Using specific heat capacity

Atomosinchemistry SG

Deleted for now. Will update with correct info later.

UltimaOnline SG

Here's an example of an exam qn from this topic :

Atomosinchemistry SG

Hi there, thanks for the info.

My clarifications is with regards to this JC paper question.

When 11.7 g of sodium chloride (Mr 58.5) dissolves in 100 g of water, the temperatur of the solution falls by 2.4 oC.

Given the specific heat capacity of the solution to be 4.2 J g-1 K-1, what is the enthalpy chage of solution of sodium chloride?

My understanding would be the mass for mcT is (100 + 11.7) g, but the given answer uses only the mass of water, 100 g.

UltimaOnline SG

Hi there, thanks for the info.

My clarifications is with regards to this JC paper question.

My understanding would be the mass for mcT is (100 + 11.7) g, but the given answer uses only the mass of water, 100 g.

The JC prelim paper's given answer is correct. While the overall mass of the solution would indeed be (100 + 11.7) g, however the mass of the water which absorbs the heat (and which therefore must be used in our calculations) remains as 100g.

Atomosinchemistry SG

I see.

Cause I took the term of "solution" literally and viewed it as "it takes 4.2 J to raise 1g of this mixture by 1 k.

But I also get your approach. Thanks!

UltimaOnline SG

I see.

Cause I took the term of "solution" literally and viewed it as "it takes 4.2 J to raise 1g of this mixture by 1 k.

But I also get your approach. Thanks!

Students are expected (though not many JCs teach this well) to be able to discern that out of the (100 + 11.7) g of the solution that absorbed or released the heat, it is to be regarded that only the 100g of water that registers this thermal transfer process (ie. that allows us to determine via calculations the magnitude of heat transfer), and not the 11.7g of the salt (which for all relevant A level purposes and intents, is identical to an external heat source, say combustion of a fuel).