# Rate constant

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#1
I’m confused. I read that the rate constant only depends on temperature but what about catalysts, concentration etc
and then I read that a change in concentration results in a change in reaction so doesn’t that mean that the rate constant must be affected by concentration?
0
7 months ago
#2
A reaction rate constant, as its name suggests, should not vary (at least, not too much) with the concentration of reactants otherwise there would be little point in defining it. The relationship between reaction rate (r), rate constant (k) and concentrations of reactants ([A], [B]) in a simple binary reaction A + B → C is given by:
r = k(T) [A] [B],
where k(T) means that the rate constant is a function of temperature and we have assumed the reaction is first order with respect to both reactants (i.e. it is linearly proportional to concentrations of A and B).

The rate constant k depends on temperature because it is related to the frequency with which reactants collide with each other and, having collided, whether or not they have sufficient energy to overcome the activation barrier to react (most reactions have a large positive activation barrier even though the final product is lower in energy). There are some relatively simple expressions which can be used to estimate k (e.g. Arrhenius law) and some more complex ones (e.g. Eyring equation from transition state theory) which improve on them, but I won't go into these here.

Catalysts can increase the rate of reaction, usually by lowering the activation barrier (i.e. reducing k). You could argue that this means k is no longer a constant, but I would argue it is still constant for a given reaction pathway. By adding a catalyst, you change the reaction pathway, but the k for each pathway is still largely independent of concentration.

Hope this helps.
0
7 months ago
#3
I think perhaps the easiest way to explain this is;
Increasing the rate constant will increase the rate of reaction
Increasing the rate of the reaction does not always mean an increase in the rate constant
In other words, the rate of reaction depends on the rate constant alongside the other factors you mention (concentration etc...) Those factors do not contribute to the rate constant, only the temperature does.
Hope that helps 👍😄
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7 months ago
#4
Look, I don't want to trash the previous answers, but although they are correct they are a little confusing.

When you introduce a catalyst the reaction proceeds via a different mechanism - it is no longer the same reaction.

Hence, a catalysed reaction has a different rate constant, because it's no longer the same reaction.

Let me give you an analogy.

If I want to drive from Madrid to Barcelona via Sevilla it will take me a long time because the journey is more than 1000km.

However, if I hitch a lift with a car that's going to Barcelona via Zaragoza the journey is now only 500km. I get there much faster.

In both cases I've gone from Madrid to Barcelona, but in the second case it was much faster. Why? because I went via a different route.

That's what catalysts do. They allow the reactants to go to the products via a different route (mechanism)
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#5
So why isn’t rate constant determined by concentration
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