Equilibrium and Reaction Rates.doc

Equilibrium and Reaction Rates

Factors That Affect Reaction Rates

For the SAT II Chemistry test, you’ll have to be familiar with certain aspects of chemical reactions, such as equilibrium and reaction rate. The reaction rate is a measure of the change in the concentration of reactants or products over time in a chemical reaction. Four main external conditions affect reaction rate. The first is the concentration of reactants. Generally speaking, if we increase the concentration of one or more reactants, the reaction will go more quickly. This is simple because the more molecules, the more collisions between molecules, and the faster the reaction will go.

The second factor that influences reaction rate is temperature. The higher the temperature of the reaction, the more quickly it will proceed. At higher temperatures, the molecules are moving around more quickly (they have more kinetic energy); this means they will collide with each other with more energy, and it’s more likely that they will overcome the activation energy needed to start the reaction. It’s a general rule of thumb that a 10˚C increase in temperature will double the reaction rate.

The addition of a catalyst will also speed up a chemical reaction. A catalyst speeds up the rate of reaction by lowering the activation energy. Biological catalysts are known as enzymes. The only other important thing you need to remember about catalysts is that they are not consumed in the course of the reaction.

The final factor that affects certain reactions is the physical state of the reactants. For example, if you mix two gases or two liquids, this represents a homogenous reaction, but if reactants are in different phases, for example, if one is a gas and one is a liquid, then the reaction area is limited to the area where they touch each other, and the larger this area, the faster the reaction will proceed. For example, consider a teaspoon of salt dissolving in water. If you were to dump the salt into the beaker of water and let it float to the bottom without stirring it, it would take much longer for it to dissolve than if you stirred the solution.

Now let’s quickly go through those factors that influence reaction rate again:

1.       Concentration of the reactants

2.      Temperature

3.      Presence of a catalyst

4.      Physical state of the reactants

Energy Diagrams

We know that in order for a reaction to occur, reactant molecules must collide and that both an increase in the concentration of reactant molecules and an increase in the temperature of the system can cause an increase in reaction rate. But it takes more than just a regular collision to cause a chemical reaction to occur—in fact, only a very small fraction of collisions that occur in the solution lead to a reaction. This is true for two reasons. First of all, for a reaction to occur, the colliding molecules must be oriented in exactly the correct way: they must be oriented in suitable way for the product molecule bonds to be formed. Second, the two molecules must collide with sufficient energy to overcome the activation energy of the reaction. The activation energy is defined as the minimum energy needed to initiate a chemical reaction, and it is symbolized by Ea.

Now let’s talk about the energy diagram below.

This energy diagram is a graph of the progress of a chemical reaction, versus the total energy of the system. The reactant in this case is BrNO, and the products are NO and Br2. As you can see, after the reaction occurs, the energy of the system is lower than it was before the reaction. This energy diagram shows an exothermic reaction, one in which energy is given off. In the energy diagram for an endothermic reaction, the energy of the products would be higher than that of the reactants.

In this diagram, the activation energy is signified by the hump in the reaction pathway and is labeled. At the peak of the activation energy hump, the reactants are in the transition state, halfway between being reactants and forming products. This state is also known as an activated complex.

The figure below shows the energy diagram for a reaction in the presence of a catalyst and in the absence of a catalyst. As you can see, the catalyst has decreased the activation energy of the reaction, which means that more molecules are able to surmount it and react.

Equilibrium

Chemical equilibrium has been reached in a reaction when the rate of the forward reaction is equal to the rate of the reverse reaction. When a chemical reaction has reached equilibrium, collisions are still occurring: the reaction is now happening in each direction at the same rate. This means that reactants are being formed at the same rate as products are being formed, and this is indicated by double arrows, . At equilibrium, the reaction can lie far to the right, meaning that there are more products in existence at equilibrium, or far to the left, meaning that at equilibrium there are more reactants. The concentration of the reactants and products in a reaction at equilibrium can be expressed by an equilibrium constant, symbolized K orKeq:

For the general reaction

aA + bB  cC + dD

In the above expression, the brackets, as always, symbolize the concentration of the reactants and products in molarity. However, while in the above expression we used the plain symbol K to symbolize the equilibrium constant, there are several types of equilibrium constants. For example, Kc symbolizes the equilibrium constant in an aqueous solution, Kp symbolizes the partial pressures of gases in equilibrium, andKsp symbolizes the solubility product of solids classified as insoluble. K values have no units, and a K > 1 means that the reaction favors the products at equilibrium, while a K < 1 means that the reaction favors the reactants at equilibrium.

Here are a couple of rules to follow when using equilibrium constant expressions on the exam:

1.       Pure solids do not appear in the equilibrium expression.

2.      Pure liquids do not appear in the equilibrium expression.

3.      Water, either as a liquid or solid, does not appear in the equilibrium expression.

4.      When a reactant or product is preceded by a coefficient, its concentration is raised to the power of that coefficient in the Keq expression.

5.       When the Keq of a reaction has been multiplied by a number, the K is raised to the power of the multiplication factor (Kn), so if it has been multiplied by 2, Kis squared, if it has been multiplied by 3, K is cubed, and so on.

6.      The Keq of a reaction occurring in the reverse direction is simply the inverse of the Keq of the reaction occurring in the forward direction (1/Keq).

7.       The Keq of a net reaction that has two or more steps is found by the product of the Keq s for each of the steps: Ks = (K1K2K3 . . .).

Let’s work through an example now of an equilibrium question.

Example

Write the equilibrium expression for the following equation:

H2(g) + I2(g)  2HI(g)

If K is calculated to have a value of 2.5 for the reaction above, what is the value of the equilibrium constant for the following reaction?

4HI(g)  2H2(g) + 2I2(g)

Explanation

The equilibrium constant expression for the reaction is

The reaction has been doubled and reversed, so the new K is the reciprocal of the oldK squared (since the reaction coefficients are doubled):

Le Chatelier’s Principle

You may see an equilibrium question that asks you to use or apply Le Chatelier’s principle on the SAT II Chemistry exam. Le Chatelier’s principle basically states that if stress is applied to a system at equilibrium, the position of the equilibrium will shift in the direction that reduces the stress to reinstate equilibrium. For example, if more reactants are added to the system, the reaction will shift in the forward direction, and if more products are added, the reaction will shift in the reverse direction. If heat is added to the system and the reaction is exothermic, heat should be thought of as a product and the reaction will shift to the left; if the reaction is endothermic and heat is added, the reaction will shift to the right. The addition of pressure will cause a shift in the direction that results in the fewer number of moles of a gas, while if pressure is relieved, the reaction will shift in the direction that produces more moles of a gas.

Practice Questions