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# Calorimetry Watch

1. I'm slightly confused on calorimetry questions that have different amounts of moles

for example:
As the moles of silver nitrate is 0.005 and moles of zinc in 0.003 I don't understand if I minus them and see the excess or if I take the smaller value as all of it will react? Thanks!
2. (Original post by Secret.)
I'm slightly confused on calorimetry questions that have different amounts of moles

As the moles of silver nitrate is 0.005 and moles of zinc in 0.003 I don't understand if I minus them and see the excess or if I take the smaller value as all of it will react? Thanks!
which is why the question asks you which reagent is in excess first?
the balanced eqn shows that 1 mole of Zn requires 2 mole of silver nitrate to react.
if you have 0.003 mol of Zn, you would require 2 x 0.003 = 0.006 mole of silver nitrate to react.

but in reality you have only 0.005 mole of silver nitrate => this implies silver nitrate is limiting, hence the other reactant (zinc) must have been in excess.

so you always use the value for the limiting reactant as that is the amount that totally reacts (assume 100% percentage yield, etc)

so mole of Zn reacted = 1/2 mole of AgNO3 reacted = ?

you can calculate the heat change for the reaction and the question asks you to calculate the enthalpy change per mole of Zn that reacts.
so that is what you should do next.

clear?
3. (Original post by shengoc)
which is why the question asks you which reagent is in excess first?
the balanced eqn shows that 1 mole of Zn requires 2 mole of silver nitrate to react.
if you have 0.003 mol of Zn, you would require 2 x 0.003 = 0.006 mole of silver nitrate to react.

but in reality you have only 0.005 mole of silver nitrate => this implies silver nitrate is limiting, hence the other reactant (zinc) must have been in excess.

so you always use the value for the limiting reactant as that is the amount that totally reacts (assume 100% percentage yield, etc)

so mole of Zn reacted = 1/2 mole of AgNO3 reacted = ?

you can calculate the heat change for the reaction and the question asks you to calculate the enthalpy change per mole of Zn that reacts.
so that is what you should do next.

clear?
Aah right yeh I understand now just was confused whether to minus them or use the one that isn't in excess, thanks!

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Updated: April 6, 2013
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