Quantity of reactants

[lilietta2000]

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How is it that the quantity of reactants does not affect the rate of the reaction but the concentration of the reactants does? 😕

 

[seraph524]

How is it that the quantity of reactants does not affect the rate of the reaction but the concentration of the reactants does? 😕

uhhhhhhhh lol what? doesnt seem possible...

 

[lilietta2000]

uhhhhhhhh lol what? doesnt seem possible...

OK here's the question:

Which of the following does NOT affect the rate of a reaction?
A. Temperature
B. Presence of a catalyst
C. Quantity of reactants
D. Concentration of reactants

Answer: C,...but how is that?

 

[seraph524]

Oooooh I know what its asking now, and it's pretty ridiculous. If the quantity of a reactant increases, with a parallel increase in volume, then the concentration will not change. In this case, the change in quantity of the reactant will not change the rate.

So in other words, the sole increase in absolute magnitude of reactant is not enough to determine whether or not the rate will change.

 

[RPedigo]

By quantity, they assume that you don't change the concentration. For instance, having 10000L of a 1M solution versus having 1L of a 1M solution. In both cases there is one mole of reactant per liter of solution, and since rate is measured in mol/L*s then changing the quantity (if concentration is constant) will not affect the rate, since the rate is normalized for molarity since it's in units of mol/L*s.

 

[lilietta2000]

Oooooh I know what its asking now, and it's pretty ridiculous. If the quantity of a reactant increases, with a parallel increase in volume, then the concentration will not change. In this case, the change in quantity of the reactant will not change the rate.

So in other words, the sole increase in absolute magnitude of reactant is not enough to determine whether or not the rate will change.

Still confusing, I thought by quantity they meant the number of reactant..right?

 

[lilietta2000]

By quantity, they assume that you don't change the concentration. For instance, having 10000L of a 1M solution versus having 1L of a 1M solution. In both cases there is one mole of reactant per liter of solution, and since rate is measured in mol/L*s then changing the quantity (if concentration is constant) will not affect the rate, since the rate is normalized for molarity since it's in units of mol/L*s.

I'm so sorry but I still don't understand how quantity is different from concentration.....😕

 

[seraph524]

For example, if I have 5 grams of X in 5 L of water. That would give me 1 g/L as the concentration. This yields a certain reaction rate.

Now, there are two possibilities:

A: grams increase, without an increase in volume.

or

B: grams increase, WITH an increase in volume.

So in case A, if I now have 10 grams of X in 5 L of water, that would give me 2 g/L as the concentration. Now as you know, an increase in concentration increases rate, which happens.

However, in the case of B, if I now have 10 grams of X but I also have 10 L of water, I still have 1 g/L. The grams of substance (or in otherwords, the quantity of reactants) increased, but the concentration stayed the same. In this situation, there would be NO increase in rate.

 

[lilietta2000]

For example, if I have 5 grams of X in 5 L of water. That would give me 1 g/L as the concentration. This yields a certain reaction rate.

Now, there are two possibilities:

A: grams increase, without an increase in volume.

or

B: grams increase, WITH an increase in volume.

So in case A, if I now have 10 grams of X in 5 L of water, that would give me 2 g/L as the concentration. Now as you know, an increase in concentration increases rate, which happens.

However, in the case of B, if I now have 10 grams of X but I also have 10 L of water, I still have 1 g/L. The grams of substance (or in otherwords, the quantity of reactants) increased, but the concentration stayed the same. In this situation, there would be NO increase in rate.

Wow, you're so good with your explanation! Thanks a ton!

 

[AsuKa]

that question is absolutely terrible

 

[seraph524]

that question is absolutely terrible

yep, pretty lame

 

[Kaustikos]

that question is absolutely terrible

Actually, with process of elimination, you should get the right answer without even worrying about what they mean by "quantity". You know the rate = k [X]

k is affected by temp and catalyst. rate is affected by k and [X] and we all know that [X] is concentration.

That's the logic I used sometimes on the mcat and it saves a lot of time. 👍