Lab 11A – Factors that affect reaction rates Post Lab Considerations: A chemical reaction occurs when the reacting particles have an effective collision that results in the formation of new particles. A simple collision is not enough – there also has to be sufficient energy involved in the collision for it to become effective and cause a reaction to occur. If the number of effective collisions can be controlled, then the rate of the reaction can be controlled. Some of the ways that can be used to change the number of collisions include changing the concentrations of the reactants or changing the surface area of the reactants. Temperature changes affect the speed of the molecules and therefore the energy they contain. The higher the temperature, the higher the average kinetic energy. As a result, the temperature determines how effective a collision will be. The effect of surface area can only be seen if the reaction is heterogeneous, that is the reactants are in more than one phase. Examples would include a solid reacting with a solution (in this case) or a liquid reacting with a gas. A general statement that can be made concerning the comparison between the rates of different chemical reactions is that in most cases, reactions that require the breaking of covalent bonds are slower than ones which only involve electron transfer or occur as a result of electrostatic attraction between ions. The interpretation of the results of Part IV will be aided with a knowledge of what actual reactions are occurring. The reactions in Part IV are given by the net ionic equations: 5Fe2+(aq) + MnO4-(aq) + 8H+ 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) 2MnO4-(aq) + 5C2O42-(aq) + 16H+(aq) 10CO2(g) 2 Mn2+(aq) + 8H2O(l) The second equation is also used in Part V in which Mn2+ is added as a catalyst. A catalyst speeds up a reaction by lowering the amount of energy required by the molecules in order to have an effective collision. Analysis of Results: Part I: Effect of Concentration on Reaction Rate 1. Calculate the average reaction rate in each case by determining the grams of magnesium used per second and record in Table 1 2. On a sheet of graph paper, graph the reaction rate vs. concentration of HCl results from Part I 3. Look at your results from part I. Does doubling the concentration of hydrochloric acid double the reaction rate? Explain your answer in terms of collision theory. 4. Use your graph to predict the reaction rate and then calculate the reaction time for a 1cm magnesium strip in 4.5M HCl solution under the same conditions as in Part I Part II: Effect of Temperature on Reaction Rate 1. Calculate the average reaction rate in each case by determining the grams of magnesium used per second and record in Table 2 2. On another sheet of graph paper, graph the reaction rate vs. Temperature results from part II. Look at your results. Which reaction’s rate was the fastest? Explain your answer in terms of collision theory. 3. Use your graph to predict the reaction rate and then calculate the reaction time for a 1cm magnesium strip in 1.0M HCl solution, at a temperature of 75oC. Part III: Effect of Surface Area on reaction rate 1. Calculate the average rate of reaction in each case from the mass of the CaCO3 reacted divided by the time elapsed in seconds and record in Table 3. (Note that if not all the marble chip dissolved after 5 min, use the loss of mass of the chip divide by the 300s elapsed.) Follow-up Questions: 1. Based on your results in Parts I, II and III, which of the following sets of conditions do you expect to result in the fastest reaction? a. Powdered CaCO3, 2.0M HCl, and 50oC b. CaCO3 chip, 6.0M HCl, and 25oC c. Powdered CaCO3, 6.0M HCl, and 50oC d. CaCO3 chip, 2.0M HCl, and 50oC 2. Explain why you will be more successful in lighting a fire made from kindling wood than in lighting a log directly 3. Explain why blowing on a smoldering fire may make it burn better. 4. Enzymes are catalysts made of protein, which are necessary for almost every reaction occurring in living cells. Find out why the rate of enzyme-catalyst reactions increase with increasing temperature only up to about 37oC (body temperature), then decrease above that temperature.
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