Chemistry AS Question

The standard enthalpy change of combustion of heptane, C7H16, is -4817kJmol^-1.

Calculate the energy transferred when the following quantities of heptane are burnt.
i) 10g
ii) 10kg

Any help would be appreciated.

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convert mass into moles using the M_r of heptane :smile:

:smile:

enrgy transferred/number of moles = enthalpy change

moles = mass/Mr

energy transferred = enthalpy change * number of moles

does it make sense now?

OP

hodgey90

energy transferred/number of moles = enthalpy change
energy transferred = enthalpy change * number of moles

My teacher never told me that bit. :s-smilie:

:s-smilie:

:confused:

OP

EierVonSatan

convert mass into moles using the M_r of heptane :smile:

:smile:

Should i work in g or kg?

aboy

Should i work in g or kg?

grams (because M_r is in grams per mole), once you've found it for 10g all you have to do is multiply it by 1000 to get the second answer anyway :smile:

:smile:

Mass to mole.

aboy

Should i work in g or kg?

grams

aboy

My teacher never told me that bit. :s-smilie:

:s-smilie:

:confused:

never told you that? what the hell has he/she been teaching you!

OP

hodgey90

never told you that? what the hell has he/she been teaching you!

She told me the mole = mole/Mr
But not the other bit.

aboy

She told me the mole = mole/Mr
But not the other bit.

mole = mole/Mr??????

mole = mass/Mr

aboy

She told me the mole = mole/Mr
But not the other bit.

so she presumably gave you this homework, and didnt tell you that enthalpy change = energy transferred/number of moles?

OP

DrVas

mole = mole/Mr??????

mole = mass/Mr

Sorry, typo.

OP

hodgey90

so she presumably gave you this homework, and didnt tell you that enthalpy change = energy transferred/number of moles?

Yup

OP

Am i being really silly here. But is the equation for hydrogen burned in oxygen to form water.

h2 + 02 -> h2o

2h2 + O2 -> 2h2o

OP

10cm3 of hydrogen are burned in oxygen to form water.
What volume of oxygen is needed to burn the hydrogen completely?

aboy

10cm3 of hydrogen are burned in oxygen to form water.
What volume of oxygen is needed to burn the hydrogen completely?

You need two moles of hydrogen molecules for every oxygen molecule so...

OP

EierVonSatan

You need two moles of hydrogen molecules for every oxygen molecule so...

5cm^3. Tbh i havent a clue. Cant remember ever doing this. :frown:

:frown:

aboy

5cm^3. Tbh i havent a clue. Cant remember ever doing this. :frown:

:frown:

and yet you got it right :smile:

:smile:

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