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Rate-concentration graphs
Can be plotted from measurements of the rate of reaction at different concentrations.
Offer route into link between rate and concentration in the rate equation.
Zero order rate-concentration graph
Produce horizontal straight-line with zero gradient
Rate = k[A]⁰ so rate = k
Intercept on y-axis gives the rate constant
Rate doesn’t change with increasing concentration
First order rate-concentration graph
Produces straight-line graph through the origin
Rate = k[A]1 so rate = k[A]
Rate is directly proportional to concentration for first order
Rate constant determined by measuring the gradient of graph
Second order rate-concentration graph
Produces an upward curve with an increasing gradient
Rate=k[A]2
Rate constant cannot be obtained from this graph directly as it is a curve
By plotting a second graph of the rate against concentration squared, the result is a straight line through the origin and the gradient of this line is rate constant, k.
initial rates
Is the instantaneous rate at the start of a reaction when the time, t = 0
If the graph is curved the initial rate can be found using tangent drawn at t=0 on a concentration-time graph.
The initial rates method: clock reactions
More convenient way of obtaining initial rates by taking a single measurement. It is an approximation but reasonably accurate.
The time, t, from the start of an experiment is observed, often a colour or a precipitate.
As there is no significant change in rate, it’s assumed that the average rate of a reaction over this time will = initial rate.
Initial rate is proportional to 1/t
Clock reaction is repeated with different concentrations and values of 1/t calculated for each run
Iodine clocks
Common clock reaction relying on formation of iodine.
As aqueous iodine is coloured orange-brown, the time from the start of the experiment and the appearance of the iodine can be measured.
Starch is usually added with iodine (forming a complex) which is an intense blue-black colour.
![<p>Produce <span style="color: red">horizontal straight-line</span> with <span style="color: red">zero gradient</span></p><p>Rate = <em>k</em>[A]⁰<span style="color: rgb(191, 191, 191)"> </span>so rate = <em>k</em></p><ul><li><p><span style="color: red">Intercept on </span><em><span style="color: red">y-</span></em><span style="color: red">axis </span>gives the <span style="color: red">rate constant</span></p></li><li><p><span style="color: red">Rate</span> <span style="color: red">doesn’t change </span>with<span style="color: red"> increasing concentration</span></p></li></ul>](https://knowt-user-attachments.s3.amazonaws.com/1bc8b3c1-a928-424e-8c1f-711775dab759.jpg)
![<p>Produces <span style="color: purple">straight-line graph through the origin</span></p><p>Rate = <em>k</em>[A]<sup>1</sup> so rate = <em>k</em>[A]</p><ul><li><p>Rate is <span style="color: purple">directly proportional </span>to concentration for first order</p></li><li><p><span style="color: purple">Rate constant</span> determined by measuring the <span style="color: purple">gradient of graph</span></p></li></ul>](https://knowt-user-attachments.s3.amazonaws.com/fd9c009f-f156-40f5-bb16-76591996cf10.jpg)
![<p>Produces an <span style="color: purple">upward curve with an increasing gradient</span></p><p>Rate=<em>k</em>[A]<sup>2</sup></p><ul><li><p>Rate constant cannot be obtained from this graph directly as it is a curve</p></li><li><p>By <span style="color: purple">plotting a second graph of the rate against concentration squared</span>, the result is a straight line through the origin and the <span style="color: purple">gradient of this line is rate constant, </span><em><span style="color: purple">k.</span></em></p></li></ul>](https://knowt-user-attachments.s3.amazonaws.com/289531aa-d755-4caa-a369-062e3d79fe63.jpg)