Kaplan MCAT General Chemistry Review

The transition state, also called the activated complex, has greater energy than either the reactants or the products and is denoted by the symbol ‡. An amount of energy at least equal to the activation energy is required to bring the reactants to this energy level. Once an activated complex is formed, it can either dissociate into the products or revert to reactants without any additional energy input. Transition states are distinguished from reaction intermediates in that, existing as they do at energy maxima, transition states exist on a continuum rather than having distinct identities and finite lifetimes.

A potential energy diagram illustrates the relationship between the activation energy, the heats of reaction, and the potential energy of the system. The most important features to recognize in such diagrams are the relative energies of all of the products and reactants. The enthalpy change of the reaction (H) is the difference between the potential energy of the products and the potential energy of the reactants (see Chapter 6). A negative enthalpy change indicates an exothermic reaction (one in which heat is given off), and a positive enthalpy indicates an endothermic reaction (one in which heat is absorbed). The activated complex, the transition state, exists at the top of the energy barrier. The difference in potential energies between the activated complex and the reactants is the activation energy of the forward reaction; the difference potential in energies between the activated complex and the products is the activation energy of the reverse reaction.

Key Concept

-H = exothermic = heat given off +H = endothermic = heat absorbed

For example, consider the formation of HCl from H₂ and Cl₂. The overall reaction is

H₂ (g) + Cl₂ (g) 2 HCl (g)

Figure 5.2 shows that the reaction is exothermic. The potential energy of the products is less than the potential energy of the reactants; heat is evolved, and the enthalpy change of the reaction is negative.

Figure 5.2

MCAT Expertise

Kinetics and thermodynamics should be considered separately. Note that the potential energy of the product can be raised or lowered, thereby changing the value of H without affecting the value of forward E_a.

FACTORS AFFECTING REACTION RATE

If you imagine a group of children playing together in a rough-and-tumble game of tag, we can identify certain conditions that would result in a more vigorous, more fun, more effective game. We can use this analogy to remember the factors that can affect chemical reaction rates.

Reactant Concentrations

A group of children playing tag will probably have more fun the greater the number of children playing: The participation of more children equals more opportunities to chase, trip, and tag each other. The greater the concentrations of the reactants, the greater the number of effective collisions per unit time. Therefore, the reaction rate will increase for all but zero-order reactions. For reactions occurring in the gaseous state, the partial pressures of the gas reactants serve as a measure of concentration (see Chapter 7, The Gas Phase).

MCAT Expertise

We saw earlier in this chapter that increasing reactant concentrations might increase the reaction rate, but be aware of each reactant^’s order before making this assumption.

Temperature

You can imagine that children’s enthusiasm for playing tag outside increases as the temperature warms from the bone-chilling snowstorms of winter to the pleasant sun of June. For nearly all reactions, the reaction rate will increase as the temperature increases. Because the temperature of a substance is a measure of the particles’ average kinetic energy, increasing the temperature increases the average kinetic energy of the molecules. Consequently, the proportion of molecules having energies greater than E_a (thus capable of undergoing reaction) increases with higher temperature. You’ll often hear that raising the temperature of a system by 10°C will result in an approximate doubling of the reaction rate. You have to be careful with this approximation, as it is generally true for biological systems but not so for many other systems. (Don’t forget also: If the temperature gets too high, a catalyst may denature—and then the reaction rate plummets!)

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