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Kinetics - Why doesn't a change in concentration affect the rate constant?

A rise in temperature and the use of catalysts both increase the rate of reaction and therefore the rate constant, but even though an increase in concentration also increases the rate of reaction, why doesn't it affect the rate constant?

Thank you for any help!

EDIT:

never mind, i've found the answer:

"The rate equation shows the relationship between the concentration of reactants and the rate of reaction.

If the concentration of one of the reactants increases, the rate of reaction will also increase, the rate constant, k will not change.

If the pressure increases, the concentration of all the reactants will increase and so the rate of reaction will also increases. Again the rate constant will not change.

The rate constant k is thus independent of concentration and pressure.

If the temperature increases, however, or a catalyst is added, the rate of reaction increases without a change in concentration, and so it must be the rate constant, k, that is changing.

The rate constant k thus varies with temperature, and is also affected by the addition of a catalyst."

For anybody else who was having the same problem. :smile:

(edited 12 years ago)

Reply 1

Gul_harram

How the rate constant k is independent of concentration and dependent on temperature?

Reply 2

David Tan

Original post by Gul_harram

How the rate constant k is independent of concentration and dependent on temperature?

Check out Arrhenius equation. Rate constant is dependent on presence/absence of catalyst and temperature.

Hope it helps. Cheers.

Reply 3

p_helena

lnk = -Ea/RT + lnA
y = mx + c
(The Arrhenius equation).
As you can see, concentration doesn't appear in this equation.
lnk = natural log of rate constant (k)
Ea= activation energy
R= relative gas constant (8.314)
T= Temperature, kelvin.
lnA= natural log of preexponential factor A (don't worry about what this is).

You use this equation in A2 to find the activation energy (and sometimes A) from graphs.

Another way you can tell that concentration doesn't affect the rate constant is the "simpler" rate equation:

Rate =k [M]^m[N]ⁿ where ^mand ⁿ are the orders of the reactants (basically the effect that increasing concentration of these reactants has on the overall reaction). The square brackets represent concentrations of the reactants.

As you can see, rate is affected by concentration- if [M] were to increase, so would the rate because k and [N] are being multiplied by a larger number.
But, k in itself isn't affected by concentration. It is the rate constant for a reaction at a given temperature.
Think about the kinetics of a reaction. Increasing concentration means there are more molecules/atoms in a certain volume, increasing the likelihood of molecules colliding, but not affecting the number of molecules above activation energy.
However, increasing temperature means that more molecules are above activation energy (look at Boltzmann distributions).
This links back to the Arrhenius equation, which talks about activation energy but not concentration- hence catalysts and temperature does change the rate constant.

Sorry this is badly explained, it's been a while since I did this rate constant nonsense XD