# Calculating heat of combustion

• RPN
In summary, the heat of combustion for C2H6 can be calculated by balancing the reactions and using Hess's Law. The final answer is -1558.3kJ.
RPN
Hello
I am trying to solve this question and have been stumped for quite a while. Can some tell me the steps I have to take in order to solve this.
Question
Calculate the heat of combustion for C2H6 from the following information:
C2H4+ 3O2----> 2 CO2 + 2 H2O -1409.5kJ
C2H4 + H2 ----> C2H2 -136.7 KJ
H2 + 1/2 O2 -----> H2O -285.5 kJ

Thanks for the help

C2H4 + H2 ----> C2H2 -136.7 KJ

theres clearly a typo here, since the rxn isn't balanced. Is the product supposed to be C2H6?

if so flip the 2nd equation around so you have C2H6--->C2H4+H2
Now add that to the first one to get

C2H6+C2H4+O2----->CO2+H20+C2H4+H2

Balance the reactions and cancel out the C2H4's
Now add in the 3rd eq H2+O2---->H20
C2H6+H2+O2----->CO2+H20+H2

Balance and cancel out the H2's. You should be able to figure out the heat of combustion following this. I of course did this real sloppily. You will have to make sure you use the correct balanced equations when you do it.

Look up Hess's Law.
Solve for reaction: ' C2H6 -> ? '

C2H6 --> C2H4 + H2 +136.7kJ
+ C2H4 + 302 --> 2CO2 + 2H20 -1409.5kJ
= C2H6 + 3O2 --> 2CO2 + 2H2O + H2 -1272.8kJ
+ H2 + 1/2 O2 --> H2O -285.5Kj
= C2H6 + 3.5O2 --> 2CO2 + 3H2O - 1558.3kJ

So 1558.3kJ is the final answer?

## 1. What is the definition of heat of combustion?

The heat of combustion is the amount of heat released when a substance undergoes complete combustion in a sealed container at constant pressure.

## 2. How is the heat of combustion calculated?

The heat of combustion is calculated by measuring the mass of the substance and the temperature change of the surrounding environment during the combustion process. This data is then used in the formula Q = m x c x ΔT, where Q is the heat of combustion, m is the mass of the substance, c is the specific heat capacity of the substance, and ΔT is the change in temperature.

## 3. What is the unit of measurement for heat of combustion?

The unit of measurement for heat of combustion is typically in joules (J) or kilojoules (kJ) per gram (g) of the substance being combusted.

## 4. Can the heat of combustion vary for the same substance?

Yes, the heat of combustion can vary for the same substance depending on factors such as the purity of the substance, the amount of oxygen present during combustion, and the initial temperature of the substance. It is important to specify these conditions when reporting the heat of combustion.

## 5. What is the significance of calculating heat of combustion?

Calculating the heat of combustion is important in understanding the energy content and efficiency of a substance. It is also used in industries such as energy production and fuel development to determine the potential energy output of a substance and its environmental impact.

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