Question

Consider the following reversible, exothermic chemical reaction: 2C2H2(g) + 5O2(g) ↔4CO2(g) + 2H2O(g)
All BUT one of the conditions will cause equilibrium to shift right and produce more of the products. Which change will cause the reaction to shift to the left?
A) adding oxygen
B) adding CO2
C) increasing the pressure
D) decreasing the temperature

Answer 1

Adding CO2 increases Q and makes Q > K (K being the equilibrium constant for this reaction).  As a result, the reaction will favor reactants until Q=K, so adding CO2 causes the reaction to proceed to the left.  B is the correct answer.

Answer 2

Answer : The correct option is, (B) adding $CO_2$

Explanation :

Le-Chatelier's principle : This principle states that if any change in the variables of the reaction, the equilibrium will shift in the direction to minimize the effect.

Option A : adding oxygen

If the concentration of $O_2$ is increased on reactant side then the equilibrium will shift in the direction where decrease of concentration of $O_2$ takes place. Therefore, the equilibrium will shift in the right direction.

Option B : adding $CO_2$

If the concentration of $CO_2$ is increased on product side then the equilibrium will shift in the direction where decrease of concentration of $CO_2$ takes place. Therefore, the equilibrium will shift in the left direction.

Option C : increasing the pressure

If the pressure of the container is increased then the equilibrium will shift in the direction where the less number of moles of gas molecules is taking place. As the number of moles of gas molecules is greater at the product reactant side and less at product side. So, the equilibrium will shift in the right direction.

Option D : decreasing the temperature

As the given reaction is an exothermic reaction in which the heat is released during a chemical reaction. That means the temperature is decreased on the reactant side. If the temperature is decreased in the equilibrium then the equilibrium will shift in the direction where, temperature is getting increased. Thus, the reaction will shift to the right direction i.e, towards the product.