# edexcel a2 chem activation energy help

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#1
guys can someone please explain what the arrhenius constant is where there is some ln (lin? idk, havent taken it yet in c34..) equation involved with the activation energy.. or at least link me a youtube video. i cant find anything helpful.. i seriously have nooo idea what the teacher is on about and i feel so blocked out, i need help pleaseeee 0
4 years ago
#2
(Original post by pondsteps)
guys can someone please explain what the arrhenius constant is where there is some ln (lin? idk, havent taken it yet in c34..) equation involved with the activation energy.. or at least link me a youtube video. i cant find anything helpful.. i seriously have nooo idea what the teacher is on about and i feel so blocked out, i need help pleaseeee I pressume you've done order of reactions and the idea of rate constants, I think this is pretty good at explaining the Arrhenius equation give it a read and see if it makes sense http://www.chemguide.co.uk/physical/...arrhenius.html
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#3
(Original post by MexicanKeith)
I pressume you've done order of reactions and the idea of rate constants, I think this is pretty good at explaining the Arrhenius equation give it a read and see if it makes sense http://www.chemguide.co.uk/physical/...arrhenius.html
thanks very much... i just realized that i dont really understand what rate constant actually means. i thought it basically meant that the temp is kept the same through out the whole reaction so that it has no effect, can you pls clarify this?
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4 years ago
#4
(Original post by pondsteps)
thanks very much... i just realized that i dont really understand what rate constant actually means. i thought it basically meant that the temp is kept the same through out the whole reaction so that it has no effect, can you pls clarify this?
The rate of a reaction depends on the concentration of reactants, we can write the rate as proportional to the concentrations to some power, for the reaction of A with B

aA + bB ---> products

rate ∝ [A]a[B]b

You can always make a proportionality into en equality by adding a proportionality constant k.

rate = k[A]a[B]b

This proportionality constant k is the rate constant, that's all the rate constant is, a constant of proportionality.

Arrhenius simply noticed that, if you change the temperature of a reaction, the rate changes, so the rate equation must have some Temperature dependence!
[A] and [B] don't have a temperature dependance (we can choose initial concentration whatever temperature) so the temperature dependance must be contained in side the rate constant.

This means that the rate constant isn't actually constant! It depends on temperature!

The Arrhenius equation is simply the suggestion that arrhenius made as to how the rate constant might depend on temperature (he wasn't just guessing, statistical methods beyond this level give you the Arrhenius equation)

So, to conclude, the rate constant is simply something introduced so we can express the rate in terms of concentrations and the Arrhenius equation is simply a model of how rate changes with temperature and how the rate constant k can be written to accomodate the temperature dependence!

This has an a-level description of rate constants, give it a read (chemguide tends to have good explanations of most topics at a level) http://www.chemguide.co.uk/physical/...es/orders.html
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#5
(Original post by MexicanKeith)
The rate of a reaction depends on the concentration of reactants, we can write the rate as proportional to the concentrations to some power, for the reaction of A with B

aA + bB ---> products

rate ∝ [A]a[B]b

You can always make a proportionality into en equality by adding a proportionality constant k.

rate = k[A]a[B]b

This proportionality constant k is the rate constant, that's all the rate constant is, a constant of proportionality.

Arrhenius simply noticed that, if you change the temperature of a reaction, the rate changes, so the rate equation must have some Temperature dependence!
[A] and [B] don't have a temperature dependance (we can choose initial concentration whatever temperature) so the temperature dependance must be contained in side the rate constant.

This means that the rate constant isn't actually constant! It depends on temperature!

The Arrhenius equation is simply the suggestion that arrhenius made as to how the rate constant might depend on temperature (he wasn't just guessing, statistical methods beyond this level give you the Arrhenius equation)

So, to conclude, the rate constant is simply something introduced so we can express the rate in terms of concentrations and the Arrhenius equation is simply a model of how rate changes with temperature and how the rate constant k can be written to accomodate the temperature dependence!

This has an a-level description of rate constants, give it a read (chemguide tends to have good explanations of most topics at a level) http://www.chemguide.co.uk/physical/...es/orders.html
ohhh x, i still dont quite get what the A (frequency factor), Arehhnius constant is?
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4 years ago
#6
(Original post by pondsteps)
ohhh x, i still dont quite get what the A (frequency factor), Arehhnius constant is?
The exponential factor (e-(Ea/RT)) simply accounts for the fact that as you increase temperature a larger proportion of molecules have the energy required to react.

However, in order to react these molecules must collide with one another! the frequency factor, A, accounts for the frequency of collisions and can also account for the fact that collisions aren't guaranteed to lead to reaction!

You don't need to know how it accounts for these factors at A level so at A-level it is always just left as a constant, A, for simplicity.
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#7
(Original post by MexicanKeith)
The exponential factor (e-(Ea/RT)) simply accounts for the fact that as you increase temperature a larger proportion of molecules have the energy required to react.

However, in order to react these molecules must collide with one another! the frequency factor, A, accounts for the frequency of collisions and can also account for the fact that collisions aren't guaranteed to lead to reaction!

You don't need to know how it accounts for these factors at A level so at A-level it is always just left as a constant, A, for simplicity.
thank you sooo much really xxx
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