# Rates of reaction

• Created by: dgavan
• Created on: 22-05-19 11:22
• Rates of Reaction
• Orders of reactants
• Zero order: changing [reactant] has no effect on rate
• NOT included in the rate eqn
• First order: change in [reactant] is directly proportional to change in rate
• Included in rate eqn
• Rate eqn:  rate = k[A]^m [B]^n
• Second order: rate is multiplied by (change in [reactant]) squared
• Included in rate eqn
• Rate eqn:  rate = k[A]^m [B]^n
• Order of reactants can be determined through exp.al data given in a table
• Overall order: the sum of all the orders of reactants e.g. if you had one zero order, one first order and two second order, then overall = 0 + 1 + 2 + 2 =5
• Concentration-time graphs
• Must be produced from continuous monitoring  e.g. vol of gas collected, mass lost, pH and colorimetry
• Method: (1) Pick a conc for the reactant being investigated (2) Start the exp (3) Measure [R] by taking samples at regular time intervals
• Processing the data: (1) Plot a graph of conc vs time (2) Draw a line of best fit (3) Draw a tangent and find gradient to get rate at that point
• Orders
• Zero order reactants have a linear (straight-line) conc-time graph going down
• First order reactants have a constant half life (t1/2); time taken for [R] to halve. It's a curved graph
• You can find rate by drawing a tangent then measuring gradient, and you  can then rearrange to get k
• OR... you can just do k = ln2/half life
• You don't need to know how to calc rate from a second order graph
• Rate-concentration graphs
• Must be produced from initial rates; the conc of 1 thing is varied and everything else stays the same.
• It's the time taken for "something" to happen e.g. iodine clock (time taken for colourless -> blue/black once all the thiosulphate ions are used up). Could also be time taken for ppte to form, a set vol of gas to be produced etc
• Rate = 1/time i.e. rate in s^-1
• Orders
• Zero order is just a flat horizontal line so k = y-intercept
• First order is a straight line going diagonally up; k = gradient
• MUST PASS THROUGH THE ORIGIN
• You don't need to know how to calc k for a 2nd order reactant, but the graph is a curve going up
• MUST PASS THROUGH THE ORIGIN
• Arrhenius eqn
• Only temp can affect k (rate constant)
• k = Ae^-(Ea/RT)
• So... lnk = (-Ea/R) (1/T) + lnA
• i.e. y = mx + c
• Forms a straight line graph (remember if lnA = y-int, then you've gotta un-ln it to get A, i.e. A = e^y-intercept)
• k= rate constant
• A = pre-exponential frequency factor (relates to no of successful collisions
• e = exponential factor (just the e button on your calc)
• Ea = activation energy (kJmol^-1)
• R = gas constant (8.314 mol^-1K^-1)
• T = temp (K)
• Rate-determining step
• Steps happen at different speeds; the slowest one dictates the rate i.e. RDS
• Everything in the rate eqn must be in the RDS
• You can use the info from the RDs, the rate eqn and the overall eqn to piece together a potential mechanism