UNIT 7
eactivity 2.3—How far- the extent of chemical change
R
.3.1: Dynamic equilibrium
2
- A state of dynamic equilibrium is reached in a closed system when the rates of forward and backward reactions are equal.
- Describe the characteristics of a physical and chemical system at equilibrium
- Dynamic equilibrium must happen in a closed system, where reactants and products cannot escape (but heat can)
- Dynamic means that both the forward and reverse reactions are still taking place and the the rate of these are equal
- The concentration of the reactants and products remains constant in the equilibrium mixture
- At equilibrium, the macroscopic properties do not change
- Equilibrium can be reached from either direction
2.3.2- 2.3.3: Equilibrium law
- The equilibrium law describes how the equilibrium constant, K, can be determined from the stoichiometry of a reaction.
- Deduce the equilibrium constant expression from an equation for a homogeneous reaction.
- The position of equilibrium can lie to the left (higher [reactants] than [products) or to the right (higher [products] than
[reactants])
- When aA+bB <—> cC+dD
𝑐 𝑑
[𝐶] [𝐷]
- 𝐾 = 𝑎 𝑏
[𝐴] [𝐵]
- K is temperature dependent and so the value must be quoted for a particular reaction at a specified temperature
- The greater the value of K, the more products present in the mixture and therefore the further to the right the equilibrium
position lies
- A small k value means there is a higher concentration of reactants than products and the equilibrium position is to the left
- K=1 is when the concentration of products and reactants are the same
- The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent
- Determine the relationships between K values for reactions that are the reverse of each other at the same temperature.
- The relationship between K of the forward reaction and the reverse reaction are reciprocals of each other.
- Therefore the effect of dividing a reaction by 2 leads to the value of K for the forward reaction being square-rooted for
the reverse reaction?
- Include the extent of reaction for: K<<1, K<1, K = 1, K>1, K>>1.
- K<<1: Reaction barely proceeds forward
- K<1: greater concentration of reactants
- K=1: same concentration of reactants and products
- K>1: greater concentration of products
- K>>1: reaction goes to completion
2.3.4: Le Chatelier’s principle
- Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a
system at equilibrium
- Apply Le Châtelier’s principle to predict and explain responses to changes of systems at equilibrium.
- ‘When a system at equilibrium is subjected to a change, the system will respond to minimise the effect of the change’
- Concentration:
- If a reactant’s concentration in a system is increased, the forward reaction is favoured and the position of equilibrium will
move to the right
eactivity 2.3—How far- the extent of chemical change
R
.3.1: Dynamic equilibrium
2
- A state of dynamic equilibrium is reached in a closed system when the rates of forward and backward reactions are equal.
- Describe the characteristics of a physical and chemical system at equilibrium
- Dynamic equilibrium must happen in a closed system, where reactants and products cannot escape (but heat can)
- Dynamic means that both the forward and reverse reactions are still taking place and the the rate of these are equal
- The concentration of the reactants and products remains constant in the equilibrium mixture
- At equilibrium, the macroscopic properties do not change
- Equilibrium can be reached from either direction
2.3.2- 2.3.3: Equilibrium law
- The equilibrium law describes how the equilibrium constant, K, can be determined from the stoichiometry of a reaction.
- Deduce the equilibrium constant expression from an equation for a homogeneous reaction.
- The position of equilibrium can lie to the left (higher [reactants] than [products) or to the right (higher [products] than
[reactants])
- When aA+bB <—> cC+dD
𝑐 𝑑
[𝐶] [𝐷]
- 𝐾 = 𝑎 𝑏
[𝐴] [𝐵]
- K is temperature dependent and so the value must be quoted for a particular reaction at a specified temperature
- The greater the value of K, the more products present in the mixture and therefore the further to the right the equilibrium
position lies
- A small k value means there is a higher concentration of reactants than products and the equilibrium position is to the left
- K=1 is when the concentration of products and reactants are the same
- The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent
- Determine the relationships between K values for reactions that are the reverse of each other at the same temperature.
- The relationship between K of the forward reaction and the reverse reaction are reciprocals of each other.
- Therefore the effect of dividing a reaction by 2 leads to the value of K for the forward reaction being square-rooted for
the reverse reaction?
- Include the extent of reaction for: K<<1, K<1, K = 1, K>1, K>>1.
- K<<1: Reaction barely proceeds forward
- K<1: greater concentration of reactants
- K=1: same concentration of reactants and products
- K>1: greater concentration of products
- K>>1: reaction goes to completion
2.3.4: Le Chatelier’s principle
- Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a
system at equilibrium
- Apply Le Châtelier’s principle to predict and explain responses to changes of systems at equilibrium.
- ‘When a system at equilibrium is subjected to a change, the system will respond to minimise the effect of the change’
- Concentration:
- If a reactant’s concentration in a system is increased, the forward reaction is favoured and the position of equilibrium will
move to the right
