Chemistry Insight 2011 MC Q12

[dimmy18]

The question is:
Magnesium reacts with oxygen according to the thermochemical equation
2Mg + O₂ -->  2MgO     H=-1200 kJ mol

The mass , in g, of magnesium that would need to be burned to release 4000kJ of energy is
A. 7.29
B. 14.6
C. 81.0
D. 162

The answer said D
However I think this is wrong. I think it should be C.

Energy required is simply 4000/1200, but what they've done is doubled it since there is a 2 in front of Mg.
However it is 2 mol for every 1200, so doubling would be finding 4 mol, hence the answer should be C.

Am I right?

(This is a pretty bad paper)

Edit: I posted this at 1 am so ignore everything I said above.

[kenny_]

You have to put the 2 there cause the delta H is per mol

[dimmy18]

But it's technically per reaction...

If it were per mol of Mg it would be -600

[kenny_]

Look at page 11 of your data book and write out balanced equations for each of the combustion reactions. The delta H is per mol regardless of the number in front of the fuels

[Zebra]

Look at page 11 of your data book and write out balanced equations for each of the combustion reactions. The delta H is per mol regardless of the number in front of the fuels

wtf? energy given in data book can be taken as per mole.... no?

[dimmy18]

The data booklet give delta H for one mol but when you write out the equation you need to adjust the delta H to match the number before the substance...

[kenny_]

Look at page 11 of your data book and write out balanced equations for each of the combustion reactions. The delta H is per mol regardless of the number in front of the fuels

wtf? energy given in data book can be taken as per mole.... no?

yes...?

[Zebra]

sorry its late any my brains cookin'

[dimmy18]

This is a quote from wikipedia (reliable)

The enthalpy of combustion for hydrogen is −286 kJ/mol:[12]
2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)[

check yourself if you want http://en.wikipedia.org/wiki/Hydrogen

[kenny_]

The question is:
Magnesium reacts with oxygen according to the thermochemical equation
2Mg + O₂ -->  2MgO     H=-1200 kJ mol

The mass , in g, of magnesium that would need to be burned to release 4000kJ of energy is
A. 7.29
B. 14.6
C. 81.0
D. 162

The answer said D
However I think this is wrong. I think it should be C.

Energy required is simply 4000/1200, but what they've done is doubled it since there is a 2 in front of Mg.
However it is 2 mol for every 1200, so doubling would be finding 4 mol, hence the answer should be C.

Am I right?

(This is a pretty bad paper)

Wait a second, to find the energy released you do a ratio.

x/2 = 4000/1200
x = 2 x 4000/1200 mol

[dimmy18]

The question is:
Magnesium reacts with oxygen according to the thermochemical equation
2Mg + O₂ -->  2MgO     H=-1200 kJ mol

The mass , in g, of magnesium that would need to be burned to release 4000kJ of energy is
A. 7.29
B. 14.6
C. 81.0
D. 162

The answer said D
However I think this is wrong. I think it should be C.

Energy required is simply 4000/1200, but what they've done is doubled it since there is a 2 in front of Mg.
However it is 2 mol for every 1200, so doubling would be finding 4 mol, hence the answer should be C.

Am I right?

(This is a pretty bad paper)

Wait a second, to find the energy released you do a ratio.

x/2 = 4000/1200
x = 2 x 4000/1200 mol

Ok.. let me explain it.

2 mol of Mg will give me 1200 KJ
then x mol will give me 4000KJ

Therefore I need...

Oh wait... I see now! (my bad)

[kenny_]

The question is:
Magnesium reacts with oxygen according to the thermochemical equation
2Mg + O₂ -->  2MgO     H=-1200 kJ mol

The mass , in g, of magnesium that would need to be burned to release 4000kJ of energy is
A. 7.29
B. 14.6
C. 81.0
D. 162

The answer said D
However I think this is wrong. I think it should be C.

Energy required is simply 4000/1200, but what they've done is doubled it since there is a 2 in front of Mg.
However it is 2 mol for every 1200, so doubling would be finding 4 mol, hence the answer should be C.

Am I right?

(This is a pretty bad paper)

Wait a second, to find the energy released you do a ratio.

x/2 = 4000/1200
x = 2 x 4000/1200 mol

Ok.. let me explain it.

2 mol of Mg will give me 1200 KJ
then x mol will give me 4000KJ

Therefore I need...

Oh wait... I see now! (my bad)

Yeah, the way you did it first, you took it as per mol, then it confused me

[Panicmode]

2   gives 1200

x   gives  4000

Cross multiply

2 x 4000 = 1200x

x = 8000/1200


Figure out mass from there...


Ah crap. Beaten to the punch.