# Enthalpy Question - URGENT

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#1
Hi,
I was attempting this test paper. I got this question wrong but I don't understand the maths of the mark scheme could someone help explain it to me?
3. The table below contains some mean bond enthalpy data.

Bond H––H C––C C=C N≡N N––H
Mean bond enthalpy / kJ mol–1 436 348 612 944 388

(a) (i) Write an equation for the formation of one mole of ammonia, NH3, from its elements.
.............................................................................................................
(ii) Use data from the table above to calculate a value for the enthalpy of formation of ammonia.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b) Use the following equation and data from the table above to calculate a value for the C–H bond enthalpy in ethane.

......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)

Markscheme:
3. The table below contains some mean bond enthalpy data.

Bond H––H C––C C=C N≡N N––H
Mean bond enthalpy / kJ mol–1 436 348 612 944 388

(a) (i) Write an equation for the formation of one mole of ammonia, NH3, from its elements.
.............................................................................................................
(ii) Use data from the table above to calculate a value for the enthalpy of formation of ammonia.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b) Use the following equation and data from the table above to calculate a value for the C–H bond enthalpy in ethane.

......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)

Also, if the data given to you is the mean bond enthalpy but it asks you to work out the enthalpy of formation do you use products- reactants or bonds broken - bonds formed
0
4 months ago
#2
(Original post by Sclaremonths)
Hi,
I was attempting this test paper. I got this question wrong but I don't understand the maths of the mark scheme could someone help explain it to me?
3. The table below contains some mean bond enthalpy data.

Bond H––H C––C C=C N≡N N––H
Mean bond enthalpy / kJ mol–1 436 348 612 944 388

(a) (i) Write an equation for the formation of one mole of ammonia, NH3, from its elements.
.............................................................................................................
(ii) Use data from the table above to calculate a value for the enthalpy of formation of ammonia.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b) Use the following equation and data from the table above to calculate a value for the C–H bond enthalpy in ethane.

......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)

Markscheme:
3. The table below contains some mean bond enthalpy data.

Bond H––H C––C C=C N≡N N––H
Mean bond enthalpy / kJ mol–1 436 348 612 944 388

(a) (i) Write an equation for the formation of one mole of ammonia, NH3, from its elements.
.............................................................................................................
(ii) Use data from the table above to calculate a value for the enthalpy of formation of ammonia.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b) Use the following equation and data from the table above to calculate a value for the C–H bond enthalpy in ethane.

......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)

Also, if the data given to you is the mean bond enthalpy but it asks you to work out the enthalpy of formation do you use products- reactants or bonds broken - bonds formed
Check out this video on Hess' law:

0
4 months ago
#3
(Original post by Sclaremonths)
Hi,
I was attempting this test paper. I got this question wrong but I don't understand the maths of the mark scheme could someone help explain it to me?
3. The table below contains some mean bond enthalpy data.

Bond H––H C––C C=C N≡N N––H
Mean bond enthalpy / kJ mol–1 436 348 612 944 388

(a) (i) Write an equation for the formation of one mole of ammonia, NH3, from its elements.
.............................................................................................................
(ii) Use data from the table above to calculate a value for the enthalpy of formation of ammonia.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b) Use the following equation and data from the table above to calculate a value for the C–H bond enthalpy in ethane.

......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)

Markscheme:
3. The table below contains some mean bond enthalpy data.

Bond H––H C––C C=C N≡N N––H
Mean bond enthalpy / kJ mol–1 436 348 612 944 388

(a) (i) Write an equation for the formation of one mole of ammonia, NH3, from its elements.
.............................................................................................................
(ii) Use data from the table above to calculate a value for the enthalpy of formation of ammonia.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b) Use the following equation and data from the table above to calculate a value for the C–H bond enthalpy in ethane.

......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)

Also, if the data given to you is the mean bond enthalpy but it asks you to work out the enthalpy of formation do you use products- reactants or bonds broken - bonds formed
Once you have the balanced equation: 2H2(g) + N2(g) --> 2NH3(g) you use the bonds broken - bonds made. The final answer needs to be for one mole of ammonia though so you will need to half the result you get as the equation forms 2 moles.
1
4 months ago
#4
for enthalpy change you do products - reactants
and for energy change you do bonds broken- bonds made
1
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