yr 13 kinetics 2
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rate equation
change in concentration of reactant/product / time for change to take place (s)
what is zero order
a change in the concentration of a zero order reactant has no effect on the rate Rate is directly proportional to []^0 conc x2, rate x 1
what does a rate against conc graph look like for zero order
flat horizontal line
what is first order
if [reactant] is doubled, the rate of reaction doubles directly proportional Rate is directly proportional to []^1 conc x2, rate x2
what does a rate against conc graph look like for first order
a diagonal line upwards
what is second order
rate is directly proportional to []^2 conc x2, rate x4
what does a rate against conc graph look like for second order
quadratic graph, exponential
describe initial rates method
• set up several reaction mixtures using different concentrations of the reactant determining the order of • measure time taken for a particular event to happen for each concentration e.g. gas produced, colour change, precipitate • plot a graph of conc against time • look at relationship
describe continuous monitoring method
• set up one reaction mixture • take samples at regular time intervals and measure the concentration of one reactant/product at that time • quench the reaction in the sample before determining concentration
quench definintion
slow down reaction to the point of any changes in concentration being negligible
how to quench
• cool in ice • add a reagent to react with and remove something in the reaction e.g. NaHCO3 to remove H+
what methods can be used to measure concentration
• volume of gas • change in mass • colourimetry • titration • electrical conductivity
how to measure volume of gas and draw
• use gas syringe • ensure no gaps between delivery tube, bung etc • make sure openings are actually open • label apparatus including delivery tubes • indicate a scale on gas syringe • include precisions such as concentrations or volumes
how to work out the rate from a continuous method
draw a concentration-time graph, then find out the half-life for the reaction at different concentrations
what does a conc time graph look like for a zero order reaction
rate is constant straight line with a negative gradient
what does a conc time graph look like for a first order reaction
downwards shallow curve length of half life is constant
what does a conc time graph look like for a second order reaction
deeper downwards curve length of half life increases with decreasing concentration
what is the rate determining step
the slowest step in the reaction
which reactants are included in the rate equation
only reactants in the RDS and before the RDS
what does the order of a reaction tell us
the number of moles of reactant in the steps up to and including the RDS
what does a high order tell us
there are several particles needing to collide with sufficient energy and correct orientation which is unlikely so therefore it must be a multistep reaction
describe Sn2 mechanism in terms of rate
primary halogenoalkanes, 2 chemical species and second order reaction
describe Sn1 mechanism in terms of rate
tertiary halogenoalkanes, first order overall, must be more than one step as nucleophile not included in rate equation so must react after the RDS
describe a clock reaction
easier way of obtaining the initial rate of a reaction by taking a single measurement time is measured from start of an experiment for a visual change to be observed small amount of reagent that reacts with one of the products is added to delay the end of reaction so it can be timed if there is no significant change in rate then it can be assumed that the average rate will be the same as the initial rate initial rate is then proportional to 1/T