Thermodynamics tells us whether a reaction will occur. But knowing a reaction is spontaneous doesn’t tell us anything about how fast the reaction occurs. Kinetics is the study of reaction rates. Reaction rates tell us how fast the reaction occurs. A reaction rate is the change in concentration of a reactant or product with time. Typical concentration units are molar, and typical time units are seconds.

We can determine a reaction's speed by measuring the rate at which it occurs. When A is converted to B, the concentration of A decreases and the concentration of B increases at the same rate. Thermodynamics tells us that sugars, such as sucrose and glucose, decompose spontaneously. We know from personal experience that we get energy from eating sugar. When we eat sugar, it decomposes into water and carbon dioxide. How long will it take sugar to decompose into carbon dioxide and water?

It depends on the conditions. If we heat the sugar, its decomposition will take just a few seconds. Sugar also won't take very long to decompose if we eat it. But if we leave it alone, sugar will take years to decompose.

Why are the reaction rates so different? Let’s use an energy diagram to answer this question. An energy diagram illustrates the energy pathway of a reaction. The products are at lower energy than the reactants. This tells us that the change in free energy for the forward reaction is negative and the reaction is spontaneous.

Notice the hill on the energy diagram. The hill represents an energy barrier — something the reacting molecules must cross in order to become products. The energy barrier is called the activation energy. The activation energy is the minimum amount of energy needed to initiate a reaction.

At room temperature, very few of the glucose molecules have enough energy to cross this barrier. When heated, the kinetic energy of the glucose molecules increases and they react. What speeds up the reaction when we eat sugar? The reactants follow a pathway with a lower energy barrier. Catalysts in our bodies guide the reactants through this new course. A catalyst is a substance that increases the reaction rate without being consumed. The catalysts used by living organisms are enzymes.

Notice that while the hill is lowered, enzymes do not affect the free energy values of either the reactants or the products. These values are related to how far the reaction proceeds before equilibrium is reached. Enzymes have no effect on the reaction equilibrium.

When we need quick energy, we eat a candy bar. It’s a good thing we don’t have to wait years for the metabolism of sugar to occur!

In this activity, we'll learn how enzymes work as catalysts, how they catalyze specific reactions, and how reaction conditions affect reaction rates.

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