# Half Life Graphs - Order of Reactions

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#1
This graph shows a zero order reaction. [H2] falls and the rate is unchanged by the change in [H2] concentration so the rate (gradient) remains constant.

BUT, does this graph ignore everything else in the reaction ? Because to my understanding , if [X] falls, this is because it is reacting with other reactants therefore [other reactant ; Y ] falls. But what if Y is a first or even second order reactant , surely this would mean that graph above would not be linear, but would actually be non-linearly and the gradient would be a curved L shape ?

Please can someone explain ? Thanks
Last edited by lhh2003; 1 month ago
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#2
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1 month ago
#3
(Original post by lhh2003)
This graph shows the constant half-life of a zero order reaction. [H2] falls and the rate is unchanged by the change in [H2] concentration so the rate (gradient) remains constant.

BUT, does this graph ignore everything else in the reaction ? Because to my understanding , if [X] falls, this is because it is reacting with other reactants therefore [other reactant ; Y ] falls. But what if Y is a first or even second order reactant , surely this would mean that graph above would not be linear, but would actually be non-linearly and the gradient would be a curved L shape ?

Please can someone explain ? Thanks
Hey OP, I may have misunderstood your question but I’ll give it my best go at explaining!

Often when we talk about rates, we usually say the phrase ‘with respect to’. So in this example, the rate is zeroth order ‘with respect to’ H2.

You are absolutely correct that if Y was first or second order, then the graph would be non linear, curved! In that case, we would be drawing a graph of Y against time, and we would say the reaction is first / second order ‘with respect to’ Y.

However as in this example we have specifically drawn a graph of H2 conc. against time, and the rate is zeroth order with respect to H2, we get this constant gradient graph.

I hope that sort of answers your question, let me know if not
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#4
(Original post by depreshun master)
Hey OP, I may have misunderstood your question but I’ll give it my best go at explaining!

Often when we talk about rates, we usually say the phrase ‘with respect to’. So in this example, the rate is zeroth order ‘with respect to’ H2.

You are absolutely correct that if Y was first or second order, then the graph would be non linear, curved! In that case, we would be drawing a graph of Y against time, and we would say the reaction is first / second order ‘with respect to’ Y.

However as in this example we have specifically drawn a graph of H2 conc. against time, and the rate is zeroth order with respect to H2, we get this constant gradient graph.

I hope that sort of answers your question, let me know if not
The gradient represents the rate of the entire reaction, but if the reaction proceeded , then the rate would not decrease linearly as there would be a first order reactant reacting with a zero order , but the first order would lead to overall a decreasing rate of reaction ; not a constant rate shown by a linear graph . This leads me to believe that the "with respect to H2" assumes the H2 for example is considered to be in total isolation to everything else reacting. Would I be right in saying this ?
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1 month ago
#5
(Original post by lhh2003)
The gradient represents the rate of the entire reaction, but if the reaction proceeded , then the rate would not decrease linearly as there would be a first order reactant reacting with a zero order , but the first order would lead to overall a decreasing rate of reaction ; not a constant rate shown by a linear graph . This leads me to believe that the "with respect to H2" assumes the H2 for example is considered to be in total isolation to everything else reacting. Would I be right in saying this ?
Two things:

1. if [Y] is VERY high, then it will barely change as X gets used up and hence the very small change in [Y] has realistically no change on rate.
2. if [X[ gets very low, then changes in [X] will start to affect rate, i.e. the straight line shouldn't go all the way to the X-axis.
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#6
(Original post by Pigster)
Two things:

1. if [Y] is VERY high, then it will barely change as X gets used up and hence the very small change in [Y] has realistically no change on rate.
2. if [X[ gets very low, then changes in [X] will start to affect rate, i.e. the straight line shouldn't go all the way to the X-axis.
I see, so we assume [Y] is huge. Makes sense. Thank you !
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1 month ago
#7
(Original post by lhh2003)
I see, so we assume [Y] is huge. Makes sense. Thank you !
Just to add to that, pigster is completely right that the concentration of ALL the other reactions is VERY VERY large, when we do experiments to determine the rate with respect to a specific reactant. In fact, my textbook said it is at least tenfold times in excess as the reactant you are determining! So whilst Y may be first order or second order, in theory it will have NO effect on the shape of the graph of H2 against time, as it is in such LARGE excess it remains constant. Hope that clears it up
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