Rates of Reaction

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  • Created on: 19-11-08 16:41
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The effect of concentration on rate
Rate of reaction = rate at which reactants are turned into products
For example-
The decomposition of hydrogen peroxide to form water and oxygen.
We could measure the rate or this reaction in the moles of product (H2O or O2) formed per a second or
moles of hydrogen peroxide used up per a second.
-1 -1
0.0001 mol (O2) s-1 or 0.0002 mol (H2O) s or -0.0002 mol (H2O2) s
Measuring the rate of reaction
To measure the rate of reaction you have to measure the change in the amount of a reactant or
product in a certain time.
Step 1- decide on a property of a reactant or product which you can measure (e.g. volume of gas
given off, concentration of a product or pH)
Step 2- Measure the change in the property in a certain time
Step 3- Find the rate in terms of
Depending on the property measured the units will change but most can be converted into mol s-1-
e.g. (cm3 of gas) s-1 when measuring the amount of gas produced or mol dm-3 s-1 if measuring the
concentration change
Investigating how the rate depends on concentration
The decomposition of hydrogen peroxide is greatly speeded up by catalysts, especially the enzyme
catalase.
To measure the rate of reaction you can measure the volume of oxygen given off at different
intervals throughout the experiment.

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Oxygen can be collected and measured using an inverted burette.
volume of 02/ cm 3
Time /s
Looking at the graph you can see-
o The graph is steep at first, when the concentration of hydrogen peroxide is highest, before
any has been used up. The rate of reaction at t=0, the gradient of the tangent to t=0, gives
the initial rate.
o The graph gradually flattens out as hydrogen peroxide is used up, and the concentration
falls.…read more

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Gives the general rate equation rate = k [A]m [B]n
Rate [A]
Order of reaction 0 (zero) with respect to A ­ means no change in the rate of reaction when I change [A]
Order of reaction 1 with respect to A ­ means that if [A] is doubled, then the rate is doubled (12)
Order of reaction 2 with respect to A ­ means that if [A] is doubled, then the rate is x4 (22)
Order of reaction 3 with respect to A ­…read more

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Half-lives method- By using the progress curve to find half-lives for the
reaction you can see that if the half-lives are constant, the reaction is first order.
Rate-determining Steps
Many reactions occur in series of simple individual steps. If one step in a reaction mechanism is
much slower that the others, it is called the rate-determining step.…read more

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