# Rates of reaction

- Created by: dgavan
- Created on: 22-05-19 11:22

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- 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

- Included in rate eqn
- Second order: rate is multiplied by (change in [reactant]) squared
- Included in rate eqn
- Rate eqn: rate = k[A]^m [B]^n

- Included in rate eqn
- 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

- Zero order: changing [reactant] has no effect on rate
- 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

- Must be produced from initial rates; the conc of 1 thing is varied and everything else stays the same.
- 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)

- So... lnk = (-Ea/R) (1/T) + lnA

- 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

- Orders of reactants

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