[H2 CHEM] Chapter 8 - Reaction Kinetics

Chapter 8 Reaction Kinetics ELAINE PEH (COPYRIGHTED) ©

CHAPTER ANALYSIS FOCUS EXAM WEIGHTAGE ELAINE PEH (COPYRIGHTED) ©

Reaction Kinetics Terminology Rate Equation & Order of Reaction Methods to determine Order of Reaction Mechanism Factors affecting Rate of Reaction Experimental studies of Kinetics KEY CONCEPT ELAINE PEH (COPYRIGHTED) ©

ELAINE PEH (COPYRIGHTED) © Reaction Kinetics Terminology RATE OF REACTION Units = mol dm-3 s-1, mol dm-3 min-1, mol dm-3 h-1 Note: When using 𝛥[reactant], rate will be (-) Note: When using 𝛥[product], rate will be (+) INSTANTANEOUS RATE The rate at a particular time.. INITIAL RATE The instantaneous rate at time=0. ACTIVATION ENERGY Minimum energy which reacting particles must possess in order to overcome the activation energy barrier before becoming products.

ELAINE PEH (COPYRIGHTED) © ENERGY PROFILE DIAGRAM In a Single-Step Reaction In a Multiple-Steps Reaction

ELAINE PEH (COPYRIGHTED) © Rate Equation & Order of Reaction RATE EQUATION aA + bB → cC + dD • k = rate constant • a = order of reaction with respect to A • b = order of reaction with respect to B • a+b = overall order of reaction Note: Stoichiometric equation is the overall chemical equation, rate equation involves only the rate determine step (the slow step). RATE CONSTANT • Rate Constant (k) is the constant of proportionality in the reaction. • It is affected by both temperature and catalyst. • The larger the k, the faster the reaction.

ELAINE PEH (COPYRIGHTED) © Rate Equation & Order of Reaction ORDER OF REACTION • The order of reaction with respect to a given reactant is the power to which the concentration of the reactant is raised to in the experimentally determined rate equation. • The overall order of reaction is the sum of the powers of all the reactant’s order in the experimentally determined rate equation. ZERO ORDER REACTION • Rate is independent to the reactant’s concentration. • k = mol dm-3 s-1

ELAINE PEH (COPYRIGHTED) © Rate Equation & Order of Reaction FIRST ORDER REACTION • Rate is directly proportional to the reactant’s concentration. • k = s-1 • Constant half-life Note: it is the only reaction with constant t1/2 HALF LIFE Time taken for the [A] to decrease to half its initial value

ELAINE PEH (COPYRIGHTED) © Rate Equation & Order of Reaction SECOND ORDER REACTION • Rate is proportional to the concentration of a single reactant raised to the power of two or to the product of the concentrations of two reactants. • k = mol-1 dm3 s-1 (2nd order) PSEUDO FIRST ORDER REACTION • When a 2nd order kinetics is approximated to a 1st order kinetics whereby all the other reactants are in large excess except the one being studied • It only occurs if: 1) presence of large excess of a reactant 2) solvent used is also the reactant 3) presence of a catalyst in the rate equation

ELAINE PEH (COPYRIGHTED) © Determine Order of Reaction Method to Determine Order of Reaction Initial Rate Method Inspection Method Calculation Method Graphical Method Rate against concentration graphs Concentration against time graphs