Kaplan MCAT General Chemistry Review

In this decomposition reaction, diatomic hydrogen gas is cleaved to produce mono-atomic hydrogen gas. For each mole of H₂ cleaved, 436 kJ of energy is absorbed by the system in order to overcome the bonding force. Since energy is absorbed, the bond-breaking reaction is endothermic.

If you have a hard time remembering whether bond formation or dissociation is endothermic, think about bonds as if they were two bar magnets stuck together. You have to exert pulling forces (invest energy) to pull apart two bar magnets (endothermic). On the other hand, the two bar magnets, once separated, “want” to come back together because they exert an attractive force between their opposite poles. Allowing them to stick together reduces their potential energy (exothermic).

Example: Calculate the enthalpy change for the following reaction:

C(s) + 2 H₂(g) CH₄(g) H = ?

Bond dissociation energies of H–H and C–H bonds are 436 kJ/mol and 415 kJ/mol, respectively.

H_f of C( g) = 715 kJ/mol

Solution: CH₄ is formed from free elements in their standard states (C in solid and H₂ in gaseous state).

Thus here, H_rxn = H_f.

The reaction can be written in three steps:

a) C (s) C ( g)H₁b) 2 [H₂ (g) 2 H (g)]2H₂c) C (g) + 4 H (g) CH₄ (g)H₃

and H_f = [H₁ + 2 H₂] + [H₃].

H₁ = H_f C ( g) = 715 kJ/mol

H₂ is the energy required to break the H–H bond of one mole of H₂. So,

H₂ = bond energy of H₂ = 436 kJ/mol

H₃ is the energy released when 4 C–H bonds are formed. So,

H₃ = -(4 × bond energy of C–H) = -(4 × 415 kJ/mol) = -1,660 kJ/mol

(Note: Because energy is released when bonds are formed, H₃ is negative.)

Therefore,

H_rxn = H_f = [715 + 2(436)] - (1,660) kJ/mol = -73 kJ/mol

MCAT Expertise

With practice, you’ll become accustomed to the patterns and shortcuts that will make things easier on Test Day.

Key Concept

The larger the alkane reactant, the more numerous the combustion products.

Heats of Combustion

One more type of standard enthalpy change that you should be aware of for the MCAT is the standard heat of combustion, H°_comb. Because measurements of enthalpy change require a reaction to be spontaneous and fast, combustion reactions are the ideal process for such measurements. Most combustion reactions presented on the MCAT occur in the presence of O₂ in the atmosphere, but keep in mind that there are other combustion reactions in which oxygen is not the oxidant. Diatomic fluorine, for example, can be used as an oxidant, and hydrogen gas will combust with chlorine gas to form gaseous hydrochloric acid and, in the process, will evolve a large amount of heat and light characteristic of combustion reactions. The reactions listed in the C₃H₈ (g) example shown earlier are combustion reactions with O₂ (g) as the oxidant. Therefore, the enthalpy change listed for each of the three reactions is the H_comb for each of the reactions.

Entropy

Many, many students are genuinely perplexed by the concept of entropy. Enthalpy makes, perhaps, intuitive sense, especially when the energy change from reactants to products is large, fast, and dramatic (as in combustion reactions involving explosions). Entropy seems to be less intuitive. Except that it isn’t. In fact, our understanding of what constitutes normal life experience and even the passage of time is based, fundamentally, upon the property of entropy. Consider, for example, how “normal” each of the following seems to you: hot tea cools down, frozen drinks melt, iron rusts, buildings crumble, balloons deflate, living things die, and so on.

Key Concept

Entropy changes that accompany phase changes (see Chapter 8) can be easily estimated, at least qualitatively. For example, freezing is accompanied by a decrease in entropy, as the relatively disordered liquid becomes a well-ordered solid. Meanwhile, boiling is accompanied by a large increase in entropy, as the liquid becomes a much more disordered gas. For any substance, sublimation will be the phase transition with the greatest entropy change.