Reversible Reactions Flashcards
(15 cards)
What is a reversible reaction?
A reversible reaction is a chemical reaction where reactants react to form products, which can in turn react to form the reactants again under suitable conditions.
Note that the concentration of reactants does not decrease to zero due to the backwards reaction as products react to form reactants.
What is dynamic equilibrium in reversible reactions?
A dynamic equilibrium occurs when the rates of forward and backward reaction have become equal.
As a result, there is no change in the amount/concentration of the reactants and products.
What does Le Chatelier’s Principle state?
The principle states that if a system in equilibrium is subjected to a change in conditions (e.g increase in temperature, decrease in pressure) which disturbs the equilibrium, the system responds in a way to counteract the effects of the change.
E.g SO2 (g) + O2 (g) –> 2SO3 (g)
Should there be an increase in SO2, the equilibrium position would shift right, such that a greater amount of SO2 would be reacted with oxygen gas to form SO3 gas.
What would happen to the position of an equilibrium if a catalyst is added? Explain your reasoning.
There is no change to the position of the equilibrium. The catalyst only serves to lower the activation energy of the forwards and reverse reactions by the same extent. Hence, adding a catalyst only increases the rate of forwards and reverse reactions by the same extent. Therefore, catalysts only allow reaction to reach equilibrium at a faster rate.
What are the general guidelines for answering questions on equilibrium shift?
- Whether forward/backward reaction is favoured, must include reason. For changes in temperature, need to also indicate if the forward/backward reaction is endothermic/exothermic.
- So equilibrium position will shift left/right.
- To minimise change due to equilibrium shift.
E.g C2H4 (g) + H2O (g) —> C2H5OH (l)
When C2H4 is added, forward reaction is favoured so equilibrium position shifts to the right to decrease the amount of C2H4 as the system resists change.
What are some further examples of answering questions on equilibrium shift?
C2H4 (g) + H2O (g) —> C2H5OH (l)
delta H is positive, meaning that the forward reaction is an endothermic reaction.
When temperature is increased, forward reaction is favoured so equilibrium position shifts to the right to decrease the temperature. As this is an endothermic reaction, as C2H4 and H2O react, temperature decreases as heat energy is absorbed.
What is the chemical reaction for the Haber Process?
N2 (g) + 3H2 (g) —> 2NH3 (g)
Category: Haber Process
What property of NH3 allows it to be effectively separated from N2 and H2?
NH3 has a higher melting point.
Category: Haber Process
Why is NH3 removed from the products before N2 and H2 is passed before the catalyst again?
To prevent backwards reaction of NH3 reforming into N2 and H2. By removing NH3, the system attempts to increase NH3, thus favouring forward reaction of N2 and H2 into NH3, thereby increasing product yield.
Category: Haber Process
Why is high pressure used during reaction of N2 and H2?
High pressure is used. By Le Chatelier’s Principle, using a high pressure would shift equilibrium position to the right, thus favouring the forward reaction, which produces less moles of gas. Hence, this would increase ammonia yield, and thus increase the amount of product formed.
Category: Haber Process
Why is a moderate temperature (not too high and not too low) temperature is used even though such a temperature may decrease ammonia yield?
A higher temperature increases the rate of reaction. By Collision theory, as temperature increases, particles have greater kinetic energy, thereby leading to an increase in the number of effective collisions between particles. However, as the reaction of N2 and H2 into NH3 is an exothermic reaction, equilibrium position shifts to the left and the reverse reaction is favoured as the system attempts to the reduce the temperature. Thus, this would also decrease the yield of NH3. In order to compensate and find a balance between increasing rate of reaction and decreasing the decrease in product yield, a moderate temperature is used.
Category: Haber Process
Why is a catalyst used?
A catalyst is able to increase the rate of reaction, and at the same time, it does not affect the equilibrium position of the system. As the catalyst has no effect on the equilibrium position, it therefore does not have any effect on ammonia yield. As such, the presence of a catalyst would only increase the rate of reaction.
Category: Factors affecting chemical equilibrium
What are the factors which affect chemical equilibrium?
- Change in Temperature
- Change in the Amount of Substance in the reaction (be it products or reactants)
- Change in Pressure (encompasses change in volume)
Category: Factors affecting chemical equilibrium
How does temperature affect the equilibrium position?
Taking the forward reaction to be exothermic,
As temperature increases, the equilibrium position shifts to the left as the system attempts to decrease the temperature. Thus, the rate of backward reaction increases in order to decrease temperature and reduce change to the system.
Category: Factors affecting chemical equilibrium
How does change in pressure affect the equilibrium position?
Taking forward reaction to cause a decrease in pressure due to less moles of gas forming.
An increase in pressure would cause the equilibrium position to shift to the right as the system attempts to decrease the pressure. As the forward reaction forms less moles of gas, thus forward reaction reduces pressure and hence the forward reaction is favoured.
