7 Chemical Equilibria Flashcards
Explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium.
A reversible reaction is one that proceeds in both the forward and backward directions.
Dynamic equilibrium refers to a state in a reversible system in which the rates of the forward and backward reactions are continuing at the same rate, resulting in no net change in the macroscopic properties (concentration, partial pressure) of the reactants and products.
State Le Chatelier’s principle and apply it to deduce qualitatively the effects of changes in concentration, pressure or temperature, on a system at equilibrium.
Le Chateliers’s principle states that when a system at equilibrium is subjected to a change, the system will react to counteract the change imposed so as to re-establish the equilibrium. It is used to predict the direction in which equilibrium position will shift when equilibrium is disturbed.
Explain what is meant by the reaction quotient.
At any given time, the reaction quotient, Qc, gives a measure of the relative amounts of products and reactants in a reaction mixture.
It is given by the ratio of the concentration of the reactants and products raised to their stoichiometric ratios.
As the reaction proceeds towards equilibrium, Qc changes as the concentration of the reactants and products change continuously until equilibrium is reached.
State how (a) equilibrium constants of the forward and backward reactions are related, (b) how to find the equilibrium constant for an overall reaction where there are two reversible reaction steps, (c) how to find the new equilibrium constant for a reaction with coefficients multiplied by a common factor n.
(a) The equilibrium constant of the forward reaction is the reciprocal of the equilibrium constant of the forward reaction and vice versa.
(b) K overall = K1 time K2 times …
(c) K is raised to the power of the same factor, if the reaction is reversed, include a negative sign in front of the factor.
State how the equilibrium constant of the reaction is related to the rate constants of the forward and backward reactions.
Pg 6 of notes
Kc = Kf/Kb
Explain the factors that affect the equilibrium constants. Explain the significance of equilibrium constants.
Since the rate constants of the forward and backward reaction are INDEPENDENT of concentration or partial pressure but DEPENDENT on temperature only*, Kc is a constant at a specific temperature and it varies with temperature. (The value of equilibrium constants is not affected by changes in concentrations of reactants and products, changes in the total pressure of the reactant system/partial pressures of the reactants and products, presence/absence of the catalyst.)
K is the measure of the extent of the reaction (i.e. gives an indication of how far a reaction proceeds towards the product side at a given temperature).
A small value of K indicates that the reaction yields little products when equilibrium is reached and the reaction favours the reactants over the products at equilibrium. If K is very small, the POE lies very far left and there is no reaction/the forward reaction does not proceed to any appreciate extent.
A large value of K indicates that very little reactants remain when equilibrium is reached and the reaction favours the products over the reactants at equilibrium. If K is very large, the POE lies very much to the right and the reaction goes into completion or the forward reaction is almost complete.
An intermediate value of K indicates that there are significant amounts of reactants and products present at equilibrium. (e.g. 5)
Does the value of K give information about the rates of the forward and backward reactions?
No, it does not.
Define homogeneous equilibrium and heterogeneous equilibrium. Explain why the concentration or partial pressures of pure solids and LIQUIDS are not included in the Kc/Kp expression of heterogeneous equilibrium.
Homogeneous equilibrium refers to an equilibrium system in which all the substances involved are in the same phase.
Heterogeneous equilibrium refers to an equilibrium system involving substances that are not in the same phase (e.g. Sublimation, Dissolution)
Solids have fixed density and hence, fixed concentration. Adding more solid will not increase its concentration. Hence, the concentrations and partial pressures of pure solids are constant and the Kc expression should exclude solids.
The Kc expression also excludes the concentration of water when it is present in large amounts (as a solvent) in an AQUEOUS solution/medium as the concentration of water is approximately constant. (Note: When water is not a solvent, its concentration should be included as its concentration is varying!)
Explain dissociation and what is meant by the degree of dissociation.
Dissociation occurs when gases break down to form smaller gaseous fragments. At equilibrium, the mixture would contain both the undissociated gas and the gases which it has dissociated into. The total number of gaseous molecules is, therefore, greater than that present before dissociation.
Degree of dissociation is the fraction of reactant that has dissociated at a particular temperature and can be expressed as a fraction of percentage; Degree of dissociation = amount dissociated/initial amount
Explain how the change in Gibbs free energy of a reaction can be used to predict if a reaction is spontaneous at a particular composition. Explain how Gibbs free energy G is related to equilibrium constant K.
In a reversible reaction under constant temperature and pressure, the reaction mixture tends to adjust its composition until its Gibbs free energy, G is at a minimum.
If the change in Gibbs free energy is less than 0, the forward reaction tending towards equilibrium is spontaneous and the rate of the forward reaction is faster than that of the backward reaction. If the change in Gibbs free energy is more than 0, the backward reaction tending towards equilibrium is spontaneous and the rate of the backward reaction is faster than that of the forward reaction. If the change in Gibbs free energy is equal to 0, the system is a dynamic equilibrium where the forward rate is equal to the backward rate.
Delta G = -RTlnK
In reactions where the change in Gibbs free energy<0, K>1. This means that the POE lies more to the right, favouring products over reactants. In cases where K»1, the reaction goes to completion or is product favoured.
