Hard Enthalpy Change Question - Help Needed

lhh2003

Hi guys , can anyone help me with this question :

An excess of magnesium is added to 100cm^3 of 2.00 mol dm^-3 CuSO4(aq) . The temperature increase is 20.0 °C to 65.0 °C .

Given that the specific heat capacity of water is 4.18 j g-1 K-1 and the density of water is 1 g cm-3 . Calculate the standard Enthalpy change of reaction in kJ mol-1 for the reaction above.

I know I have to use q = mc Δt .

I have calculated Δt = 45.0 °C

I have calculated there are 0.2 moles of Cu2SO4.

I don't know what to do from here though.

Thanks guys.

Reply 1

hj!gz

12

assuming water density,

mass of cuso4 = 100g

Q = 100 X 4.18 X 45

temperature increases, so this is an exothermic reaction

deltaH = - Q / moles of CuSo4 (0.2)

Reply 2

lhh2003

OP

18

Original post by hj!gz

assuming water density,

mass of cuso4 = 100g

Q = 100 X 4.18 X 45

temperature increases, so this is an exothermic reaction

deltaH = - Q / moles of CuSo4 (0.2)

Why did you use the mass of CUSO4 ? Why no the mass of Mg ? Or why didn't you add the sum of these , seeing as they're both reacting ?

Reply 3

hj!gz

12

you measure the energy transferred:

can you get energy transferred to magnesium solid?
no, only energy transferred to the solution, which is copper sulfate.

if there were 2 solutions, then you would add their volumes together, but with these calculations, always ignore solid mass!!

Reply 4

lhh2003

OP

18

Original post by hj!gz

you measure the energy transferred:

can you get energy transferred to magnesium solid?
no, only energy transferred to the solution, which is copper sulfate.

if there were 2 solutions, then you would add their volumes together, but with these calculations, always ignore solid mass!!

Thank you so much. You are a beast man

Reply 5

lhh2003

OP

18

Original post by hj!gz

you measure the energy transferred:

can you get energy transferred to magnesium solid?
no, only energy transferred to the solution, which is copper sulfate.

if there were 2 solutions, then you would add their volumes together, but with these calculations, always ignore solid mass!!

Wait, looking back on what you said though, if the density of water is 1g cm^-3 , then surely the 100g mass is of the water. Therefore, you have the mass of the water of 100g and the mass of CuSO4 of 100g (which I calculated previously) ? Surely if the whole solution is important, then you would want the combined mass of water and ions ? Or is the mass of the ions only of interest ? If so, why ?

(edited 4 years ago)

Reply 6

Tolgash

21

Original post by lhh2003

Wait, looking back on what you said though, if the density of water is 1g cm^-3 , then surely the 100g mass is of the water. Therefore, you have the mass of the water of 100g and the mass of CuSO4 of 100g (which I calculated previously) ? Surely if the whole solution is important, then you would want the combined mass of water and ions ? Or is the mass of the ions only of interest ? If so, why ?

The mass of the water solution is 100g. The calculation you made was for aqueous copper sulphate, which means that it's already in the solution. You didn't compute the mass of the copper-sulphate ions in isolation.

(edited 4 years ago)

Reply 7

lhh2003

OP

18

Original post by Tolgarda

The mass of the water solution is 100g. The calculation you made was for aqueous copper sulphate, which means that it's already in the solution. You didn't compute the mass of the copper-sulphate ions in isolation.

This doesn’t make sense to me. How can the 100g account for both of the masses of water and ions if the formula for density is 1g / 1 cm ^3 ? This formula is for water so why would it include the mass of the copper sulphate too ?

Reply 8

Tolgash

21

Original post by lhh2003

This doesn’t make sense to me. How can the 100g account for both of the masses of water and ions if the formula for density is 1g / 1 cm ^3 ? This formula is for water so why would it include the mass of the copper sulphate too ?

If the volume of the aqueous copper sulphate (i.e. the solution) is 100 centimetres cubed, and the density of water (the solvent) is 1 gram per centimetre cubed, we can safely say that the mass of the aqueous copper sulphate must also be 100 grams. The ‘water’ here really means the solution of copper-sulphate ions in water.