Rates of Chemical Reactions

1. Rates of Chemical Reactions And the factors that affect them

2. What is the rate of a chemical reaction? • The rate of a chemical reaction is the speed at which the chemical reaction occurs. • Some reactions occur quickly, such as decomposition of sodium azide to form nitrogen gas and sodium NaN3  Na + N2 + heat * This is the reaction that is triggered in your air bag when your car hits something. Nitrogen gas fills the air bag. • Other reactions occur slowly, such as the chemical reaction between baking soda and cream of tartar that causes a cake to rise.

3. Factors that affect the rate of a chemical reaction • Quite often it is advantageous for scientists to control the rate of a chemical reaction. What good would an air bag be if it took 5 minutes to inflate? • There are several factors that affect the rate of a chemical reaction that we can control, and they are: • Temperature • Concentration • Surface area

4. Temperature • The temperature of a solution is a measure of the average energy of the molecules that make up that solution. • We can measure temperature using several scales, including: °C °F K (in this class we will only use °C ) • Perhaps the most obvious point of the whole slide: As the temperature of a solution increases, it gets hotter.

5. Concentration • The concentration of a solution is a measure of the amount of solute present in a given volume of solution. • Imagine that you poor 5 g of salt in 1 L of water, then you take 10 g of salt and poor it in a different 1 L of water. The second 1 L of water has a higher concentration of salt in it. • For acids, we typically measure concentration as moles/L instead of g/L. The symbol for moles/ L is M.

6. Surface Area • Surface area is the amount of area of a sample of matter that is visible and able to react. • For example, the surface area of a whole orange is the outer peel; more surface area is exposed if we cut the orange in half. • When we crush solids into powder, we drastically increase the surface area.

7. How do Temperature, Concentration, and Surface Area affect the rate of a Chemical Reaction? • Answering this question is the goal of the experiment that you are going to design. • You will get into groups of 4 (your choice) and together you will design an experiment to test how ONE of the 3 factors affects the rate of a chemical reaction. • You will need to have your experiment approved by myself before you perform it. • After conducting the experiment, you should discover how your assigned factor affects the rate of a reaction.

8. Things to Keep in Mind When Designing an Experiment: • If your goal is to see how temperature affects the rate of a chemical reaction, then for each trial of a chemical reaction you perform every factor should stay the same except the temperature. This is the only way to see that temperature is the only thing affecting your altered reaction rate. • Keep safe laboratory practices in mind when designing your experiment.

9. All of the following will be available to you for your experiment: • Any glassware in Room 25 including beakers, flasks, stir rods or anything else you may need equipment-wise. • Different concentrations of hydrochloric acid: 0.2 M, 1 M, and 2 M. • 1 cm long strips of magnesium metal (which reacts and dissolves in hydrochloric acid) • Alka-Seltzer tablets • Salt • Water • Hot plate (which can increase the temperature of a solution)

10. How will you be evaluated? • After performing your experiment and determining how your assigned factor affects the rate of a chemical reaction, each person will submit a copy of the report. • The experiment and submitted information will be graded using a rubric that is attached to the handout that you will receive. GOOD LUCK 