In reactions where the change in Gibbs free energy>0, K<1. This means that the POE lies more to the left, favouring reactants over products. In cases where K«1, we can say that there is no reaction.
Explain what happens to a system of equilibrium when the concentration of reactants/products is changed by analysising the rates of forward and backward reactions and by comparing the reaction quotient with the equilibrium constant.
When the concentration of reactants increases, the rate of the forward reaction increases (forward reaction is favoured). When the concentration of products increases, the rate of the backward reaction increases (backward reaction is favoured).
When QcKc, the backward reaction is favoured.
When Qc=Kc, equilibrium is reached.
Explain the effect of (a) concentration changes, (b) pressure changes, (c) volume changes, (d) temperature changes,
Pg 21 and 22 of notes
(a) By Le Chatelier’s principle, the equilibrium position will shift right/left (where) to partially offset the increase/decrease (why) in concentration of X by removing/producing more X until a new equilibrium is reached. (Note: This is different from equilibrium position lies to the right/left)
(b) Recall: When the partial pressures of the gaseous reactants and porducts (and hence total pressures) are changed due to changes in volume, the equilibrium system will be disturbed. The effect of changing the total pressure of a gaseous system depends on the stoichiometry of the reaction (whether more or less or equal number of gaseous particles are produced after a reaction). When the total pressure of the equilibrium mizture is increased, by Le Chatelier’s principle, the system will try to counteract the increase in total pressure by favouring the reaction that decreases the total pressure. Hence, the forward reaction/backward reaction is favoured as it produces fewer gas particles, decreasing total pressure and the POE shifts to the left/right. (Note: Change is only partially offset)
When the numebr of gaseous particles on both sides of the system is equal, the change in total pressure does not favour the forward or backward reaction hences the POE does not shift and the ssytem remains at equilibrium with no change to the composition of the equilibrium mixture. This is because the increase or decrease of the numerator in the Qp expression is the same as the increase or decrease of the denorminator since the power of the terms are the same. Hence, Qc still = Kc and there is no shift in the equilibrium position.
(c) When the volume of reaction system is increased under constant T, the total pressure decreases. According to Le Chatelier’s principle, the ssytem will counteract the change in pressure by favouring the forward/backward reaction which produces more gas particles. Hence, the POE shifts right/left. When volume increases, the partial pressures of all gases decrease. Since partial pressure is proportional to the concentration of the gas, concentration decreases, rate of both the forward and backward reactions decreases and the reaction takes longer to reach equilibrium.
(d) When temperature is increased, according to the Le Chatelier’s principle, the system will try to counteract the increase in temperature by favouring the forward/backward endothermic reaction in order to obsorb the heat. The POE shifts right/left, favouring the formation of more products/reactants. Hence, the equilibrum mixture contains more X and less X’ and the Kc value increases/decreases. Additionally, when temperature is increased, the rate constants of both the forward and backward reactions increase since the number of particles with energy greater than or equal to activation energy increases and dynamic equilibrium is reached more quickly. (Draw Maxwell Botlzmann distribution curve)
(e) A catalyst lowers the activation energy of the forward and backward reactions to the same extent hence the rates of both the forward and backward reactions are increased to the saemextent and a catalyst has no effect on equilibrium constant and the composition of the reaction mixture. However, equilibrium is reached more quickly.
Explain how the conditions of the Haber process achieve the aim of producing the maximum amount of product/yield in the shortest time and at the minimum cost.
N2 + 3H2 ->< 2NH3
Since the forward reaction in the Haber process is exothermic, a lower temperature will result in a higher yield of ammonia. However, the rate of production is too slow at low temperature and high temperature will result in lower yield and higher production cost. Thus, a moderately high temperature of 450 degree celsius is used to ensure a reasonable rate of production and yield.
The forward reaction takes place with a reduction in the number of gaseous particles. A high temperature will favour the desired reaction but increases the COP as more expensive and stronger pipes need to be used to withstand the high pressure and there are more safety concerns. Hence, a moderate pressure of 200 atm is used.
A catalyst is used to increase the proudction rate without affectingthe percentage of NH3 in the equilibrium mixture (yield is not affected).Hence, finely divided iron catalyst with aluminium oxide as a promoteris used.
The removal of ammoina shifts the POE towards the right, increasing the yield of ammonia. This is done by cooling the reaction mixture to -50 degree celsius to liquefy ammonia formed.
The molar ratio used is similar tothat of the stoichiometric ratio to minimise wastage.
Explain what happens during the addition of an inert gas at constant volume and at constant pressure.
Recall that partial pressure = nRT/V
At constant volume, the total pressure of the gaseous system is increased. However, the partial pressures remain unchanged and Qc still = Kc and POE does not change.
When an inert gas is added to the system in equilibrium at constant pressure, then the total volume will increase. Hence, the number of moles per unit volume of various reactants and products will decrease. Hence, the equilibrium will shift towards the direction in which there is increase in number of moles of gases.
Eexplain the significance of the position of equilibrium.
The relative composition of the products and reactants present in a reaction mixture at equilibrium is known as the position of equilibrium.
When POE lies to the left, there are more reactants than products. When POE lies to the right, there are more products than reactants,
