MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Equilibrium
Answers to Concept Checks

6.1

1. First calculate the value of Q from the given concentrations:

Image

Q can now be compared to each value of Keq to predict the direction of the reaction.

Keq

Direction of Reaction

ΔG

5.0 × 10−2

At equilibrium: no net reaction

0

5.0 × 10−3

Qc > Keq: proceeds toward reactants (left)

Positive

5.0 × 10−1

Qc < Keq: proceeds toward products (right)

Negative

2. Image

Image

3. The concentration of a reactant that converts to product can be considered negligible if it is two or more orders of magnitude less than the initial concentration of the reactant.



Keq

Initial

Concentration of A (M)

Is the amount

reacted negligible?

1 x 10-12

1

Yes

1 x 10-2

0.1

No

1 x 10-3

0.001

No

1 x 10-15

0.001

Yes

4. The first step in solving is to write the equation for Keq for the reaction:Image

Note that the equation for Keq does not include product D because D is a solid.

Next, using the initial concentrations for A and B and x for the amount that has reacted, plug into the equation for Keq: Image

Given that Keq = 2.1 x 10—7, the concentrations of A and B are sufficiently large that x can be considered negligible in comparison to both. This allows the equation for Keq to be simplified and solved:

Image

The value of x = 4.2 x 10—9 is equal to both the equilibrium concentration of C and the amount of A and B that have reacted. The approximation that x is negligible compared to the initial concentrations of A and B is valid.

· 6.2

1.

§ Increasing pH of H2SO4 (aq) ⇌ H+ (aq) + HSO4− (aq): [H+] decreases, shifting reaction to the right.

§ Decreasing pressure of 2 C (s) + O2 (g) ⇌ 2 CO (g): Reaction shifts right, favoring the side with more moles of gas.

§ Warming CH4 (g) + 2O2 (g) ⇌ CO2 (g) + 2 H2O (l) + heat: Reaction shifts left, using the additional heat energy to produce more reactants.

§ Removing water from H3PO4 (aq) + H2O (l) ⇌ H3O+ (aq) + H2PO4− (aq): Reaction shifts left. All concentrations would increase proportionately; because there are more products than reactants (and the stoichiometric coefficient is 1 for each reactant and product), the value of Q will increase.

· 6.3

1. Kinetic products are favored at low temperatures with low heat transfer. Thermodynamic products are favored at high temperatures with high heat transfer.

2. Kinetic pathways require a smaller gain in free energy to reach the transition state. They also have a higher free energy of the products, with a smaller difference in free energy between the transition state and the products.