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AS Enthalpy of formatipn/combustion

Hey guys

Recently learned this in class however im finding it really difficult when working out the enthalpy changes from formation or combustion. Could someone guide me through this question so that I could get the general way to work them out please ?

Calculate the standard enthalpy change for the following reaction:

MgO(s) + 2HCl(g) --> MgCl2(s) + H20(l)

Delta Hf0/kjmol-1:
MgO = -602
HCl = -92
MgCl2 = -642
H2O= -286
Original post by Chris654
Hey guys

Recently learned this in class however im finding it really difficult when working out the enthalpy changes from formation or combustion. Could someone guide me through this question so that I could get the general way to work them out please ?

Calculate the standard enthalpy change for the following reaction:

MgO(s) + 2HCl(g) --> MgCl2(s) + H20(l)

Delta Hf0/kjmol-1:
MgO = -602
HCl = -92
MgCl2 = -642
H2O= -286


Read through this interactive on Hess law
Reply 2
Avatar for RRC
RRC
1
use can use the hess law or another way is to use Fpr (product-reactant)when the data foe formation is given or Crp (reactant - product) when combustion data is provided u would have to multiply the values by the no. of moles because the values given are only for one mole
for the above reaction you would multiply the enthalpy for HCL by 2
hope this helps
Reply 3
Avatar for ak96_x
ak96_x
1
Hi, I'm having a similar problem to you. I'm finding that topic difficult, too. Let me calculate it and once I think I have the answer I'll post another reply.

Isit true that:

Enthalpy of combustion is always negative and enthalpy of formation is always positive? Or is enthalpy of formation both + and - ?! Please help, thank you :smile:


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Reply 4
Avatar for Chris654
Chris654
OP
5
Hey there

I finally got my head around it few weeks back hahha :smile:

Combustion is an exothermic reaction, therefore it has a negative enthalpy value, which means energy is released
The enthalpy of formation can be either positive or negative as it can in certain reactions release heat to form bonds (exothermic), whilst in other reactions heat must be supplied to form bonds (endothermic)
Correct me if im wrong in saying this please haha!

hope that helped :smile:
Original post by Chris654
Hey there

I finally got my head around it few weeks back hahha :smile:

Combustion is an exothermic reaction, therefore it has a negative enthalpy value, which means energy is released
The enthalpy of formation can be either positive or negative as it can in certain reactions release heat to form bonds (exothermic), whilst in other reactions heat must be supplied to form bonds (endothermic)
Correct me if im wrong in saying this please haha!

hope that helped :smile:


The highlighted phrase is wrong.

Bond formation is always exothermic.

Bond cleavage is always endothermic.

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