[jamesbrown]

If the specific heat capacity of water is 4.2 J/g/°C and the mass of 1cm³ of water is 1g

Then In an experiment, 0.40g of butane (C₄H₁₀) was completely burned in air. The heat was given off raised the temperature of 100g of water by 54.2°C.

So calculate the heat released by burning 0.40g of fuel in KJ.

Would the answer be 0.40*54.2*4.2 = 91.056 KJ
or did I do something wrong? because it said put it in KJ but I didn't account for that I don't think?

[sjb]

If the specific heat capacity of water is 4.2 J/g/°C and the mass of 1cm³ of water is 1g

Then In an experiment, 0.40g of butane (C₄H₁₀) was completely burned in air. The heat was given off raised the temperature of 100g of water by 54.2°C.

So calculate the heat released by burning 0.40g of fuel in KJ.

Would the answer be 0.40*54.2*4.2 = 91.056 KJ
or did I do something wrong? because it said put it in KJ but I didn't account for that I don't think?

If you had less water to heat, would you expect the temperature rise to be more, less, or the same?

[jamesbrown]

Less?

[sjb]

Not less, no. Consider the energy as fixed, like e.g. having a pound of rice to share amongst first 8, then 4 people. Each person gets more in the second case.

You haven't taken into account the mass of water in your calculation.

[jamesbrown]

What is the equation that I am suppose to use then I will be able to work it out!

[Burner]

If the specific heat capacity of water is 4.2 J/g/°C and the mass of 1cm³ of water is 1g

Then In an experiment, 0.40g of butane (C₄H₁₀) was completely burned in air. The heat was given off raised the temperature of 100g of water by 54.2°C.

So calculate the heat released by burning 0.40g of fuel in KJ.

Would the answer be 0.40*54.2*4.2 = 91.056 KJ
or did I do something wrong? because it said put it in KJ but I didn't account for that I don't think?

Hint: Why do you need to measure the temperature of water?

[jamesbrown]

You need to measure the temperature to get the temperature rise when the reaction has completed then you can put it in the equation q = mcΔt
so m is the mass of a substance in which the heat is being transferred.
so c is the specific heat capacity (which is 4.2 in this case)
so Δt is the temperature rise
and q is the heat energy given out/released

so q = 100*4.2*54.2
so q = 22764 KJ

Is this right? I think I was getting confused with the 0.4g of fuel as the next part is calculating moles!

[Burner]

You need to measure the temperature to get the temperature rise when the reaction has completed then you can put it in the equation q = mcΔt
so m is the mass of a substance in which the heat is being transferred.
so c is the specific heat capacity (which is 4.2 in this case)
so Δt is the temperature rise
and q is the heat energy given out/released

so q = 100*4.2*54.2
so q = 22764 KJ

Is this right? I think I was getting confused with the 0.4g of fuel as the next part is calculating moles!

You get the mass right now. But there's another problem. The specific heat capacity(c) that you are using is 4.2 J/g/°C, note the unit. So, why are you using kJ as your unit in the calculation? what shall you do so as to use the unit kJ as required in the question?

Once you handled this, you can go to the next part.

Edit: I overlooked the original question that told us to use kJ as the energy unit.

[jamesbrown]

So are you saying I need to change the specific heat capacity to KJ rather than J and then work it out again so:
4.2J =  0.0042KJ

So then it would be q = 100*0.0042*54.2
so q = 22.764 KJ

is that right or was my answer before right?

[Burner]

So are you saying I need to change the specific heat capacity to KJ rather than J and then work it out again so:
4.2J =  0.0042KJ

So then it would be q = 100*0.0042*54.2
so q = 22.764 KJ

is that right or was my answer before right?

right.

[jamesbrown]

So question 1 was:
Calculate the heat released by burning 0.40g of fuel in KJ

And we found that the answer was 22.764KJ

The question 2 is:
Calculate the heat released in KJ/g of fuel burned

Could someone explain what to do for this one?

Thanks!

[Burner]

The question 2 is:
Calculate the heat released in KJ/g of fuel burned

When you burn 0.4g of the fuel you get 22.764KJ

So what amount of energy will you get when you burn 1g of the fuel?

[jamesbrown]

so would you get 56.91KJ of energy produced?

[Burner]

That's what I got

[jamesbrown]

then for question 3 it says calculate the moles of fuel burned:
which is 0.4/58
moles = 0.0069
I think

then question 4 says calculate the heat released in KJ/mol of fuel burned
so for that would it be 22.76/0.0069 or is that wrong?