CR4 Identify factors that affect the rates of chemical reactions

Suggested time: 4-6 hours

Learning Objectives

1. Identify how factors such as temperature, concentration, and surface area can affect the rate of a chemical reaction.
   
2. Use the collision model to explain changes in chemical reaction rates.
   
3. Design and perform an experiment to determine how various factors affect chemical reaction rates, identifying and controlling major variables. (CCT)
   
4. Carry out procedures controlling the major variables and adapting or extending procedures where required.
   
5. Compile and organize data, using appropriate formats and data treatments to facilitate interpretation of the data. (COM, NUM)
   
6. Interpret patterns and trends in data, and infer or calculate linear and nonlinear relationships among variables. (NUM)
   
7. Value the processes for drawing conclusions in science.
   

Key Questions

1. What is the rate of a chemical reaction?
   
2. What factor(s) might influence the rate of a chemical reaction?
   
3. What is the role of a catalyst in a chemical reaction?
   
4. How can the collision model explain rates of chemical reactions?
   
5. Why are rates of chemical reactions often controlled in industry?
   
6. What are some examples of chemical reactions that people want to speed up or slow down?
   

Key Concepts

• The rate of a chemical reaction is a measure of how quickly or slowly the reaction occurs.
  
• Measuring the rate of a chemical reaction involves measuring how much product(s) forms or how much reactant(s) is used up in a time interval.
  
• Factors that influence the rate of chemical reactions include: nature of the reactant(s), temperature of the reactant(s), concentration of the reactant(s), surface area of the reactant(s), and the presence or absence of a catalyst or inhibitor.
  
• Increasing the temperature of the reactants generally increases the rate of a chemical reaction.
  
• Increasing the concentration of one or more reactants generally increases the rate of a chemical reaction.
  
• Increasing the surface area of one or more reactants generally increases the rate of a chemical reaction.
  
• A catalyst is a substance that changes the rate of a chemical reaction but is not changed in the reaction.
  
• The collision model states that the number of effective collisions (above activation energy) of reactant molecules affects the rate of a chemical reaction.
  
• Scientific knowledge is based on experimentation and observation.
  
• Science is based on evidence that could be obtained by other people working in a different place and at a different time under similar conditions.
  
• Controlling variables in an experiment is done to isolate factors that may influence a situation or event.
  
• Hypothesizing is stating a tentative generalization which may be used to explain a relatively large number of events. Hypotheses are subject to testing by experiments.
  
• Interpreting data is based on finding a pattern in a collection of data that then leads to generalizations.
  

Pre-Instructional Questions

1. Are students able to describe examples of chemical reactions that occur rapidly or slowly?
   
2. Do students understand the concept 'rate of change'?
   
3. Do students know how to measure a rate of change?
   
4. Are students able to identify when a chemical reaction is complete?
   
5. Are students able to suggest factors that might influence the rate of a chemical reaction?
   
6. Are students able to describe examples of chemical reactions that are purposely controlled?
   

Suggested Teaching Strategies and Activities

1. Students could brainstorm a list of common chemical reactions and categorize them according to the rate of the reaction. Students should suggest reasons for why these reactions occur at such different rates. (COM)
   
   
2. Students could discuss methods of determining the rate of chemical reactions. They could suggest methods that they could use in the classroom to determine the rate of a reaction as well as methods that industry might use. Students should also be able to explain how to identify when a chemical reaction is complete.
   
   
3. Students could describe chemical reactions that are controlled in domestic or industrial processes (e.g., cooking, food preservation, refrigeration, explosives, pharmaceuticals, manufacturing, and air bags). Students should suggest reasons why these reactions are controlled and possible design requirements for the rate (e.g., an air bag must inflate within 15 - 20 ms after impact).
   
   
4. Students should design an experiment to investigate factors that may influence the rate of a chemical reaction. Typical factors to investigate include: temperature of the reactant(s), concentration of the reactant(s), surface area of the reactant(s), and the presence or absence of a catalyst. It is not necessary that each student conduct an experiment to determine the relationship of each factor. Instead, groups may investigate different factors and share the results, along with supporting documents and visuals, with classmates. Open sharing of group results helps to demonstrate the public nature of science and the need for obtaining reproducible experimental results. Students could use this set of experiments as a mini-science fair activity.
   
   
5. Students could investigate how to change the rate of a chemical reaction. For example, they could vary the amounts of baking soda and vinegar mixed together to simulate an explosion. Students could collect data in order to determine whether factors such as the amount of each reactant or the ratio of the reactants control the reaction rate. Students should also be able to notice a decrease in reaction rate as the reactants get used up. For an extension to this activity, consider providing students with a much larger quantity of the reactants and asking students to predict the reaction rate based on their previously graphed data. This provides an opportunity to discuss the limits of the best-fit graph specifically and the limits of extrapolation generally. (NUM)
   
   
6. Students could identify the role of catalysts in common chemical reactions and in industrial processes (e.g., catalytic converters in automobiles, decomposition of hydrogen peroxide, biological enzymes, manufacture of ammonia by the Haber process, the Contact process for the manufacture of sulphuric acid, and the destruction of ozone in the atmosphere). The focus of student research at this grade should be to explain how and why these processes are controlled, not to understand every step of these complex processes.
   
   
7. Enrichment: Students could conduct an experiment to determine whether a substance is a catalyst or a reactant in a chemical reaction.