Volume

Decreasing the volume increases the concentration of gasses, and the system shifts in the direction of the smaller number of moles to compensate

Stress on Equilibrium: Increase Volume

Increasing the volume decreases the concentration of gasses, and the system shifts in the direction of the larger number of moles to compensate

Stress on Equilibrium: Pressure

* Increase and Decrease

Think of it like volume and do the opposite :D

* Yes, I’m too lazy to write it but anyway

In A(g) + B(g) ⇌ C(g), where does the reaction shift?

* Decrease Pressure
* Increase Pressure
* Increase Volume
* Decrease Volume

Decrease Pressure = reactants

Increase Pressure = products

Increase Volume = reactants

Decrease Volume = products

Where is the heat in an endothermic reaction vs an exothermic reaction?

* In the equation

Endothermic: Reactant Side

Exothermic: Product Side

Stress on Equilibrium: Decreasing Temperature

* Exothermic

Causes the system to respond by shifting the reaction toward more heat

* Shifts toward products, increasing heat in the system

Stress on Equilibrium: Increasing Temperature

* Exothermic

Causes the system to respond by shifting the reaction toward removing heat

* Shifts toward reactants, decreasing heat in the system

Stress on Equilibrium: Decreasing Temperature

* Endothermic

Causes the system to respond by shifting the reaction toward more heat

* Shifts toward reactants, increasing heat in the system

Stress on Equilibrium: Increasing Temperature

* Endothermic

Causes the system to respond by shifting the reaction toward removing heat

* Shifts toward products, using up the heat