Question

At 25°C the following heats of reaction are known:

2C2H2 + 5O2 ---> 4CO2 + 2H2O; ΔH = -2600 kJ

C + O2 ---> CO2 ; ΔH = -394 kJ

2H2 + O2 ---> 2H2O; ΔH = -572 kJ

At the same temperature, calculate ΔH for the following reaction:

2C + H2 ---> C2H2; ΔH = ??

I don't understand how to solve this, what equation would i use?

Answer 1

you need to combine the given reactions
... some partial and/or reversed
... to get the desired reaction

2CO2 + H2O --> C2H2 + 5/2 02 ΔH = 1300 kJ

2C + 2O2 --> 2CO2 ΔH = -788 kJ

H2 + 1/2 O2 --> H20 ΔH = -286 kJ

add the 3 reactions, along with the ΔH's
... cancel like terms on opposite sides of the result