# Rates of reactions

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• Rates of reactions
• the rate of reaction all depends on
• how fast the products are made
• how fast the reactants are used up
• as such the equation is rate=change in conc of product or reactant/time taken for the change to happen
• this has the unit of mol dm-3 s-1
• measuring rates
• as the reactions continues on the rate deceases and the collisions between the reactants
• you can measure the rate of reaction through experiment you then draw a graph
• you then draw a tangent on the graph draw a triangle divide the vertices side by the horizontal this will give you the gradient this is the rate of reaction
• some changes you can make to a reaction is changing the concentration of the reaction or the pressure
• orders of reactions
• this is all about the effect the reactant has on the rate of reaction
• a zero Oder reactant means it has no effect on the overall rate of reaction this is usually a catalysis
• a first oder reactant has a directly proportional effect on the rate of reaction if you double the amount of the reactant you double the rate
• a second order reaction has the effect of squaring the rate of  reaction if you double the amount of a second order reactant you incre the rate by 4 if you x 3 the amount of the reactant you x 9 the rate
• rate constants and half lives
• the rate constant is k is the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation
• if you have a zero order reactant this dosent need to be included as it has no effect on the rate
• the overall order of a reaction is the sum of  the individual orders of the reactions
• the units of the reaction are worked out by a simple equation the initial rate in mol dm s/ the sum of the concentrations e.g two first oders would (mols dm)^2
• the half life changes depending on the order of the reaction
• in a second order reaction the half life increases as time moves on in a concentration time graph
• determining the rate constant from experimental data
• you will often be given a table the table will have up to three reactants and up too six rows
• from this table you must determine the rate of reaction
• here is an example of a table
• intial rate mols dm^-3 S^-1
• initial [ O2] mols  dm^3
• initial concentration of [NO] mol dm^3
• experiment
• 1
• 2
• 0.00100
• 1
• 2
• 0.00100
• 0.00200
• 3
• this is a second order reaction you must note that the concentration for the value of O2 has returned to its original value so you need to compare the rate of this experiment to the first this can be checked the same way
• 7.28x10^-5
• 0.00100
• 0.00200
• 3
• this is a second order reaction you must note that the concentration for the value of O2 has returned to its original value so you need to compare the rate of this experiment to the first this can be checked the same way
• 7.28x10^-5
• 0.00100
• 5.46x10^-5
• 0.00300
• 0.00100
• this is a first order reactant as when you triple the concentration of it you triple the rate you can check this by x3 the rate of the first experiment
• 5.46x10^-5
• 0.00300
• this is a first order reactant as when you triple the concentration of it you triple the rate you can check this by x3 the rate of the first experiment
• remember to always check both values this will make it easier to work out the orders
• remember to always check both values this will make it easier to work out the orders
• 0.00100
• 0.00100
• 1.83x10^-5
• 0.00100
• from this you can make a K equation in this case the equation would be rate=K[NO]^2[O2]
• concentration time graphs and orders of reactions
• for a zero order reaction the rate decreases at a constant rate
• for first order reaction the concentration halfs in equal time intervals e.g the concentration halfs every 5 mins
• for a second order reaction the rate will decrease rapidly after this rapid decrease it will start to slow down
• from a concentration time graph you can make a concentration rate graph
• concentration rate graphs
• a second order reaction has a directly proportionate line so if you double the concentration reactant you double the rate or if you triple the concentration of the reactant you triple the rate
• the rate concentration graph for a zero order reaction will be a horizontal line as the rate is unaffected by changes inn concentration
• a second order reaction will be a curve as the rate is = to [concentration]^2
• if you double the concentration of the reactant you quadruple the rate