Question

Question about calculating enthalpy?

Answer 1

Foam cup A has 36 mL of water that contains 2 moles of HCl at 20 degrees Celsius. Foam cup B has 36 mL of water that contains 2 moles of NaOH at 20 degrees Celsius. The contents of foam cup A are added to foam cup B and a chemical reaction occurs. The final temperature of the contents of cup B is 31 degrees Celsius.
I need to calculate the enthalpy change of the reaction. The density of both solutions is 1 g/mL. I really don't understand how to calculate enthalpy, and my textbook doesn't really say how to. I don't need the exact answer, but if someone could teach me the steps with this problem, that would be great.

Answer 2

are you familiar with the equation \[Q = m*C*\Delta T\]?

Answer 3

Not really

Answer 4

I've looked it up and it is mass x specific heat capacity x temperature change, right?

Answer 5

yep yep that's right

Answer 6

But at @JFraser shouldn't the equation be Q= ms delta T ?
As to what I know C is the heat capacity and S is specific heat capacity.
C=mS
So there's no way that C and S are equal right ?

Answer 7

So this is the formula to calculate joules, but do I combine the amount of mass of both solutions, or do I stick with 36 mL?

Answer 8

Not exactly but wait till he replies ?

Answer 9

you use the combined mass, because the reaction only happens when you combine the two solutions

Answer 10

And the mass is 72 g, right?

Answer 11

first u have to find the masses of both reactants HCl and NaoH
We know
density = mass divided by volume
therefore
mass = density * volume right ?
So now quickly find the masses ( we know the density and volumes of the reactants)

Answer 12

right

Answer 13

yeah so 72 is right !

Answer 14

Is there a specific formula to find specific heat capacity?

Answer 15

Now use the firstly mentioned equation to calculate the heat

Answer 16

So we would write out C = 72g x 11?

Answer 17

nop nop ! Wait haven't they given a specific heat capacity ?

Answer 18

I didn't see it in the question, but I might be wrong

Answer 19

just see if they have or not

Answer 20

They didn't give it in the question but I can look it up

Answer 21

no no then we can just assume that thye have the same specific heat as of water ! :)

Answer 22

which is 4.184Jg^-1C^-1

Answer 23

Oh ok

Answer 24

well delta T is the temperature difference since both starts from the same temperature and they both attain a common temperature at the end we can sue it that way

Answer 25

so 72g x 4.184Jg^-1C^-1 x 11 degrees C
Is that right?

Answer 26

yep that's the heat

Answer 27

heat is in Joules, enthalpy is in Joules per moles

Answer 28

So we need to divide the heat by the no. of moles in the system !

Answer 29

Did u understand now ? :) @bloonscat

Answer 30

I think so, all I need to do is (72g x 4.184Jg^-1C^-1 x 11 degrees C)/4
right?

Answer 31

yep that's right

Answer 32

Thank you so much! This was very helpful, now I understand!
Thank you!

Answer 33

No problem :)

Answer 34

I hope you understood that