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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
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
B. Conditions That Affect Reaction Rates
• Concentration – increases rate because more molecules lead to more collisions
• Temperature – increases rate – Why?
Section 17.1
Reaction Rates and Equilibrium
B. Conditions That Affect Reaction Rates
• Activation energy – minimum energy required for a reaction to occur
Section 17.1
Reaction Rates and Equilibrium
B. Conditions That Affect Reaction Rates
• Catalyst – a substance that speeds up a reaction without being consumed – Enzyme – catalyst in a biological system
Section 17.1
Reaction Rates and Equilibrium
C. Heterogeneous Reactions
• Homogeneous reaction – all reactants and products are in one phase – Gas – Solution
• Heterogeneous reaction – reactants in two phases
Section 17.1
Reaction Rates and Equilibrium
C. Heterogeneous Reactions
Section 17.1
Reaction Rates and Equilibrium
D. The Equilibrium Condition
• Equilibrium – the exact balancing of two processes, one of which is the opposite of the other
Section 17.1
Reaction Rates and Equilibrium
D. The Equilibrium Condition
• Chemical equilibrium – a dynamic state where the concentrations of all reactants and products remain constant
Section 17.1
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
E. Chemical Equilibrium: A Dynamic Condition
• Why does equilibrium occur?
Section 17.1
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
B. Heterogeneous Equilibria
• Heterogeneous equilibria – an equilibrium system where the products and reactants are not all in the same state
Section 17.1
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
A. Le Chatelier’s Principle
• Effect of a Change in Concentration
Section 17.1
Reaction Rates and Equilibrium
A. Le Chatelier’s Principle
• 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
Reaction Rates and Equilibrium
A. Le Chatelier’s Principle
• 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
Reaction Rates and Equilibrium
A. Le Chatelier’s Principle
• Effect of a Change in Volume
– Decreasing the volume
– The system shifts in the direction that gives the fewest number of gas molecules.
Section 17.1
Reaction Rates and Equilibrium
A. Le Chatelier’s Principle
• Effect of a Change in Volume
– Increasing the volume
– The system shifts in the direction that increases its pressure.
Section 17.1
Reaction Rates and Equilibrium
A. Le Chatelier’s Principle
• 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
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
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
Reaction Rates and Equilibrium
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).