Homework Help: Chemistry

Posted by a Canadian on Wednesday, February 20, 2013 at 7:38pm.

You know how when you do calculations for the enhaloy change of a reaction you start sort of like this?:

NH3(aq) + HCl(aq) → NH4Cl(aq)
25 mL 25 mL
1.0 mol/L 1.0 mol/L

n = c x v
= 1 mol/L x 0.025L
= 0.025 mol

How come you only need the moles of one of the reactants? I just always did it but didn't really know why... I just know I'll remember better for a test or something if I understand, so I'd like to know. And for all our sample problems the concentrations and volumes of both reactants were conveniently the same. So we never had to choose which reactant to get the moles for. But what if the concentrations and volumes were different? Would that ever happen? Or maybe that's too advanced or something for right now? I'm just scared something like that will pop up on the quiz tomorrow and I'm just going to have to guess which reactant to use to find moles...

Chemistry - bobpursley, Wednesday, February 20, 2013 at 7:40pm
The coefficents on a balanced equation are in mole ratios. If you know one, you know all.
Chemistry - a Canadian, Wednesday, February 20, 2013 at 7:46pm
So if you find the enthalpy change for one reactant, it's the enthalpy change for the other reactant (and products??), i.e. you know the enthalpy change for the entire reaction?

So enthalpy change of one reactant (or product also?) = enthalpy change of entire reaction?
Chemistry - bobpursley, Wednesday, February 20, 2013 at 7:49pm
No. Enthlapy change is from the total reaction. Remember Enthalpy is in kJ/molereactant
Chemistry - a Canadian, Wednesday, February 20, 2013 at 8:01pm
That's what I don't really understand. If it's of the total reaction, shouldn't you add, in my example, the moles of both reactants? i.e. 0.025 +0.025 = 0.05 mol. Then use that value when doing deltaH = -Q/n?

How come you only use The moles of one?

Answer this Question

Related Questions

Chemistry 152 - For a certain reaction, the enthalpy change is -148.5 kJ, the ...
chemistry ASAP please!! - The reaction of CO(g) + NO(g) is second-order in NO2 ...
Chemistry - HI I'd like to ask if anyone would give me a website that ...
chemistry - Draw an energy profile describing an exothermic reaction and label ...
Chemistry - A gaseous reaction occurs at a constant pressure of 50.0 atm and ...
english ms. sue - ms. sue i change up my story about switch on and off from ...
Chemistry - 35. In a calorimeter, the energy content of a substance is ...
chemistry - A reaction is 50% complete in 34.0 min. 1.) How long after its start...
Chemistry - A chemical system within a sealed 1 L reaction vessel is described ...
science(chem) - I was just wondering if it was necessary to remember each branch...

For Further Reading

First Name:
School Subject:
Answer: