Factors Affecting Reaction Rates: Concentration, Temperature, and Catalysts, Study notes of Analytical Chemistry

Download Factors Affecting Reaction Rates: Concentration, Temperature, and Catalysts and more Study notes Analytical Chemistry in PDF only on Docsity! (a) Reaction rates (i) Following the course of a reaction Reactions can be followed by measuring changes in concentration, mass and volume of reactants or products. Measuring a change in mass Measuring a change in volume g M as s o f b ea ke r an d c o n te n ts Time Time V o lu m e o f ga s p ro d u ce d The rate is highest at the start of the reaction because the concentration of reactants is highest at this point. The steepness (gradient) of the plotted line indicates the rate of the reaction. = 0.001moll-1s-1 Rate = 0.05-0.00 50-0 0.05 50 = Rate = 0.075-0.05 100-50 0.025 50 = = 0.0005moll-1s-1 C o n ce n tr at io n m o ll-1 The rate of a reaction, or stage in a reaction, is proportional to the reciprocal of the time taken. Rate  1 time Rate is inversely proportional to time. i.e. Rate = 1 time and Time = 1 rate Units: s-1, min-1 etc. Units: s, min etc. See Unit 1 PPA 1 and Unit 1 PPA 2 for examples of ‘clock reactions’ that use the relationship described above. (ii) Factors affecting rate The rates of reactions are affected by changes in concentration, particle size and temperature. The rate of reaction can be increased by - decreasing the particle size of a solid reactant - increasing the concentration of a reactant in solution - increasing the temperature at which the reaction occurs. The factors which affect reaction rate can be understood more fully by examining the conditions needed for a reaction to take place. For a chemical reaction to occur, reactant particles must collide – this is the basis for the Collision theory. The collision theory can be used to explain the effects of concentration and surface area on reaction rates. The Effect of Temperature on the Rate of Reaction Rate Temperature We discovered that the rate of reaction is not directly proportional to the temperature, instead a 10°C rise in temperature roughly doubles the rate. The increase in the number of collisions at a higher temperature is not enough on its own to account for this increase in the rate. Activation Energy and Energy Distribution Unsuccessful collision: reactants collide not enough energy to break bonds reactants move apart Successful collision: reactants collide enough energy to break bonds products move apart ≥ Activation energy The activation energy is the minimum kinetic energy required by colliding particles before reaction will occur. Temperature is a measure of the average kinetic energy of the particles of a substance. Energy distribution diagrams can be used to explain the effect of changing temperature on the kinetic energy of particles. T1 T2 The shaded areas represent the total number of molecules which have sufficient energy to react. Endothermic changes cause absorption of heat from the surroundings. In an endothermic reaction the reactants absorb energy from the surroundings so that the products possess more energy than the reactants. The enthalpy change is the energy difference between the reactants and the products. The enthalpy change can be calculated from a potential energy diagram (if drawn to scale). The enthalpy change has a negative value for exothermic reactions. The enthalpy change has a positive value for endothermic reactions. Enthalpy changes are usually quoted in kJmol-1. If the total energy change for the bond breaking step is less than for the bond making step, the overall reaction will be exothermic. If the reverse is true then the reaction will be endothermic. The activation energy is the energy required by colliding molecules to form an activated complex. The activation energy can be calculated from potential energy diagrams (if drawn to scale). In potential energy diagrams the activation energy appears as an ‘energy barrier’. The rate of a reaction will depend on the height of this barrier. The higher the barrier the slower the reaction. Note: the rate of reaction does not depend on the enthalpy change. Catalytic convertors are fitted to cars to catalyse the conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen. Cars with catalytic converters only use ‘lead-free’ petrol to prevent poisoning of the catalyst. The convertors contain honeycomb ceramic material covered with a metal catalyst such as platinum or rhodium. The honeycomb structure is used to increase the surface area of the catalytic convertor. Enzymes Enzymes catalyse the chemical reactions which take place in the living cells of plants and animals. For example, amylase catalyses the hydrolysis of starch and catalase catalyses the decomposition of hydrogen peroxide in the blood. The molecular shape of an enzyme usually plays a vital role in its function. substrate enzyme Products Enzymes are usually highly specific. Enzymes are used in many industrial processes. Enzymes operate most effectively at a certain optimum temperature and within a narrow pH range. Above this temperature or outwith this pH range the enzyme changes shape (the lock and key no longer fit together) and the enzyme is denatured.