When oxygen gas is reacted with hydrogen bromide, bromine gas is produced and heat is evolved according to the equation:
4HBr(g) + O2(g)
Br2(g) + 2H2O(g)
ΔH = –155 kJ mol–1
Predict the effect on the equilibrium position and the equilibrium constant of the following changes.
a Extra HBr is added to the container.
b The vessel is heated.
c Some unreactive argon gas is pumped into the container.
d The volume of the reaction vessel is decreased.
e A catalyst is employed.
a) If extra HBr is added to the container, the equilibrium position will shift to the right, favoring the formation of more products (Br2 and H2O). This is because according to Le Chatelier's principle, when more reactant (HBr) is added, the equilibrium will shift in the direction that consumes the added reactant. The equilibrium constant, however, will remain unchanged.
b) If the vessel is heated, the equilibrium position will shift to the left, favoring the formation of more reactants (HBr and O2). This is because the reaction is exothermic (heat is evolved), and according to Le Chatelier's principle, increasing the temperature will favor the endothermic direction to consume the excess heat. The equilibrium constant will not be affected.
c) If unreactive argon gas is pumped into the container, it will have no effect on the equilibrium position or the equilibrium constant. This is because argon is unreactive and does not participate in the chemical reaction.
d) If the volume of the reaction vessel is decreased, the equilibrium position will shift to the side with fewer moles of gas. In this case, it will shift to the right, favoring the formation of more products (Br2 and H2O). This is because according to Le Chatelier's principle, when the volume is decreased, the equilibrium will shift in the direction that reduces the number of moles of gas. The equilibrium constant will not be affected.
e) If a catalyst is employed, it will increase the rate of the forward and reverse reactions equally, but it will not affect the equilibrium position or the equilibrium constant. This is because a catalyst provides an alternate pathway for the reaction, reducing the activation energy for both the forward and reverse reactions. The equilibrium position is determined by the balance of forward and reverse reactions, which is not affected by the presence of a catalyst. The equilibrium constant will remain unchanged.
To predict the effect on the equilibrium position and the equilibrium constant of the given changes, we need to consider Le Chatelier's principle. This principle states that if a system in equilibrium is subjected to a change, it will adjust itself in order to minimize the effect of that change.
Here's how each change will affect the equilibrium position and the equilibrium constant:
a) Extra HBr is added to the container:
According to Le Chatelier's principle, adding more of a reactant will shift the equilibrium in the direction that consumes the additional reactant. In this case, adding more HBr will shift the equilibrium to the right, favoring the formation of more bromine gas (Br2). As a result, the concentration of Br2 will increase while the concentration of HBr will decrease. However, the equilibrium constant (K) remains the same because it only depends on the temperature.
b) The vessel is heated:
Increasing the temperature will favor the endothermic reaction to absorb the excess heat. In this reaction, the forward reaction is endothermic, meaning it absorbs heat. So, by increasing the temperature, the equilibrium will shift to the right to consume the excess heat. As a result, more bromine gas (Br2) will be formed. The equilibrium constant (K) will also change as it depends on the temperature.
c) Some unreactive argon gas is pumped into the container:
Adding an inert gas, like argon, does not have any effect on the equilibrium position or the equilibrium constant. This is because an inert gas does not participate in the reaction and does not affect the concentration of reactants or products.
d) The volume of the reaction vessel is decreased:
According to Le Chatelier's principle, decreasing the volume will shift the equilibrium in the direction that reduces the number of moles of gas. In this reaction, the forward reaction produces fewer moles of gas (Br2 and H2O) compared to the reactants (HBr and O2). So, decreasing the volume will shift the equilibrium to the side with fewer moles of gas, which is the left. As a result, the concentration of HBr and O2 will increase while the concentration of Br2 and H2O will decrease. However, the equilibrium constant (K) remains the same as it only depends on the temperature.
e) A catalyst is employed:
Adding a catalyst does not affect the equilibrium position or the equilibrium constant. A catalyst speeds up the rate of both the forward and reverse reactions but does not alter the position of the equilibrium.
Note: The equilibrium constant (K) is a constant value that depends only on the temperature. It is calculated using the concentrations of the reactants and products at equilibrium and does not change with changes in concentrations, pressure, or the presence of a catalyst.
a) When extra HBr is added to the container, according to Le Chatelier's principle, the equilibrium will shift to the left to consume the additional HBr. This will result in an increase in the concentration of HBr and a decrease in the concentrations of Br2 and H2O. Therefore, the equilibrium position will shift towards the reactants, decreasing the concentration of products. The equilibrium constant, K, will remain the same as it depends on the temperature only.
b) When the vessel is heated, the reaction is exothermic, meaning heat is produced as a product. According to Le Chatelier's principle, the equilibrium will shift to the left to consume the excess heat. This will result in an increase in the concentrations of HBr and O2, while decreasing the concentrations of Br2 and H2O. Therefore, the equilibrium position will shift towards the reactants, decreasing the concentration of products. The equilibrium constant, K, will remain the same as it depends on the temperature only.
c) When unreactive argon gas is pumped into the container, it does not participate in the reaction and therefore does not affect the equilibrium position or the equilibrium constant. The presence of argon gas has no impact on the concentration of the reactants and products.
d) When the volume of the reaction vessel is decreased, the pressure inside the vessel will increase. According to Le Chatelier's principle, an increase in pressure will cause the equilibrium to shift in the direction that reduces the total number of gas molecules. In this case, the equilibrium will shift to the right to decrease the pressure. This will result in an increase in the concentrations of Br2 and H2O, while decreasing the concentrations of HBr and O2. Therefore, the equilibrium position will shift towards the products, increasing the concentration of products. The equilibrium constant, K, will remain the same as it depends on the temperature only.
e) When a catalyst is employed, it will increase the rate of both the forward and reverse reactions, but it will not affect the equilibrium position or the equilibrium constant. The catalyst provides an alternative pathway with lower activation energy, allowing the reaction to reach equilibrium faster. However, it does not have any effect on the concentrations of the reactants and products at equilibrium.