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# Confused about law of mass action watch

1. Hello!

So I'm currently doing a maths bio course and I am on the maths behind enzyme kinetics.

I've been given this reaction:

Now according the Law of mass action: The law of mass action states that the rate of a reaction is proportional to the product of the concentrations of the raction partners involved

So for this reaction that give an equation for the change in [A] which is:

Now I may be overcomplicating things but why is this the case? It's more that there's the [A][B] term that has seriously confused me now.

Any explanations welcome.

Thanks
2. Hi there,

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3. (Original post by Slowbro93)
Hello!

So I'm currently doing a maths bio course and I am on the maths behind enzyme kinetics.

I've been given this reaction:

Now according the Law of mass action: The law of mass action states that the rate of a reaction is proportional to the product of the concentrations of the raction partners involved

So for this reaction that give an equation for the change in [A] which is:

Now I may be overcomplicating things but why is this the case? It's more that there's the [A][B] term that has seriously confused me now.

Any explanations welcome.

Thanks
The d[A]/dt term refers to the rate of change of the concentration of A with respect to time.

There are two reactions that affect the concentration of A: either A can be consumed or it can be produced. A is consumed when it reacts with B to produce C; A is produced along with B when C degrades.

The d[A]/dt term is equal to the sum of the rate of production of A and the rate of consumption of A. The rate of production of A is equal to the rate constant for that reaction multiplied by the concentration of all of the reactants in that reaction. Likewise for the consumption of A.

As to why the [A][B] term is used for the rate of consumption of A, think of it this way: there are three factors that affect the probability that A will react with B to produce C. The first is the concentration of A, the second is the concentration of B, and the third is the rate constant for that reaction, which encapsulates the probability that A and B will collide with enough energy to react with each other to produce C. As you can imagine, increasing either the concentration of A or B will increase the probability that they will come into contact and then react with each other. For example, if you had a solution containing both A and B and then you decided to multiply the number of B molecules by three, then the production of C would be three times more likely as A and B would three times more likely to collide. You could think of it as each molecule in solution being an independent event, and then probability of two different molecules co-occurring with each other in the same space in solution (i.e. colliding) is the product of the probabilities for each of the independent events.

If you scale up this principle across many millions/billions of molecules, then the probability (stochastic) of A's consumption then becomes a deterministic rate equation encapsulated by the product of k+, [A], and [B].

I hope this makes sense.
4. Sorry didn't get around to replying back. I think I get it now thanks

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