Essential idea: Many reactions are reversible. These reactions will reach a state of equilibrium when the rates of the forward and reverse reaction are equal. The position of equilibrium can be controlled by changing the conditions.
7.1 Equilibrium
UNDERSTANDINGS:
U7.1.1 A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.
Most chemical reactions are reversible, meaning that the products can turn back into the reactants. This is represented with the symbol
⇌ in chemical equations:
A + B ⇌ C + D
When dynamic equilibrium is reached, the rates of the reactants of the forwards and reverse reactions are equal, so there is no overall change in the concentrations of the reactants or products. Both reactions are still occurring but at the same rate.
NOTE: dynamic equilibrium is the state of balance in a reversible process when the forward and reverse changes are proceeding at equal rates; there is no overall change in the concentrations of the reactants or products.
Equilibrium can ONLY be set up in a closed system i.e. a container that does not allow anything to enter or escape. A physical dynamic equilibrium occurs when the rate of vapourisation of water is equal to the rate of condensation. Another example of physical dynamic equilbrium is a mixture of ice and water at the freezing point of water.
In a chemical equilibrium, the position of equilibrium can be reached either by starting with the reactants or by starting with the products.
EQUILIBRIUM CHARACTERISTICS:
- the forward and reverse reactions are occurring at equal rates
- concentration of reactants and products remain constant but not equal
- no change in macroscopic properties
- equilibrium can be achieved in either direction
U7.1.2 The equilibrium law describes how the equilibrium constant (Kc) can be determined for a particular chemical reaction.
The law of equilibrium states that:
At any given temperature, the ratio of concentration of products to the power of their molar coefficients to the concentration of reactants raised to the power of their molar coefficients is a constant called equilibrium constant.
For a homogenous reaction, a constant can be deduced from the balanced chemical equation. The constant is called the concentration 'equilibrium constant', Kc, and is constant for a reaction at a particular temperature.
For the reaction aA + bB ⇌ cC + dD
the 'equilibrium expression' can be written as:
Kc =([C]^c x [D]^d) / ([A]^a x [B]^b)
NOTE: Kc CAN have units, however it depends on the reaction. Sometimes Kc has no unit at all.
Inversing reaction = 1/Kc
Doubling reaction = Kc squared
Tripling reaction = Kc cubed
Halving reaction = root Kc
Adding together two reactions = Kc^i x Kc^ii
U7.1.3 The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent.
Kc >> 1 = reaction almost goes to complete i.e. mostly products, less reactants
Kc << 1 = equilibrium mixture is mostly reactants
Kc = 1 = product and reactant concentrations are equal
ONLY temperature can affect the value of Kc.
U7.1.4 The reaction quotient (Q) measures the relative amount of products and reactants present during a reaction at a particular point in time. Q is the equilibrium expression with non-equilibrium concentrations. The position of the equilibrium changes with changes in concentration, pressure, and temperature.
Reaction quotient = a measure of the relative amount of products and reactants present in a reaction that is not in a state of equilibrium; equilibrium constant with non-equilibrium concentrations.
If a system has not reached equilibrium, the ratio of concentration of product to reactants will not equal Kc. This ratio is called the reaction quotient Q and this helps you to determine the progress of the reaction and the direction of the reaction that is favoured to eastablish equilibrium.
U7.1.5 A catalyst has no effect on the position of equilibrium or the equilibrium constant.
UNDERSTANDINGS:
U7.1.1 A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.
Most chemical reactions are reversible, meaning that the products can turn back into the reactants. This is represented with the symbol
⇌ in chemical equations:
A + B ⇌ C + D
When dynamic equilibrium is reached, the rates of the reactants of the forwards and reverse reactions are equal, so there is no overall change in the concentrations of the reactants or products. Both reactions are still occurring but at the same rate.
NOTE: dynamic equilibrium is the state of balance in a reversible process when the forward and reverse changes are proceeding at equal rates; there is no overall change in the concentrations of the reactants or products.
Equilibrium can ONLY be set up in a closed system i.e. a container that does not allow anything to enter or escape. A physical dynamic equilibrium occurs when the rate of vapourisation of water is equal to the rate of condensation. Another example of physical dynamic equilbrium is a mixture of ice and water at the freezing point of water.
In a chemical equilibrium, the position of equilibrium can be reached either by starting with the reactants or by starting with the products.
EQUILIBRIUM CHARACTERISTICS:
- the forward and reverse reactions are occurring at equal rates
- concentration of reactants and products remain constant but not equal
- no change in macroscopic properties
- equilibrium can be achieved in either direction
U7.1.2 The equilibrium law describes how the equilibrium constant (Kc) can be determined for a particular chemical reaction.
The law of equilibrium states that:
At any given temperature, the ratio of concentration of products to the power of their molar coefficients to the concentration of reactants raised to the power of their molar coefficients is a constant called equilibrium constant.
For a homogenous reaction, a constant can be deduced from the balanced chemical equation. The constant is called the concentration 'equilibrium constant', Kc, and is constant for a reaction at a particular temperature.
For the reaction aA + bB ⇌ cC + dD
the 'equilibrium expression' can be written as:
Kc =([C]^c x [D]^d) / ([A]^a x [B]^b)
NOTE: Kc CAN have units, however it depends on the reaction. Sometimes Kc has no unit at all.
Inversing reaction = 1/Kc
Doubling reaction = Kc squared
Tripling reaction = Kc cubed
Halving reaction = root Kc
Adding together two reactions = Kc^i x Kc^ii
U7.1.3 The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent.
Kc >> 1 = reaction almost goes to complete i.e. mostly products, less reactants
Kc << 1 = equilibrium mixture is mostly reactants
Kc = 1 = product and reactant concentrations are equal
ONLY temperature can affect the value of Kc.
U7.1.4 The reaction quotient (Q) measures the relative amount of products and reactants present during a reaction at a particular point in time. Q is the equilibrium expression with non-equilibrium concentrations. The position of the equilibrium changes with changes in concentration, pressure, and temperature.
Reaction quotient = a measure of the relative amount of products and reactants present in a reaction that is not in a state of equilibrium; equilibrium constant with non-equilibrium concentrations.
If a system has not reached equilibrium, the ratio of concentration of product to reactants will not equal Kc. This ratio is called the reaction quotient Q and this helps you to determine the progress of the reaction and the direction of the reaction that is favoured to eastablish equilibrium.
U7.1.5 A catalyst has no effect on the position of equilibrium or the equilibrium constant.
APPLICATION:
A7.1.1 The characteristics of chemical and physical systems in a state of equilibrium.
See U7.1.1
A7.1.2 Deduction of the equilibrium constant expression (Kc) from an equation for a homogeneous reaction.
NOTE: Notice that the value is just '4'. There are no units for Kc in this particular example as there are two mol/dm3 units on both the top and bottom of the ratio, thus they cancel out.
SL chemistry is not required to calculate Kc.
A7.1.3 Determination of the relationship between different equilibrium constants (Kc)
for the same reaction at the same temperature.
check...
A7.1.4 Application of Le Châtelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant. - see U7.1.4
"If dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change."
NOTE AGAIN: Kc is a constant that ONLY changes with temperature. Therefore, if a change happens that results in less products being made, the top of the equilibrium expression will decrease. This would mean that Kc would also decrease, but because Kc is a constant, the bottom of the equilbrium expression must also decrease to counteract this change.
check...
A7.1.4 Application of Le Châtelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant. - see U7.1.4
"If dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change."
NOTE AGAIN: Kc is a constant that ONLY changes with temperature. Therefore, if a change happens that results in less products being made, the top of the equilibrium expression will decrease. This would mean that Kc would also decrease, but because Kc is a constant, the bottom of the equilbrium expression must also decrease to counteract this change.
The Haber Process
The Haber process is used to manufacture ammonia on a large scale. It is an essential process as the ammonia made is the key ingredient for the manufacture of chemical fertilisers, without which, large-scale crop production would be impossible.
The Haber process is used to manufacture ammonia on a large scale. It is an essential process as the ammonia made is the key ingredient for the manufacture of chemical fertilisers, without which, large-scale crop production would be impossible.
The Contact Process
The Contact process is used for the manufacture of sulphuric acid.
2SO2(g) + O2(g) ⇌ 2SO3(g)
The equation above is the second stage of the Contact process and ∆H = -196 KJ/mol, making it an exothermic reaction.
The Contact process is used for the manufacture of sulphuric acid.
2SO2(g) + O2(g) ⇌ 2SO3(g)
The equation above is the second stage of the Contact process and ∆H = -196 KJ/mol, making it an exothermic reaction.