ACSI HL Chemical Kinetics 2022 (Teacher)

IBDP Chemistry HL/ Chemical Kinetics Page 1 Anglo − Chinese School (Independent) Year 5 (2022) IBDP Chemistry HL (IBDP syllabus Topic 6 ) 6 .1 Collision theory and rates of reaction - Essential Idea: the greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction . (IBDP syllabus Topic 1 6 ) 1 6 .1 Rate expression and reaction mechanism − Essential Idea: rate expressions can only be determined empirically and these limit possible reaction mechanisms. In particular cases, such as a liner chain of elementary reactions, no equilibria and only one significant activation barrier, the rate equation is equivalent to the slowest step of the reaction . 1 6 . 2 Activation energy − Essential Idea: the activation energy of a reaction can be determined from the effect of temperature on reaction rate. TEACHER COPY – WITH SUGGESTED SOLUTIONS TOPIC 6 CHEMICAL KINETICS

IBDP Chemistry HL/Chemical Kinetics Page 2 6.1 Collision theory and rates of reaction Nature of science: The principle of Occam’s razor (also known as Law of ecnonmy) is used as a guide to developing a theory – although we cannot directly see reactions taking place at the molecular level, we can theorise based on the current atomic models. Collision theory is a good example of this principle (2.7) It states that Pluralitas non est ponenda sine necessitate, “Plurality should not be posited without necessity.” (Encyclopedia Britannica). Newer theories need to remain as simple as possible while maximizing explanatory power. Understandings: • Species react as a result of collisions of sufficient energy and proper orientation. • The rate of reaction is expressed as the change in concentration of a particular reactant or product per unit time. • Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour. • Activation energy (Ea) is the minimum energy that colliding particles need in order to have successful collision leading to a reaction. • By decreasing E a, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed. Applications and skills: • Description of the kinetic theory in terms of the movement of particles whose average kinetic energy is proportional to temperature in Kelvin. • Analysis of graphical and numerical data from rate experiments. • Explanation of the effects of temperature, pressure/ concentration and particle size on rate of reaction. • 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. • Investigation of rates of reaction experimentally and evaluation of the results. • Sketching and explanation of energy profiles with and without catalysts. Guidance: • Calculation of reaction rates from tangents of graphs of concentration, volume or mass vs time. • Familiarisation with the interpretation of graphs of changes in concentration, volume or mass vs time.

IBDP Chemistry HL/Chemical Kinetics Page 3 16.1 Rate expression and reaction mechanism Nature of science: The principle of Occam’s razor (also known as Law of ecnonmy) is used as a guide to developing a theory – although we cannot directly see reactions taking place at the molecular level, we can theorise based on the current atomic models. Collision theory is a good example of this principle (2.7) It states that Pluralitas non est ponenda sine necessitate, “Plurality should not be posited without necessity.” (Encyclopedia Britannica). Newer theories need to remain as simple as possible while maximizing explanatory power. Understandings: • Reaction may occur by more than one step and the slowest step determines the rate of reaction (rate–determining step/ RDS) • The molecularity of an elementary step is the number of reactant particles taking part in that step. • The order of reaction can be either integral 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. • Rate equations can only be determined experimentally. • The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of the reaction. • Catalysts alter a reaction mechanism, introducing a step with a lower Ea. Applications and skills: • Deduction of the rate expression for an equation from the experimental data and solving problems involving the rate expression. • Sketching, identifying and analysing graphical representations for zero, first and second order reactions. • Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data. Guidance: • Calculation of order of reactions with whole number values. • Interpretation of concentration–time and rate–concentration graphs. • Using potential energy level profiles to show multi –step reactions; identifying the rate – determining step as the one with the highest Ea. • Catalysts are involved in the rate–determining step. • Reactions where the rate–determining step is not the first step should be considered. • Any experiment which allows students to vary concentrations to see the effect upon the rate and hence determine a rate equation is appropriate.

IBDP Chemistry HL/Chemical Kinetics Page 4 16.2 Activation energy Nature of science: 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) Understandings: • The Arrhenius equation uses the temperature dependence of the rate constant to determine the Ea. • A graph of lnk vs 1 T is linear plot with gradient −Ea RT and the y–intercept lnA. • The frequency factor (or pre– exponential factor), A, takes into account the frequencies of collisions with proper orientations. Applications and skills: • Analysing graphical representation of the Arrhenius equation in its linear form, lnk = −Ea RT + lnA • Using the Arrhenius equation, k = A𝑒𝑒 −Ea RT . • Describing the relationships between temperature and rate constant; frequency factor and complexity of molecules colliding. • Determining and evaluating values of activation energy and frequency factors from data. Guidance: • Using energy level diagrams to illustrate multi–step reactions showing the rate–determining step in the diagram. • Consider various data sources in using the linear expression lnk = −Ea RT + lnA. The expression ln k1 k2 = Ea R ( 1 T2 − 1 T1 ) is given in the data booklet.

IBDP Chemistry HL/Chemical Kinetics Page 5 6.1 Rates of Reaction Overview • Reaction kinetics is concerned with measuring the rate of a reaction and the sequence of step(s) by which it occurs. • The study of reaction kinetics would allow us to: o determine how fast a reaction would take place, o determine conditions required for optimum reaction rates, o propose a reaction mechanism by which the reaction takes place. 6.1.1 Definition of the Term Rate of Reaction The rate of reaction is defined as the decrease in concentration of a reactant per unit time or the increase in concentration of a product per unit time. • Mathematically, rate = − d[reactant] dt = d[product] dt The negative sign is placed in front of the differential expression