Question on heat of reaction...Can someone show me how to do this?

Question

A 35.6 g sample of ethanol (C2H5OH) is burned in a bomb calorimeter according to the following reaction. If the temperature rose from 35.0 to 76.0°C and the heat capacity of the calorimeter is 23.3 kJ/°C, what is the value of DH°rxn? The molar mass of ethanol is 46.07 g/mol.

 

C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)  ΔH°rxn = ?

 

Can someone please show the steps for this? I believe I used Q=mcat formula but I do not have specific heat, so how do I use heat capacity?

 

Thank you!!

Answer 1

Hi, Sara. Bomb calorimetry calculations are a little different than solution calorimetry calculations. Instead of using Q=m*C*deltaT, you will use Q=C*deltaT where C is the heat capacity of the calorimeter. Note that your heat capacity of 23.3 has units of kj/degree C. This means that all we need to do is multiply it by the temperature change to get a number with units of kj. Hence, Q for the calorimeter is (23.3 kj/degree C)*(76.0-35.0)degrees C = 955 kj (rounded to 3 sig figs). Note that Q for the calorimeter = -Q for the reaction. So, the heat released by the sample is -955 kj. Now, we need to convert that to kj/mole. We have 35.6 g of ethanol. Divide that number by the molar mass (46.07 g/mol) to get 0.773 moles. 955kj/0.773 moles = 1240 kj/mole (rounded to 3 sig figs). We can verify this with a Google seach of the phrase "heat of combusion of ethanol". Wikipedia gives the heat of combustion of ethanol as 1300 kj/mole, which is similar. It's common for textbooks to present numbers that are just a little "off" so that students can't get the right answer just by looking up a number in a table. I hope that helps! Alix

Answer 2

The temperature change is 41°C.

The heat capacity of the calorimeter is 23.3 kJ/°C (not specific heat, so mass is already included)

We can calculate the heat released by the reaction by multiplying: 41*23.3=955.3 kJ

 

The next step is to calculate the amount of substance of ethanol

n(Ethanol)=35.6/46.07=0.7727 mol

 

To get the molar heat of reaction divide the 955.3kJ by the calculated amount of substance.

 

Hope this helps!