10 Rate of Reaction

Learning Outcomes:

At the end of the chapter, students must understand that:

   -  Rate of change is a ratio of two quantities, of which one is usually time (s). For example, volume of gas evolved per second.

   - Rate of change is affected by concentration, pressure, particle size and temperature. (Not required: effect of concentration and pressure)

At the end of the chapter, students must be able to:

     - Describe the effects of particle size and temperature on the rate of change.
     -Explain the effects in terms of collisions between particles.

     - Interpret data obtained from experiments concerned with rate of change. This includes the interpretation of
    graphs and calculation.

      1. What is the speed/rate of reaction?

a. Speed of reaction =   volume of gas evolved

                                                time taken 

b. Speed of reaction =  mass of product produced or amount of a reactant remaining 

                                                                        time taken 

   

 2. Measuring speed of reaction 

a. Measuring the volume of gas evolved 

•Graph shows:

–The gradient is greatest at the start.

–The gradient decreases with time.

–The gradient becomes zero when the reaction has completed. 

b. Measuring the volume of gas evolved 

•Measuring the change in mass of the reaction mixture

Speed of graph is measured by the gradient of the graph. 

Examples of fast or slow reaction:

1. Gummy bear Experiment

2. Iodine Clock 

3. Corrosion of metals

Exercise: Plot the graph of Mass of Reactants remaining against time for these three examples on the same graph. Assuming the initial and final of the reactants of the three examples are the same.

3. Factors affecting speed of reaction - particle size 

•Most chemical reactions of solids with liquids or gases go faster when the solids are broken into smaller pieces.

•Small pieces of solids have a larger total surface area than a large piece of the same mass.

•Therefore, there is more surface area for the acid particles to react with at any one time, more collisions between the reacting particles can occur, leading to a faster reaction.

•The larger the surface area of a reactant, the faster the reaction.

4. Factors affecting speed of reaction - temperature

•The speed of a chemical reaction increases when temperature increases.

•To react, particles must collide at great speed with a lot of energy. 

•At a higher temperature, reactant particles move faster. There is an increase in the number of effective collisions between the reacting particles. A larger number of the particles will therefore have the minimum activation energy to react when they collide, resulting in a faster reaction.

•The speed of a reaction is approximately doubled for every 10˚C rise in temperature.

5. End of Lesson Quiz