Speed of Reaction PDF - Past Paper

Speed of Reaction 16. Speed of Reaction Learning Outcomes 1. Describe the effects of particle size, concentration, pressure and temperature on the speeds of reactions. 2. Suggest a suitable method for investigating the effect of a given variable on the spe...

Speed of Reaction 16. Speed of Reaction Learning Outcomes 1. Describe the effects of particle size, concentration, pressure and temperature on the speeds of reactions. 2. Suggest a suitable method for investigating the effect of a given variable on the speed of a reaction. 3. Interpret data obtained from experiments concerned with speed of reaction (including graphical data). 4. Explain speeds of reactions in terms of collisions between reacting particles. 5. Define the term catalyst. 6. State that enzymes are biological catalysts. 7. Describe the effect of catalysts (including enzymes) on the speeds of reactions. 8. Explain how pathways with lower activation energy account for the increase in speeds of reactions. 9. State that some elements and their compounds act as catalysts in industrial processes. 10. Give examples from daily life of these effects on speeds of chemical reactions. 1  Speed of Reaction Start of Pre-Lesson Reading 1. Reactions Proceed at Different Rates Very Fast Moderately Fast Very Slow 2. Measuring the Speed of a Reaction  This can be through one of the following ways: (1) Measure the amount of product formed over a period of time; (2) Measure the amount of reactant used up over a period of time; (3) Measure the changes that occur in the reaction over a period of time. Volume of gas produced against time Mass of reaction mixture against time CaCO3(s) + HCl(aq) CaCl2(aq) + H2O(l) + CO2(g) CaCO3(s) + HCl(aq) CaCl2(aq) + H2O(l) + CO2(g)  As the reaction proceeds, the total volume of  As the reaction proceeds, the total mass of gas collected will increase. the reaction mixture will decrease as carbon dioxide escapes. Volume of gas/ cm3 Data logger  We can measure the rate of evolution of gas by setting up the apparatus shown above,  We can measure the rate of change in and taking readings off the gas syringe at mass of the reaction mixture with the regular time intervals. apparatus shown above, and taking mass  The data can be plotted into a volume of gas readings at regular time intervals. evolved against time graph as shown.  The data can be plotted into a mass against time graph as shown. 1  Speed of Reaction Mass/ g 3 1 2 3 2 50 1 Time/ min 5 Time/ min (1) The gradient of the graph is the steepest at (1) The gradient of the graph is the steepest at the start of the reaction as the speed of the start of the reaction as the speed of reaction is the fastest. reaction is the fastest. (2) The gradient decreases as the reaction (2) The gradient decreases as the reaction proceeds as the reactants are used up. The proceeds as the reactants are used up. speed of reaction decreases with time. The speed of reaction decreases with time. (3) The gradient becomes 0 after 4.5 min as the (3) The gradient becomes 0 at 5 min as the reaction has stopped. The speed of reaction reaction has stopped. The speed of = 0. No more gas is being produced. One reaction = 0. No more change in mass is or more of the reactants has been used up. taking place. Final mass = 50 g. Total volume of gas evolved = 70 cm3. Pre-Check 1 Draw two graphs on the axes below to show a fast and a slow reaction between calcium carbonate and dilute hydrochloric acid, but with the same volume of carbon dioxide evolved at the end of the reaction. Volume of gas/ cm3 Time/ min 3. Factors Affecting the Speed of a Reaction 1  Speed of Reaction  For a chemical reaction to occur, the reactant particles must first collide. Collision Theory Assumption 1 Assumption 2 A reaction only takes place when the reacting A reaction only takes place when the reacting particles collide. particles collide with sufficient energy. The more collisions per unit time, the faster the The energy is required to overcome the reaction. activation energy. The more energy is provided to the reaction mixture, the greater the likelihood that the particles can collide with energy equals to or greater than activation energy.  5 main factors affect the speed of a chemical reaction. Speed of  Increase in concentration of reactants theReaction number of reactant particles per unit volume.  