# chemistry: calorimetry/enthalpy question, help please :)

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#1
A vessel and its contents of total heat capacity 120 J K–1 were heated using a methane burner. Calculate the maximum theoretical temperature rise when 0.10 g of methane was completely burned. The standard enthalpy of combustion of methane is –890 kJ mol–1

this seems like a simple calorimetry q and I assumed you would just find the moles of methane (6.25 x10^3) and then times it by 890 to give you 5.56kJ which is 5.56 x 10^3 J. This value is Q.

then I would sub it into the equation to give: delta T = 5.56 x 10^3 / 120 x 0.1g

However, the mark scheme has ignored the m value and has just done:
delta T = 5.56 x 10^3 / 120

can someone explain why you do delta T = Q/c instead of delta T = Q/mc please?????
0
3 years ago
#2
look at the units of specific heat capacity, thats a big clue
0
3 years ago
#3
(Original post by 1dk)
A vessel and its contents of total heat capacity 120 J K–1 were heated using a methane burner. Calculate the maximum theoretical temperature rise when 0.10 g of methane was completely burned. The standard enthalpy of combustion of methane is –890 kJ mol–1

this seems like a simple calorimetry q and I assumed you would just find the moles of methane (6.25 x10^3) and then times it by 890 to give you 5.56kJ which is 5.56 x 10^3 J. This value is Q.

then I would sub it into the equation to give: delta T = 5.56 x 10^3 / 120 x 0.1g

However, the mark scheme has ignored the m value and has just done:
delta T = 5.56 x 10^3 / 120

can someone explain why you do delta T = Q/c instead of delta T = Q/mc please?????
You're not supposed to use the mass of the substance you are using as fuel. You always use the mass of the substance being heated up e.g. water.
q=mcDT
Enthalpy change= mass of substance being heated up*It's specific heat capacity*the temp change
Since the mass is nowhere in the question (or any measure of what's in the container being heated), T=Q/C is just being used instead of T= Q/mc. I assume because the mass is constant, they are letting you ignore it.
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