Reversible Reactions Flashcards
(23 cards)
Define a reversible reaction.
A reversible reaction is one that can go in either direction depending on the conditions of the reaction.
What can be said about the reactants and products when both forward and reverse reactions occur simultaneously?
The reactants and products are never fully consumed, they are each constantly reacting and being produced.
Describe the concentration of the limiting reactant in:
1: irreversible reactions
2: reversible reactions
1: Concentration of the limiting reactant decreases to zero as the reaction proceeds to completion.
2: Concentration of the limiting reactant does not decrease to zero as the equilibrium is established.
State the factor affecting the reversibility of a reaction.
The magnitude of the activation energy of the reverse reaction.
Exothermic reversible reaction: If the activation energy of the reverse reaction is too large, the reverse reaction is essentially non-occurring compared to the forward reaction. Hence the forward reaction would proceed almost to completion.
Define a dynamic equilibrium.
A dynamic equilibrium exists in a system which the forward and reverse reactions take place at the same rate, and thus the concentration of each species of reactant and product remain constant.
State 4 characteristics of a system in dynamic equilibrium.
1: Dynamic equilibrium is not static (forward and reverse reactions occurring at the same rate)
2: Dynamic equilibrium can be obtained from either direction.
3: Dynamic equilibrium can only be attained in a closed system.
4: Concentrations of reactants and products in dynamic equilibrium remain constant with time.
Define “position of equilibrium”.
The position of equilibrium refers to the relative proportion of products to reactants in an equilibrium mixture.
State the position of equilibrium when:
1: [reactant] < [product]
2: [reactant] > [product]
1: position of equilibrium lies towards the right (reaction is product-favoured)
2: position of equilibrium lies towards the left (reaction is reactant-favoured)
State what is Le Chatelier’s principle.
When a system in equilibrium is subjected to a change in conditions which disturbs the equilibrium, the position of equilibrium will shift to reduce that change.
How will the concentration of reactants and products change when a change is introduced to a system at equilibrium?
The system will readjust itself to reach a new equilibrium where the concentration of reactants and products become constant again, but these concentration values would be different from those of the previous equilibrium state.
Le Chatelier’s principle can predict a quantitative amount of reactants and products.
True/False?
False
Le Chatelier’s principle can predict the effects of a change in conditions such as concentration, temperature or pressure on a system at equilibrium.
List the factors affecting position of equilibrium.
concentration, pressure (applicable for gases only), temperature
Note: A catalyst does not affect position of equilibrium. It only increases the rates of forward and reverse reactions by the same extent and thus brings the system to equilibrium more rapidly.
Consider the equilibrium:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
When some N2 is added to the equilibrium mixture,
i) how will the position of equilibrium shift?
ii) how will concentration of H2 and NH3 change?
i) By Le Chatelier’s principle, position of equilibrium will shift to the right so as to decrease concentration of N2.
ii) Concentration of H2 will decrease while concentration of NH3 will increase.
Consider the reaction in equilibrium:
pale yellow Fe^3+ (aq) + colourless SCN^- (aq) ⇌ blood-red [FeSCN]^2+ (aq)
State and explain the observations when:
i) NaSCN (aq) is added to the above equilibrium, given that NaSCN (aq) -> Na^+ (aq) + SCN^- (aq).
ii) [Fe^3+] is decreased.
i) When NaSCN is added, [SCN^-] increases and the equilibrium is disturbed. By Le Chatelier’s principle, the system would decrease the [SCN^-] by favouring the forward reaction. The position of equilibrium will shift to the right to reduce the increase in [SCN^-]. Thus, blood-red colour deepens.
ii) The equilibrium is disturbed. By Le Chatelier’s principle, the system would increase the [Fe^3+] by favouring the reverse reaction. The position of equilibrium will shift to the left to reduce the decrease in [Fe^3+]. Thus, red solution turns yellow.
Under what condition will a gaseous equilibrium system be affected with a change in total pressure?
When there are unequal number of gaseous molecules on each side of the equation.
E.g. N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Under what condition will a gaseous equilibrium system not be affected with a change in total pressure?
When there are equal number of gaseous molecules on each side of the equation.
E.g. H2 (g) + I2 (g) ⇌ 2HI (g)
Consider the equilibrium:
2N2 (g) + 6H2 (g) ⇌ 3NH3 (g)
State and explain the how the position of equilibrium changes when total pressure is increased. Predict the new equilibrium.
When total pressure is increased, by Le Chatelier’s principle, the position of equilibrium shifts to the right so as to decrease the pressure by favouring fewer number of moles of gas, to reduce the increase in total pressure.
New equilibrium:
N2 (g) + 3H2 (g) ⇌ 5NH3 (g)
Briefly explain what type of process be favoured when:
i) temperature increases.
ii) temperature decreases.
i) By Le Chatelier’s principle, an increase in temperature favours the endothermic reaction to absorb the additional heat so as to reduce the increase in temperature.
ii) By Le Chatelier’s principle, a decrease in temperature favours the exothermic process to generate additional heat to reduce the decrease in temperature.
State and explain the change when a catalyst is added to an equilibrium system.
It increases both the forward and reverse reaction rates by the same extent. This is because the catalyst lowers the activation energy of both forward and reverse reactions to the same extent.
Does a catalyst changes the position of equilibrium?
No. It only enables the state of equilibrium to be reached more quickly.
Methylbenzene, C7H8, is required on a large scale both as an industrially important organic compound and also because it is added to high grade unleaded petrol. C7H8 is manufactured by cyclising and dehydrogenating heptane.
C7H16 (g) ⇌ C7H8 (g) + 4H2 (g)
Predict and explain the effect on the equilibrium position by an increase in pressure. With brief reasoning, suggest a value for the pressure that would be used in a typical plant
An increase in pressure shifts the equilibrium position to the left by le Chatelier’s principle to decrease the number of gas molecules so as to reduce the increase in pressure. The compromised pressure used in a typical plant is around 1 atm. Too high pressure would decrease the yield of C7H8 while too low pressure would decrease rate of reaction.
When air is heated to 2000°C, nitrogen and oxygen can react to form nitrogen monoxide.
N2 (g) + O2 (g) ⇌ 2NO (g)
ΔH> 0
Describe and explain how the equilibrium position will shift when:
i) the volume of the reaction vessel decreases.
ii) the temperature in the reaction vessel decreases.
i) A decrease in volume results in an increase in total pressure of the system. Pressure change will not affect the equilibrium position as the number of moles of gas on both sides are the same.
ii) By Le Chatelier’s principle, as the forward reaction is endothermic, an increase in temperature will favour forward reaction to absorb the additional heat so as to reduce the increase in temperature. This increases the yield of NO (g). When heat is supplied to the system, the system would like to return to its equilibrium by removing this extra heat added. Thus, the endothermic reaction is favoured.
Note: ΔH> 0 means endothermic
