Equilibrium, acid-base chemistry, and volumetric analysis

In a closed system:

• both matter and energy are exchanged with the surroundings.

• matter is exchanged with the surroundings.

• energy is exchanged with the surroundings.

• neither matter nor energy are exchanged with the surroundings.

Answer explanation

A system only needs to prevent exchange of matter in order to be defined as 'closed'.

The activation energy, E_a, of the forward reaction is:

• 50 kJ

• 150 kJ

• 200 kJ

• 250 kJ

Answer explanation

Activation energy = energy required to get from initial energy to top of energy barrier.

200 kJ is required to get from 50 kJ to 250 kJ

• +150 kJ

• -150 kJ

• +50 kJ

• -50 kJ

Answer explanation

Reaction enthalpy = energy difference between initial and final state

In the reverse direction, initial energy = 100 kJ, final energy = 50 kJ, with an overall enthalpy of -50 kJ.

The reverse reaction is exothermic.

Which statement best describes a dynamic equilibrium?

• A closed system in which the forward rate of reaction is equal to the reverse rate of reaction.

• A closed system in which the concentration of the reactants and products are equal and constant.

• An open system in which the forward rate of reaction is equal to the reverse rate of reaction.

• An open system in which the concentration of the reactants and products are equal and constant.

Answer explanation

'Dynamic' indicates movement, 'equilibrium' indicates no change over time.

Thus, in a dynamic equilibrium, the reaction is proceeding such that the rate of change in the forward direction is the same as the reverse direction, appearing as if nothing is changing. This can only occur in a closed system.

Remember that the concentrations do not have to be equal in an equilibrium, but the reaction rates do.

Le Chatelier's principle states that "if a chemical system at equilibrium is subjected to a change in conditions, the system will...

• react in a way that quickly returns all reactant and product concentrations to their original values."

• adjust to re-establish equilibrium in a way that partially opposes the change."

• adjust to re-establish equilibrium in a way that completely reverses the change."

• not respond in any way unless the temperature is altered significantly."

Answer explanation

If you disturb an equilibrium (e.g. add or remove one of the components, change the temperature or pressure), the system will oppose the change but never return back to its original state.

For example, if I add some more reactant after the system has reached equilibrium, the forward reaction will proceed in order to reduce that reactant concentration, but not all the way to its original value.

The following reaction is at equilibrium:

H₂(g) + I₂(g) ⇌ 2HI(g)

What will happen if more H₂ is added to the system?

• Concentration of H₂ will decrease, and concentration of HI will increase

• Concentration of I₂ will decrease, and concentration of HI will increase

• Concentration of H₂, I₂, and HI will all decrease

• Concentration of H₂ and I₂ will decrease, and concentration of HI will increase

Answer explanation

Le Chatelier's principle: system will react to partially oppose the change.

If H₂ is added, system will react in forward direction to remove H₂.

Note that both H₂ and I₂ will decrease, as the entire reaction proceeds in the forward direction; you can only remove H₂ by reacting it with I₂.

The following reaction is at equilibrium in a variable-volume system (e.g. a syringe):

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

What will happen if the volume of the system is reduced by half?

• Concentration of SO₂ and O₂ will decrease, and concentration of SO₃ will increase

• Concentration of SO₂ and O₂ will increase, and concentration of SO₃ will decrease

• Concentration of SO₂, O₂, and SO₃ will all decrease

• Concentration of SO₂, O₂, and SO₃ will all increase

Answer explanation

From Boyle's law: a decrease in volume will lead to an increase in pressure (also concentration of all substances)

To decrease the pressure, the system will react to decrease the total number of particles.

Left-side has 3 particles (2SO₂ + O₂), right-side has 2 particles (2SO₃); therefore, reaction will proceed to the right

The following reaction is at equilibrium:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) [ΔH = -92.4 kJ mol⁻¹]

What will be the effect of increasing the temperature?

• Concentration of reactants will decrease and products will increase

• Concentration of reactants will increase and products will decrease

• Concentration of both reactants and products will increase

• Concentration of both reactants and products will decrease

Answer explanation

Negative ΔH = exothermic = heat released (as a product); effectively:

Reactants ⇌ Products + Heat

Increasing temperature increases the amount of heat; thus, the system will react to partially oppose this change and shift to the left (increase reactants, decrease products).

2HI(g) ⇌ H₂(g) + I₂(g)

At 445 °C, K_c for the reaction is 0.020.

A mixture at 445 °C has: [HI] = 2.0 M, [H₂] = 0.50 M, and [I₂] = 0.10 M. Which of the following statements regarding the reaction quotient, Q_c, is TRUE for the above system?

• Q_c is less than K_c; more H2 and I2 will be produced

• Q_c is less than K_c; more HI will be produced

• Q_c is greater than K_c; more H2 and I2 will be produced

• Q_c is greater than K_c; more HI will be produced

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

The above reaction has come to equilibrium. Before the reaction began, there was 0.06 M of SO₂ and 0.05 M of O₂. After equilibrium was reached, there is 0.04 M of SO₃.

What is the equilibrium constant, K_c?

• 0.0075

• 0.015

• 66.67

• 133.33

Answer explanation