Chemical Kinetics – Rate of a Reaction Kinetics = Ancient Greek: κίνησις "kinesis", movement or to move ΔHRXN = ∑(aΔHProducts) – ∑ (bΔHReactants) Energy Diagram shows time course of reaction from reactants to products E = exothermic (this reaction) height = ‘activated complex’ energy height were reactant bonds are broken (Hreactants) and product bonds are formed (Hreactants) Difference is H (change in enthalpy; heat of a reaction)

Collision Theory For a reaction to occur: Sufficient energy to overcome activation energy Correct orientation Ea = peak on diagram

Energy Diagram Ea = activation energy E = energy change in reaction (Ea-Ea’) Ea’ = energy needed to activate an endothermic reaction Exo- or endothermic reaction? (exo) Ea – activation energy: minimum energy required to transform the reactants into the activated complex (e.g., match to light a fire) Ea’ – N.B. reach activated complex + energy stored in reactants (e.g., candle wax) E = Ea – Ea’ (in this case E is negative so heat is given off)

The reactant energy – on left The product energy – on right E: The energy released by an exothermic reaction or absorbed in an endothermic reaction. ¿This reaction: Endo- or exothermic? (exo) The reactant energy – on left The product energy – on right DE = energy reactant – energy product

DEforward: energy products – energy reactants 50 kJ/mol – 0 kJ/mol = 50 kJ/mol  DEreverse: energy reactant – energy products 0 kJ/mol –50 kJ/mol = –50 kJ/mol Ea: energy activated complex – energy of reactants 80 kJ/mol - 0 kJ/mol = 80 kJ/mol Ea’: energy activated complex – energy products 80 kJ/mol - 50 kJ/mol = 30 kJ/mol

Factors That Can Influence Reaction Rate: Nature of reactants. Surface area. Temperature. Concentration. Catalysts. (depends on reactant(s)) (direct) For example. E.g., hydrogen combines vigorously with chlorine under certain conditions but may only react weakly with nitrogen. Surface area. E.g., small pieces of wood kindling have a more surface area-to-volume ratio than a large piece of wood. The wood can react with more oxygen and thus, burn faster. Temperature. Temp.  KE  collision frequency & energy. (more rxn) Concentration. The more particles that are present, the higher the collision frequency, and the faster the reaction rate. Catalysts: Changes rate of a chemical reaction without itself being permanently consumed or produced. Lowers activation energy E.g., KI in H2O2 E.g., Adding ice to carbonated water H2CO3(aq)  H2O(l) + CO2(g)