The frequency of collisions increases, and this increases the frequency of effective collisions. Size Hence, the speed of the of reaction increases. chemical Concentration Pressure Temperature Presence of reacting of reactants catalyst particles Checkpoint 1 The rate of evolution of hydrogen gas was measured in two separate reactions between excess 1 mol dm-3 dilute hydrochloric acid, and excess 2 mol dm-3 dilute hydrochloric acid, with 10 g magnesium ribbon. On the axes below, plot and label the two volume against time graphs. 1  Speed of Reaction Volume of gas/ cm3 Time/ min 1. Size of Reacting Particles  When the particle size decreases, the surface area of the reactant increases, giving a larger surface area to volume ratio.  More surfaces are exposed for reactant particles to collide, increasing the frequency of collisions.  This increases the frequency of effective collisions, hence increasing the speed of reaction. 1  Speed of Reaction Checkpoint 2 100 cm3 of 1 mol dm-3 dilute hydrochloric acid was reacted with 50 g of marble as shown in the diagram above. In flask 1, the marble was broken into small chips. In flask 2, the marble was a single piece weighing 50 g. On the axes below, plot and label the two volume against time graphs. Volume of gas/ cm3 Time/ min 1  Speed of Reaction 2. Pressure  This only applies to reactions involving gases.  Increasing the pressure of a gas is similar to increasing its concentration. At higher pressures, the particles are closer together, resulting in more particles per unit volume of the gas.  The collisions between the gas particles become more frequent. This increases the frequency of effective collisions, increasing the speed of the chemical reaction, e.g., Haber process – 250 atm. 3. Temperature  Increase in temperature increases the speed of a chemical reaction.  Particles do not always collide with enough energy to overcome the activation energy barrier.  When temperature increases, the particles move faster and they possess more energy. This increases the likelihood that they collide with sufficient energy to overcome activation energy.  The frequency of effective collisions increases, increasing the speed of the chemical reaction. 1  Speed of Reaction Checkpoint 3 In which of the following beakers will the reaction be fastest? 4. Catalyst  A catalyst is a substance which increases the speed of a chemical reaction and remains chemically unchanged at the end of the reaction. Energy/ kJ 2 1 reactants products Reaction coordinate  Catalysts increase the speed of a chemical reaction by providing an alternative pathway with a lower activation energy for the reaction to proceed. (1) Activation energy of catalysed reaction < (2) Activation energy of uncatalysed reaction  Summary of the characteristics of catalysts - Only a small amount is required as it is not chemically changed. - A catalyst is not used up during the chemical reaction. - A catalyst is selective and different reactions require different catalysts. - A catalyst increases the speed of a reaction but not the yield of the reaction. 1  Speed of Reaction - A catalyst can be poisoned by impurities, and no longer works or become less effective as a catalyst. - Catalysts are usually transition metals or their compounds. 5.1 Enzymes  Enzymes are biological catalysts which increases the speed of a chemical reaction and remains chemically unchanged at the end of the reaction.  Characteristics of enzymes (1) Enzymes are proteins. (2) Enzymes are substrate specific. (3) Enzymes are sensitive to temperature changes. Most enzymes perform their function most effectively within a narrow range of temperatures. Generally, every 10oC increase in temperature doubles the rate of reaction until the enzyme is denatured. (4) Enzymes are sensitive to pH changes. Most enzymes function within a narrow pH range. 1  Speed of Reaction Checkpoint 4 A solution of hydrogen peroxide was allowed to decompose and the oxygen given off was collected. The graph shows the change in the total volume of oxygen evolved with time. A black solid was added to the solution at 5 minutes. (a) What effect has the black solid had on the rate of the reaction? What is the general name given to compounds causing this effect? Identify the black solid. (b) Write the equation, including state symbols, for the decomposition of hydrogen peroxide. (c) Calculate the maximum volume of oxygen, measured at room temperature and pressure, which can be obtained by this reaction: (i) With 1.7 g of hydrogen peroxide in the absence of the black solid. (ii) With 1.7 g of hydrogen peroxide in the presence of the black solid. 1  Speed of Reaction (d) Draw an energy profile diagram to show the energy changes during the reaction with, and without the black solid. 1