H2 and i2 react in an exothermic reaction according to the following equation:
h2(g) + i2(g) double arrow yields 2hi (g)
h2 and i2 are placed in a sealed container and are allowed to reach equilibrium. what could you change about the system that would shift its equilibrium position?
a. add a catalyst.
b. decrease the volume.
c. increase the pressure.
d. lower the temperature.

The correct answer is Option D.

Explanation:

For the given chemical reaction:

Catalyst is a chemical substance which alters the rate of the reaction and does not participate in the reaction. It does not effect the equilibrium position.

Any change in the equilibrium is studied on the basis of Le-Chatelier's principle. This principle states that if there is any change in the variables of the reaction, the equilibrium will shift in the direction to minimize the effect.

Any change in volume during equilibrium is studied on the basis of change in moles of gaseous reactants and gaseous products.

When volume is decreased, the equilibrium will shift in the direction which produces fewer moles of gas and when volume is increased, the equilibrium will shift in the direction which produces more moles of gas.

Any change in pressure during equilibrium is studied on the basis of change in moles of gaseous reactants and gaseous products.

When pressure is decreased, the equilibrium will shift in the direction, which produces fewer moles of gas and when pressure is increased, the equilibrium will shift in the direction which produces more moles of gas.

Number of moles of gases on reactant side = [1 + 1] = 2

Number of moles of gases on product side = 2

Change in number of moles = 2 -2 = 0

As, number of moles on both the side of the reaction is same. This means there will be no effect on change in volume and change in pressure.

For an exothermic reaction, heat is released during a chemical reaction and is written on the product side.

As, heat is released during a chemical reaction. This means that temperature is decreased on the reactant side. If the temperature in the equilibrium is decreased, the equilibrium will shift in the direction where, temperature is getting increased. Thus, the reaction will shift in right direction that is towards the product.

So, lowering the temperature of the system will shift the position of equilibrium.

Hence, the correct answer is Option D.