Chemistry Calorimetry

I need help with this question:
25cm^3 of 1.00moldm^-3 copper sulphate solution was put in a calorimeter and 6.0g of zinc powder added. The temperature of the solution rose by 50.6 degrees. Work out which reagent was in excess and then calculate the enthalpy change for the reaction. Assume that the density of the solution is 1.00 gcm^-3, the specific heat capacity of the solution is 4.18Jg^-1K^-1.

I know how to work out Q but when I’m working out the enthalpy would I use the moles of the copper sulphate solution or the zinc powder. Also to work out which one is in excess do I just have to find out which substance (copper sulphate or zinc) has the bigger moles.

Reply 1

USMAAN gh

Hi!
So you need to find out which reagent is in excess by calculating the number of Moles of each one using the data given, the reagent which is NOT in excess is the reagent you use to calculate the enthalpy

Hope this helps

Reply 2

Ðeggs

Calculate the moles of zinc and the moles of copper sulphate, which reagent is NOT in excess you use the moles of.
Q = mass of substance x 4.18 x temperature change
For the mass I assume you use the mass of water so this is just 25g as the density is 1g/ cm^3
Then q / mol is the enthalpy change in J mol -1, so divide by a thousand as the question usually wants delta H in kJ mol-1

Reply 3

Kian Stevens

Zn + CuSO₄

\rightarrow

ZnSO₄ + Cu
The reaction is of a 1:1 stoichiometry, so the species with the greatest number of moles is the one in excess

However, the one with the least number of moles is the limiting reagent, and this is what you use to calculate the enthalpy change; this is because since the stoichiometry is 1:1, the maximum number of moles of both species that will react are the moles of limiting reagent

This is where the terminology of ‘excess reagent’ and ‘limiting reagent’ stems from...