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# Calculation on enthalpy change of neutralisation

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1. Hi, I need help with this question:
1. since HCl is in excess, I thought that not all 50g of acid will react, so I worked out the mass of acid which will react. Why is it wrong
2. why is the mass in the q equation not the total mass of carbonate and acid that will react, why is it just 50g?

Cheers
2. (Original post by coconut64)
Hi, I need help with this question:
1. since HCl is in excess, I thought that not all 50g of acid will react, so I worked out the mass of acid which will react. Why is it wrong
2. why is the mass in the q equation not the total mass of carbonate and acid that will react, why is it just 50g?

Cheers
I am not sure, since my chemistry knowledge is vanishing. But I will give it a try and someone else may confirm it.

the system consists of the reactants/products and the medium i.e water where reaction takes place. The equation to calculate heat deals with the substance that gets affected by the changes in energy around it, and this substance is water. therefore you use mass of water.

Hope this helps
3. (Original post by Daniel Atieh)
I am not sure, since my chemistry knowledge is vanishing. But I will give it a try and someone else may confirm it.

the system consists of the reactants/products and the medium i.e water where reaction takes place. The equation to calculate heat deals with the substance that gets affected by the changes in energy around it, and this substance is water. therefore you use mass of water.

Hope this helps
only the masses of acid and the carbonate are given. THanks
4. (Original post by coconut64)
only the masses of acid and the carbonate are given. THanks
in general terms, m refers here to the solution. the same concept applies. got it?
5. (Original post by Daniel Atieh)
in general terms, m refers here to the solution. the same concept applies. got it?
But i have checked the answer, only the mass of hcl is considered
6. (Original post by coconut64)
But i have checked the answer, only the mass of hcl is considered
if you are in an isolated room and burning fire. The energy exchange occurs in the room whether you use little or more wood. .

The HCl being the solution provides the bulk of space where energy exchange occurs. It does not care about what happens in the reaction, it only deals with the energy. You need to use the 50 g in your calculation. Clear?
7. (Original post by Daniel Atieh)
if you are in an isolated room and burning fire. The energy exchange occurs in the room whether you use little or more wood. .

The HCl being the solution provides the bulk of space where energy exchange occurs. It does not care about what happens in the reaction, it only deals with the energy. You need to use the 50 g in your calculation. Clear?
Hcl is in excess though, so not all HCl will react to the carbonate. Right?
8. (Original post by coconut64)
Hcl is in excess though, so not all HCl will react to the carbonate. Right?
it will be in excess. but that does not matter. the energy released must go into the solution available
9. (Original post by Daniel Atieh)
it will be in excess. but that does not matter. the energy released must go into the solution available
What about mass of the carbonate then, thats reacting with the acid
10. (Original post by coconut64)
What about mass of the carbonate then, thats reacting with the acid
we look at the mass of solvent that is in the liquid state
11. (Original post by coconut64)
Hi, I need help with this question:
1. since HCl is in excess, I thought that not all 50g of acid will react, so I worked out the mass of acid which will react. Why is it wrong
2. why is the mass in the q equation not the total mass of carbonate and acid that will react, why is it just 50g?

Cheers
The question tells you that HCl is in excess purely so you know the carbonate is fully reacted.

Once you know that, you don't need to worry about how much HCl is used up in the reaction.Remember that the acid (HCl) is in solution and so rather than saying some mass of acid is used up, in actual fact the 50g of solvent (water) will be left over with a lower concentration of acid in it.

Hopefully that's all clear!

After that, you have to realise that in the q equation you're talking about, the mass is the mass of the surroundings (ie what is being heated up by the reaction) in this case, that is 50grams of water!The neutralisation reaction will produce some additional water as well as a salt (NaCl) but the water produced is negligible compared to the 50g you began with so can be ignored. The NaCl will be in solution and again, any effect these ions may have on heat capacity can be ignored.

Hope that clears up any questions!
12. (Original post by MexicanKeith)
The question tells you that HCl is in excess purely so you know the carbonate is fully reacted.

Once you know that, you don't need to worry about how much HCl is used up in the reaction.Remember that the acid (HCl) is in solution and so rather than saying some mass of acid is used up, in actual fact the 50g of solvent (water) will be left over with a lower concentration of acid in it.

Hopefully that's all clear!

After that, you have to realise that in the q equation you're talking about, the mass is the mass of the surroundings (ie what is being heated up by the reaction) in this case, that is 50grams of water!The neutralisation reaction will produce some additional water as well as a salt (NaCl) but the water produced is negligible compared to the 50g you began with so can be ignored. The NaCl will be in solution and again, any effect these ions may have on heat capacity can be ignored.

Hope that clears up any questions!
Hi, carbonate is heated along with the acid, so why isnt that considered? Thanks
13. (Original post by coconut64)
Hi, carbonate is heated along with the acid, so why isnt that considered? Thanks
Carbonate is being reacted, so by the end of the process no carbonate is present!On top of that, water has a very high heat capacity, so the vast majority of the energy generated goes into heating the water, this means that other things being heated can be ignored (eg reactant/products or the air around the vessel containing the water).

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