Kaplan MCAT General Chemistry Review

1) A student wants to create H₂O from its natural state elements. If he plans to use gaseous hydrogen and oxygen, what type of system stress would help the reaction progress? Does it matter whether he forms water vapor or liquid water?

2) The reaction [Co(H₂O)₆]²⁺ (aq) + 4 Cl^- (aq) CoCl₄^2- (aq) + 6 H₂O (l) describes the formation of a blue solution from one that is pink. A student isolates the solid of both cobalt compounds. What happens when he adds them to separate flasks of water?

3) A cold pack works by reacting NH₄NO₃ with water. The reaction requires energy to solvate ammonium nitrate. What is the effect of putting a cold pack on your skin? What is the effect of putting a cold pack in a 0°C freezer?

Reaction Energy Profiles

Key Concepts

Chapter 6

Thermodynamics

Kinetics

Reaction profiles

Takeaways

The goal of a reaction profile is to give you information about energy differences. Make sure that you identify the important differences and their significances, as above.

When chalcone (A) is subjected to reductive conditions with sodium borohydride, two products can result. The two products are the so-called “1,2-reduction” product (B), in which the carbonyl is reduced, and the “1,4-reduction” product (C), in which the conjugated alkene is reduced.

The reaction profiles leading to each reduction product are both shown in the plot below.

Based on the plot above, answer the following questions:

1) Which product is more thermodynamically stable? Which one forms faster?

2) Assume that A is in equilibrium with C. What will the ratio of C to A be at equilibrium?

3) How could the rate of the reaction of A to C be made closer to the rate of the reaction of A to B?

4) Which product would be favored if A were subjected to high temperatures for a long time? If A were subjected to low temperatures for only a brief period of time? Explain why for each situation.

Things to Watch Out For

Be careful to take note of the units of energy on the y-axis if you plan on doing any computations.

1) Look at the energy differences between the starting material and the product(s), as well as the differences between the starting material and the transition state leading to each product.

Notice that the energy of C is lower than that of B. Therefore, it is the more thermodynamically stable product.

The rate of formation of each product is determined by the difference in energy between the starting material A and the top of the “hump” leading to each product. Because this distance is lower for the formation of B, it forms faster.

2) Note that the difference in energy between the starting material and the product(s) determines the ratio of products to reactants at equilibrium.

G° = -RT ln K_eq

This equation provides the relationship between K_eq and G°. We need to rearrange it to solve for K_eq.

Note that G° 1,000 - 3,500 = -2,500 cal mol^-1, from the diagram, and R = 1.99 cal (mol K)^-1 2 cal (mol K)^-1 and T = 298 300 K.

Let’s say 2,500/600 is about equal to 4, and e = 2.7818 3.

Note that a negative G° gives more product than reactant, as you would expect for a spontaneous reaction.

3) Consider what role(s) the addition of a catalyst might play.

Remember: A catalyst is something that speeds up a reaction and is not consumed during a reaction. If it speeds up a reaction, it lowers the “hump” in the reaction profile. So we could increase the rate of formation of C by adding a catalyst to that reaction, making the rate closer to the rate of formation of B.

4) Consider the effects of temperature on the reaction(s).

At high temperatures for long times, A has the energy to go back and forth over and over again between B and C. Then, over time, the lowest energy product C would predominate, just as rolling a ball down a hill would cause it to fall to the lowest location.

Between the two products, B will form much faster than C because its energy of activation (height of the “hump”) is lower. At low temperatures, the products won’t have the energy to go back over the hill to get to A, so the faster-forming product will predominate (i.e., B).

Similar Questions

1) What would be the ratio of B to A at equilibrium?