If 5.100 g C6H6 is burned and the heat produced from the burning is added to 5691 g of water at 21 ∘ C, what is the final temperature of the water?

Question

The balanced combustion reaction for C6H6 is 2C6H6(l)+15O2(g)⟶12CO2(g)+6H2O(l)+6542 kJ

Answer 1

Hi Kinsui,

This question tests you on two concepts: molar conversion + ratios, and specific heat.

To solve this question, you need to first calculate the amount of energy released by 5.100g C6H6, then find out how much change in temperature, ΔT, is caused by adding this amount of energy to water.

So first think about how to calculate this energy, we need to do the following things:

1. Convert grams into moles of C6H6
2. With the amount of moles of C6H6, we need to see how much heat is released
3. With the heat released, we need to see how much the temperature of the water changes
4. Find the final temperature of the water

1.

To convert 5.100 grams into moles of C6H6 (MM = 78.11) we do the following:

5.100 g C6H6 * (1 mol C6H6/ 78.11g C6H6) = .06529 mol C6H6

2.

We see that in the reaction, 2moles of C6H6 are needed to make 6542 kJ of energy. To calculate how much energy .06529mol C6H6 makes, we do the following calculations:

.06529 mol C6H6 * (6542kJ/ 2mol C6H6) = 213.6 kJ

3.

To calculate how much change in temperature of water is caused by this energy, we use the formula:

q = m*c*ΔT

where q is the heat required, m is mass, and c is specific heat

We can rearrange this equation to solve for the change in temperature

ΔT = q/ (m*c), and plugging in our numbers we get the following:

ΔT = 213.6kJ/ (5.691kg * 4.180kJ/(kg* °C)) = 8.979 °C

(note that when I plugged in these numbers, I unit converted 5691g -> 5.691kg of water and 4180J/(kg*°C) to 4.180 kJ/(kg* °C).

4.

Now the last step (don't forget to do this!), we add 8.979 to 21 to get 30°C (or 29.98 if we are rounding to 4 significant figures)

Now you notice that I always kept my units during these calculations, this is because omitting these units can very often lead to errors in plugging in numbers or conversion. I also did my best to respect the significant figures in this problem. Depending on how antsy your chemistry teacher is, you can lose a significant amount of points not representing the right number of significant figures in your final answer!

Hope this helps,

Daniel