6 / 16 Kinetics

6.1.NoS The principle of Occam’s razor is used as a guide to developing a theory—although we cannot directly see reactions taking place at the molecular level, we can theorize based on the current atomic models. Collision theory is a good example of this principle. (2.7)

6.1.U1 Species react as a result of collisions of sufficient energy and proper orientation.

6.1.U2 The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

6.1.U3 Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.

6.1.U4 Activation energy (E_{a}) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction.

6.1.U5 By decreasing E_{a}, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.

6.1.AS1 Description of the kinetic theory in terms of the movement of particles whose average kinetic energy is proportional to temperature in Kelvin.

6.1.AS2 Analysis of graphical and numerical data from rate experiments.

6.1.AS3 Explanation of the effects of temperature, pressure/concentration and particle size on rate of reaction.

6.1.AS4 Construction of Maxwell–Boltzmann energy distribution curves to account for the probability of successful collisions and factors affecting these, including the effect of a catalyst.

6.1.AS5 Investigation of rates of reaction experimentally and evaluation of the results.

6.1.AS6 Sketching and explanation of energy profiles with and without catalysts.

6.1.G1 Calculation of reaction rates from tangents of graphs of concentration, volume or mass vs time should be covered.

6.1.G2 Students should be familiar with the interpretation of graphs of changes in concentration, volume or mass against time.

6.1.IM1 Depletion of stratospheric ozone has been caused largely by the catalytic action of CFCs and is a particular concern in the polar regions. These chemicals are released from a variety of regions and sources, so international action and cooperation have been needed to ameliorate the ozone depletion problem.

6.1.ToK1 The Kelvin scale of temperature gives a natural measure of the kinetic energy of gas whereas the artificial Celsius scale is based on the properties of water. Are physical properties such as temperature invented or discovered?

6.1.Aims1 Aims 1 and 8: What are some of the controversies over rate of climate change? Why do these exist?

6.1.Aims2 Aim 6: Investigate the rate of a reaction with and without a catalyst.

6.1.Aims3 Aim 6: Experiments could include investigating rates by changing concentration of a reactant or temperature.

6.1.Aims4 Aim 7: Use simulations to show how molecular collisions are affected by change of macroscopic properties such as temperature, pressure and concentration.

6.1.Aims5 Aim 8: The role that catalysts play in the field of green chemistry.

16.1.NoS Principle of Occam’s razor—newer theories need to remain as simple as possible while maximizing explanatory power. The low probability of three molecule collisions means stepwise reaction mechanisms are more likely. (2.7)

16.1.U1 Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate determining step/RDS)

16.1.U2 The molecularity of an elementary step is the number of reactant particles taking part in that step.

16.1.U3 The order of a reaction can be either integer or fractional in nature. The order of a reaction can describe, with respect to a reactant, the number of particles taking part in the rate-determining step.

16.1.U4 Rate equations can only be determined experimentally.

16.1.U5 The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of the reaction.

16.1.U6 Catalysts alter a reaction mechanism, introducing a step with lower activation energy

16.1.AS1 Deduction of the rate expression for an equation from experimental data and solving problems involving the rate expression.

16.1.AS2 Sketching, identifying, and analysing graphical representations for zero, first and second order reactions.

16.1.AS3 Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data.

16.1.G1 Calculations will be limited to orders with whole number values.

16.1.G2 Consider concentration–time and rate–concentration graphs.

16.1.G3 Use potential energy level profiles to illustrate multi-step reactions; showing the higher Ea in the rate-determining step in the profile.

16.1.G4 Catalysts are involved in the rate-determining step

16.1.G5 Reactions where the rate-determining step is not the first step should be considered.

16.1.G6 Any experiment which allows students to vary concentrations to see the effect upon the rate and hence determine a rate equation is appropriate.

16.1.IM1 The first catalyst used in industry was for the production of sulfuric acid. Sulfuric acid production closely mirrored a country’s economic health for a long time. What are some current indicators of a country’s economic health?

16.1.ToK1 Reaction mechanism can be supported by indirect evidence. What is the role of empirical evidence in scientific theories? Can we ever be certain in science?

16.1.Uz1 Cancer research is all about identifying mechanisms; for carcinogens as well as cancer-killing agents and inhibitors.

16.1.Aims1 Aim 7: Databases, data loggers and other ICT applications can be used to research proposed mechanisms for lab work performed and to carry out virtual experiments to investigate factors which influence rate equations.

16.2.NoS Theories can be supported or falsified and replaced by new theories—changing the temperature of a reaction has a much greater effect on the rate of reaction than can be explained by its effect on collision rates. This resulted in the development of the Arrhenius equation which proposes a quantitative model to explain the effect of temperature change on reaction rate. (2.5)

16.2.U1 The Arrhenius equation uses the temperature dependence of the rate constant to determine the activation energy.

16.2.U2 A graph of 1/T against ln k is a linear plot with gradient – Ea / R and intercept, lnA

16.2.U3 The frequency factor (or pre-exponential factor) (A) takes into account the frequency of collisions with proper orientations.

16.2.AS1 Analysing graphical representation of the Arrhenius equation in its linear form ln k = (-Ea / RT) + ln A.

16.2.AS2 Using the Arrhenius equation 𝑘 = 𝐴*𝑒^{-Ea/RT}.

16.2.AS3 Describing the relationships between temperature and rate constant; frequency factor and complexity of molecules colliding.

16.2.AS4 Determining and evaluating values of activation energy and frequency factors from data.

16.2.G1 Use energy level diagrams to illustrate multi-step reactions showing the RDS in the diagram.

16.2.G2 Consider various data sources in using the linear expression ln k = −(Ea) / RT + ln A. The expression ln k1 / k2 = (Ea) (1 / T2 − 1 / T1) is given in the data booklet.

16.2.Uz1 The flashing light of fireflies is produced by a chemical process involving enzymes

16.2.Uz2 The relationship between the “lock and key” hypothesis of enzymes and the Arrhenius equation.

16.2.Aims1 Aim 4, Aim 7: Use of simulations and virtual experiments to study effect of temperature and steric factors on rates of reaction.

16.2.Aims2 Aim 6: Experiments could include those involving the collection of temperature readings to obtain sufficient data for a graph.

16.2.Aims3 Aim 7: Graphing calculators can be employed to easily input and analyse data for Ea and frequency factor values.