To understand the collision model of chemical reactions To understand activation energy

Section 17.1

Reaction Rates and Equilibrium

1. To understand the collision model of chemical reactions

2. To understand activation energy

3. To understand how a catalyst speeds up a chemical reaction

4. To explore reactions with reactants or products in different phases

5. To learn how equilibrium is established

6. To learn about the characteristics of chemical equilibrium

Objectives

Section 17.1


A. How Chemical Reactions Occur

• Collision model – molecules must collide in order for a reaction to occur – Rate depends on concentrations of reactants and

temperature.

Section 17.1


B. Conditions That Affect Reaction Rates

• Concentration – increases rate because more molecules lead to more collisions

• Temperature – increases rate – Why?

Section 17.1



• Activation energy – minimum energy required for a reaction to occur

Section 17.1



• Catalyst – a substance that speeds up a reaction without being consumed – Enzyme – catalyst in a biological system

Section 17.1


C. Heterogeneous Reactions

• Homogeneous reaction – all reactants and products are in one phase – Gas – Solution

• Heterogeneous reaction – reactants in two phases

Section 17.1


C. Heterogeneous Reactions

Section 17.1


D. The Equilibrium Condition

• Equilibrium – the exact balancing of two processes, one of which is the opposite of the other

Section 17.1


D. The Equilibrium Condition

• Chemical equilibrium – a dynamic state where the concentrations of all reactants and products remain constant

Section 17.1


E. Chemical Equilibrium: A Dynamic Condition

Equal numbers of moles

of H2

O and CO are

mixed in a closed

container.

The reaction begins to

occur, and some products

(H2

and CO2

) are formed.

The reaction

continues as time

passes and more

reactants are changed

to products.

Although time continues to pass, the

numbers of reactant and product

molecules are the same as in (c). No

further changes are seen as time

continues to pass. The system has

reached equilibrium.

Section 17.1


E. Chemical Equilibrium: A Dynamic Condition

• Why does equilibrium occur?

Section 17.1


1. To understand the law of chemical equilibrium

2. To learn to calculate values for the equilibrium constant

3. To understand how the presence of solids or liquids affects the equilibrium expression

Objectives

Section 17.1


A. The Equilibrium Constant: An Introduction

• Law of chemical equilibrium– For a reaction of the type

• Each set of equilibrium concentrations is called an equilibrium position.

aA + bB cC + dD

– Equilibrium expression

Section 17.1


B. Heterogeneous Equilibria

• Heterogeneous equilibria – an equilibrium system where the products and reactants are not all in the same state

Section 17.1


1. To learn to predict the changes that occur when a system at equilibrium is disturbed

2. To learn to calculate equilibrium concentrations

3. To learn to calculate the solubility product of a salt

4. To learn to calculate solubility from the solubility product

Objectives

Section 17.1


A. Le Chatelier’s Principle

• Le Chatelier’s Principle – when a change is imposed on a system at equilibrium the position of the equilibrium shifts in a direction that tends to reduce the effect of that change

Section 17.1



• Effect of a Change in Concentration

Section 17.1



• Effect of a Change in Concentration

– When a reactant or product is added the system shifts away from that added component.

– If a reactant or product is removed, the system shifts toward the removed component.

Section 17.1



• Effect of a Change in Volume

The system is initially at

equilibrium.

The piston is pushed in, decreasing the

volume and increasing the pressure. The

system shifts in the direction that consumes

CO2

molecules, lowering the pressure again.

Section 17.1




– Decreasing the volume

– The system shifts in the direction that gives the fewest number of gas molecules.

Section 17.1




– Increasing the volume

– The system shifts in the direction that increases its pressure.

Section 17.1



• Effect of a Change in Temperature

– The value of K changes with temperature. We can use this to predict the direction of this

change.

– Exothermic reaction – produces heat (heat is a product)

• Adding energy shifts the equilibrium to the left (away from the heat term).

– Endothermic reaction – absorbs energy (heat is a reactant)

• Adding energy shifts the equilibrium to the right (away from the heat term).

Section 17.1


B. Applications Involving the Equilibrium Constant

– K > 1 the equilibrium position is far to the right

– K < 1 the equilibrium position is far to the left

The Meaning of K

Section 17.1


B. Applications Involving the Equilibrium Constant

• The value of K for a system can be calculated from a known set of equilibrium concentrations.

• Unknown equilibrium concentrations can be calculated if the value of K and the remaining

equilibrium concentrations are known.

Section 17.1


C. Solubility Equilibria

• The equilibrium conditions also applies to a saturated solution containing excess solid, MX(s). – Ksp = [M+][X] = solubility product constant

– The value of the Ksp can be calculated from the

measured solubility of MX(s).

To understand the collision model of chemical reactions To understand activation energy

Documents

Transcript of To understand the collision model of chemical reactions To understand activation energy