chapter 1 - rate graphs

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11 Terms

1
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what is a rate of reaction?

  • the change in concentration of a reactant or product over time

    • if you plot concentration against time, the rate is the gradient of the graph


2
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how to calculate the gradient (and the rate) of the graph?

  • you find the change in concentration divided by the change in time

    • pick 2 points and read off the x and y values

    • calculate the differences in concentration values and the time values, and then divide one by the other


<ul><li><p>you find the change in concentration divided by the change in time</p><ul><li><p>pick 2 points and read off the x and y values</p></li><li><p>calculate the differences in concentration values and the time values, and then divide one by the other</p></li></ul></li></ul><p></p>
3
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how to calculate the rate on curved graphs?

  • by drawing a tangent


<ul><li><p>by drawing a tangent</p></li></ul><p></p>
4
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what can be deduced from the shape of a concentration-time graph?

  • the order with respect to a reactant


5
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zero order reaction?

  • produces a straight line with a negative gradient

  • the reaction rate does not change at all during the course of the reaction

  • the value of the gradient is equal to the rate constant

    • the straight line graph makes a zero order relationship easy to identify


<ul><li><p>produces a straight line with a negative gradient</p></li><li><p>the reaction rate does not change at all during the course of the reaction</p></li><li><p>the value of the gradient is equal to the rate constant</p><ul><li><p>the straight line graph makes a zero order relationship easy to identify</p></li></ul></li></ul><p></p>
6
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first order reaction?

  • produces a downward curve with a decreasing gradient over time

  • as the gradient decreases with time, the reaction gradually slows down

  • in a first-order concentration time graph, the time for the concentration of the reactant to halve is constant

  • this time is called the half-life, and the rate constant of a first order reaction can be determined using its value


<ul><li><p>produces a downward curve with a decreasing gradient over time</p></li><li><p>as the gradient decreases with time, the reaction gradually slows down</p></li><li><p>in a first-order concentration time graph, the time for the concentration of the reactant to halve is constant</p></li><li><p>this time is called the half-life, and the rate constant of a first order reaction can be determined using its value</p></li></ul><p></p>
7
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second order reaction?

  • the graph for a second order is also a downward curve, steeper at the start, but tailing off more slowly


8
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what is half-life?

  • time taken for half of the reactant to be used up


9
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what type of half-lives do first order reactions have?

  • they have a constant half-life with the concentration halving every half-life

  • this pattern is called exponential decay


10
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how can a first-order relationship be confirmed from a concentration-time graph?

  • by measuring successive half-lives.

  • if they are the same, the reaction is first-order with respect to the reactant


11
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example: first order relationship confirmed from a concentration time-graph?

  • the graph shows 3 successive half-lives with a constant value of 100s

  • the constant half-life means that this reaction is first order with respect to N2O5

  • [N2O5]0 u=is the initial concentration of N2O5 at time = 0s


<ul><li><p>the graph shows 3 successive half-lives with a constant value of 100s</p></li><li><p>the constant half-life means that this reaction is first order with respect to N2O5</p></li><li><p>[N2O5]0 u=is the initial concentration of N2O5 at time = 0s</p></li></ul><p></p>