# rate equations

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## what is the rate equation

Rate = k[A]m [B]n

where m and n are the orders of reaction with respect to the reactants A and B. K is the rate constant

orderds m and n are restricted to values 0,1 and 2

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## what is the Arrhenius equation and what does it sh

k = AeEa/RT

the rate constant k varies with temperature as shown by the equation

A is the Arrhenius constant, Ea is activation energy, T is the temperature in kelvins, R is the gas constant ( be given when required), and e is a constant (on a calculator).

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## explain the effect of changes in temp on the rate

the higher the temperature the

• more kinetic energy each particle has
• therefore a greater proportion of particles have energy greater than or equal to the activation energy
• therefore there is a greater frequency of succesful collision

if the temperature gets 10 kelvin higher, the rate is approximately double

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## how would you use the Arrhenius Equation as a grap

rearrange it to

ln k = –Ea /RT + ln A

where y= ln k, x= 1/T, m= -Ea/RT and C=Ln A

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## define the rate of reaction

the change in concentration per unit of time

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## define the activation energy

the minimum energy required for a reaction to occur

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## what is a catalyst and how does it work

it is a substance which increases the rate of reaction without being used up by providing an alternate reaction route with a lower activation energy.

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## what do the different orders mean

• zero-order means the changing concentration of the reactant has no effect on the rate of reaction
• the first order is directly proportional to the concentration
• the second-order is proportional to the square of the change in concentration e.g. doubling the concentration causes the rate to increase by 4 (2 squared)
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## how do you determine the orders of reaction using

when a rate vs conc graph is a horizontal line it means the reagent is zero order

when a rate vs conc graph is a straight line it means the reagent is 1st order

when a rate vs conc graph is a line curving up it means the reagent is 2nd order

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## how do you determine the rate determining step in

the species in the rate determining step must match the species in the rate equation

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## how do you find the overall order of reaction

by adding up all the orders in the equation

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## define the terms order of reaction and rate consta

the order of reaction with respect to a particular reactant is the power to which the concentration of this reactant is raised in the rate equation

the rate constant is the proportionality constant which links the rate of reaction to the concentrations in the rate equations

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1- s-1

2- mol-1dm3s-1

3- mol-2dm6s-1

4- mol-3dm9s-1

4- mol-4dm12s-1

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## why do many chemical reactions start off with fast

• initially there is a high concentraion of reactants so a large frequency of succesful collision
• as the reactnats start to get used up succesful collision becomes less frequent
• once one or more reactants are used up there can be zero succesful collisions
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## how to draw conc x time graph of A + 2B--> C

• As A and B are both reactants their concentrations go down and as C is a product its concentration goes up
• B starts off twice as high as A because in the equation it shows that twice as much of B is required for the reaction
• all three graphs become horizontal at a certain time because no concentrations change which indicates the reaction has finished or is at equilibrium.

to meausre the initial rate a tangent is drawn at t=0 and to find the rate of reaction at a given time you use a tangent.

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