Unit 2 Chemistry Energetics Question

Hi. If anyone could help with this question it would be great thanks:

A 50.0 cm3 sample of a 0.200 mol dm–3 solution of silver nitrate was placed in a polystyrene beaker. An excess of powdered zinc was added to this solution and the mixture stirred. Zinc nitrate, Zn(NO ) , and silver were formed and a rise in temperature of 3.20 °C was recorded.

The equation is:
2AgNO + Zn → Zn(NO ) + 2Ag

I have calculated the heat energy change for the reaction (699J)
However then it goes on to ask:

Calculate the heat energy change for the reaction per mole of zinc reacted.

The mark scheme then finds the enthalpy change by dividing by the number of moles of silver nitrate (I understand that) and THEN multiplies the answer by 2.

I thought that it would to divide the answer by 2 if calculating the number of moles of zinc because the molar ratio is 2: 1 (2AgNO: 1Zn)

Thanks

Reply 1

joostan

Original post by lindseyyy

First off, I make that

\Delta q= 669J

not 699.
I'm not sure I understand your actual query though. . . :smile:

EDIT: I see where you're misunderstanding - There are twice as many moles of AgNO, so when you divided the energy by the no. of moles of AgNO, you ended up with half the energy you wanted.

(edited 10 years ago)

Reply 2

lindseyyy

Original post by joostan

First off, I make that

\Delta q= 669J

not 699.
I'm not sure I understand your actual query though. . . :smile:

EDIT: I see where you're misunderstanding - There are twice as many moles of AgNO, so when you divided the energy by the no. of moles of AgNO, you ended up with half the energy you wanted.