Chapter 17 - Kinetics

Reaction Rates – Section 17.1

What is the reaction rate?Kinetics - The branch of chemistry that is concerned with the rates of change in the concentration of reactants in a chemical reaction.

The reaction rate is a measure of the change in the concentration of reactants or products over time in a chemical reaction.

Reactions can happen very quickly or extremely slow. Examples:

fast and furious chemical rxn slower rxn rate millions of years rxn rate

Expressing Reaction Rates

Fast and slow helps us to visualize the speed of chemical reactions, but chemists, doctors and engineers need to be more specific about how fast reactions occur. For example, a knowledge of the speed at which radioactive tracking devices break down in the human body enables doctors to take x-rays to diagnose diseases.

Expressing Reaction RatesIn medicine, specific isotopes are used to observe the condition of specific organs. A common procedure is the injection of iodine-131 for the observation of the thyroid gland. A healthy thyroid will accumulate any iodine entering the body. Because the body cannot distinguish between stable iodine and its radioactive isotope, iodine-131 will also be accumulated. When a physician scans the patient, if iodine-131 is present in the thyroid, the gland is working properly. However, if the trace element has not collected in the thyroid, the physician knows the gland is failing.

Thyroid glandShowing cancer

Expressing Reaction Rates

Related technology is often used in industrial settings. Using trace elements, engineers can follow the path of a coolant or lubricant throughout a system. It is also used to identify inclusions, cracks, areas of porosity, and other flaws. When used in test situations, radioisotopes can help to detect areas of abnormal wear and corrosion.

Exothermic Reactions

An exothermic reaction is a chemical reaction accompanied by the release of heat. In other words, the energy needed for the reaction to occur is less than the total energy released. As a result of this, the extra energy is released, usually in the form of heat.

Endothermic Reactions

An endothermic reaction is a chemical reaction in which heat is absorbed. The energy needed for the reaction to occur is more than the amount of energy released. Therefore, high energy products are formed.

Factors that Affect Reaction Rate - Concentration

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.

Factors that Affect Reaction Rate - Temperature

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.

Factors that affect Reaction Rate - Catalyst

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.

Factors that Affect Reaction Rate – Physical State of Reactants

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.

Reaction Rate Calculations

The speed at which a process works is measured against time. For example, a sprinter moves in meters per second, the speed of a car in miles per hour, and the speed of light is measured by the distance it travels in a year. The rate of a chemical reaction is measured as the change in concentration (mol/L) per second.

CO (g) + NO2 (g) -> CO2 (g) + NO (g)

[NO] @ t2 – [NO] @ t1 = ∆[NO]

t2 – t1 ∆t

[ ] = molar concentration

t = time

∆ = change

Reaction Rate Calculation Example

t1 = 0.00 sect2 = 2.00 sec[NOt1] = 0.00 M[NOt2] = 0.01 M

0.01 M – 0.00 M = .01 M = .0050 M/L · s2.00 sec – 0.00 sec 2 sec

Practice Problems1. Determine the rate of reaction of a 1.0 M concentration of chlorine

when it changes during a reaction to a 0.0 M concentration in 10 seconds.

2. Determine the rate of reaction of a 2.4 M concentration of hydrogen when it changes during a reaction to a 0.00 M concentration of hydrogen in 6 seconds.

3. Determine the rate of reaction of a 1.6 M concentration of calcium when it changes during a reaction to a 0.2 M concentration of calcium in 40 seconds.

Questions you should be able to answer . . .

1. What does the reaction rate indicate about a particular chemical reaction?

2. How is the rate of a chemical reaction usually expressed?

3. What is chapter 17 about?

4. What factors affect the reaction rate?

5. What is the formula for average rate?

6. How are reaction rates expressed?

7. Are reaction rates positive or negative?

8. What is the difference between M and mol/(L· s)

9. What do brackets around a formula represent?