Rates of reaction - kinetics (A2)
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Rates from concentration
Concentration/ Time graphs
In any study of the kinetics of a reaction, you need to be able to measure the rate of the reaction at various concentrations of the particular reactant under investigation as the reaction proceeds.
Measuring rates
Measuring rates is not straightforward. There is unlikely to be a direct experimental method of measuring rate at a particular instant of time. However, as you should know, there are various methods of measuring the concentration of the reactant being investigated at precise times during the reaction. In addition, the rate of a reaction is not normally constant. It changes as the reaction proceeds. (Most commonly it gets progressively slower.)
Sometimes, when studying fairly slow reactions, you can get round this by using techniques such as the initial rate method.
This relies on the fact that rate is approximately constant at the beginning of a reaction (usually up to about 10% completion). So, if the time (t) to complete a known amount of reaction (< 10%) can be determined, then rate is proportional to: 1/t.
More commonly, however, a graph has to be plotted of 'Concentration of reactant being investigated' against 'Time' and the rate at various times determined from that.
How is this done?
Plotting a graph
Rate is defined as the change in concentration of the reactant being investigated in unit time (normally 1 s). So it is equal to the slope of the concentration against time graph.
However, except in the case of a zero order reaction where rate does not depend on the concentration of the reactant, the graph line is a curve. This is because the rate changes with time as the concentration of the reactant changes.
How do you determine the slope of a curve?
Answer: by drawing tangents to the curve at various points on it and measuring the slope of the tangent.
But what is a tangent and how do you draw one?
A tangent can be defined as a straight line which touches a curve at a point on it without crossing the curve at that point.
Tangents can be quite hard to draw accurately, but you may find this way helpful.
Draw a construction line at the point where you want to draw the tangent so that, as near as possible, it crosses the curve at right angles at that point. Then draw you tangent at right angles to your construction line to just touch the curve where the construction line crosses it and measure its slope.
Tip: You will get a more accurate value for the slope of the tangent if you draw it as long as you reasonably can so that the ‘y value’ (concentration) and ‘x value’ (time) are as large as possible.
Repeat this process at various points on the curve to get several values for the rate at different concentrations. Record these concentrations and their corresponding rates in a table.
Then add two more columns to your table and record the corresponding values of lg (rate) and lg (concentration), both of which you can determine using your calculator.
You can now plot lg (rate) against lg (concentration) which should give you a straight line. Measure the slope and this will be the order of the reaction for that particular reactant.
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updated: 07 February 2006
