Question

Can someone help explain to me why 1mol NO, 0mol H2, 4mol NH3, 4mol H20 best represent the mixture (2NO (g)+5 H2 (g)-> 2 NH3 (g) +2 H20) after the reactants have reacted completely? I can't quite grasp the concept.

Answer 1

I can try. So you will look at their number of moles at the beginning of the reaction. Then, using the balanced equation, you will look at their ratio in the reaction. Notice how the reactants are not being used up at the same rate? i.e 2 NO react with 5 H2

So you take a look at the rate for each ratio of reactant is used up by dividing the number of moles with the ratio. Get is so far?

Answer 2

Hmm. I'm trying to follow. The ratio in the beginning is 2:5?

Answer 3

Yeah because look at the coefficients of the compounds in the equation. Does that make sense? It means 2 parts of NO is needed to react with 5 parts of H2 to produce 2 parts of NH3 and 2 parts of H2O

Answer 4

So are the coefficients always are the ratio too? it's called the mole ratio right?

On your second question, reactant is the first set of reaction right? But I don't quite understand the rest of that question. diving which number of moles with what ratio? Hopefully I haven't confused you yet :) Chem is like a foreign language to me.

Answer 5

Yup!

Well, reactants are what we have before the reaction, and products are what the reaction produces. The arrow means that a reaction occurs.

So since you are not given any info on the amount of compounds, the question is asking for the BEST representation. We therefor assume we use the same amount of reactants. So if we need 5 parts of H2 for every 2 parts of NO, we know H2 will run out first. And since we know the products are made at the same rate, they will have the same amount in the end with respect to each other.

Answer 6

Oh!! I understand that now! Thank you!!!

Answer 7

Last question on this problem. So where did the 4 come from? It was just given at random as an example?

Answer 8

I would say yes for this question since you are learning about limiting reactant. But we can calculate as well. So we know that H2 will run out first and cause the reaction to stop. Therefore, the amount of products you obtain will depend on how much H2 you have. Makes sense? So here we know that when you used all H2, you get 4 mol of products and we know the ratio is 5:2 We can then calculate that we had 4/2*5=10 mol of H2 in the beginning. And since the NO to products ratio is 2:2, we know that a total of 4 mol of NO was used with 1 mol extra. We can therefore conclude that we had 5 mol of NO and 10 mol of H2 in the beginning of the reaction.

Answer 9

Thank you so much! Is there a way for me to save this question so that I may refer to it when I need to study for an exam? :)

Answer 10

You're very welcome. Well, when you close it you can always revisit it in the 'closed questions' section. Or you can go to your profile, click on 'asked questions' and then look for it.