chapter 18 rates

for a successful reaction: 1. reactants must possess at least a minimum ammouunt of energy - activation energy 2. particles must approach eachother in a certain relative way - steric effect

• increase surface area of solids • increase temperature • add a catalyst • increase the pressure of any gases • increase the concentration of reactants

• the rate of change of concentration is found from the gradient • the gradient at the start will give the initial rate and the gradient decreases as the reaction proceeds

• the order attribuuted to a reagent identifies the effect changing its concentration has on the rate of reaction

• rate = k[A]a[B]b where: • rate = mol dm-3s-1 • k = rate constant • [A] = cocncentration of A • a = the order of reagent • the overall order of a reaction is the sum of individual orders

• k can be determined for a first order reaction by using: 1. Rate (tangent) - k = rate / [A] 2. Half-life as first oders have exponential relationships) - k = ln2 / t1/2

• usually record the time it takes for an easily recognisable change to occur ealry in the reaction e.g colour change, formation of precipitate • these are often refered to as clock reactions where the concentration of one reagent is altered • the intital rate = 1/t , where t is the time taken for change to occur

• the rate determining step is the slowest step in a reaction mechanism - it determines the overall rate of reaction • there is a direct relationship between the order and the rate determining step: 1. zero order - not involved 2. first order - 1 mole of reactant 3. second order - 2 moles of reactant

• increase ROR: 1. boltzman distribution shifts to the right so more particles have the required activation energy 2. increased kinetic energy more frequent collisions • aprox every 10 degree increase results in doubling the rate constant k, so rate doubles

• the effect of temperature on rate can be explained by using arrhenius equation