Posted by **mohd** on Sunday, April 11, 2010 at 3:57pm.

these are my three thermochemical equations

2Cu(s)+S(s) give Cu_2S(s) delta H -79.5 KJ

S(s)+O_2 give SO_2(g) -297 KJ

Cu_2S(s)+2O_2(g) give 2CuO(s)+SO_2(g)

how i calculate standard enthalpy of formation in Kj per mol of CuO(s)?

please dont give the answer only guid me please

- chemistry -
**DrBob222**, Sunday, April 11, 2010 at 4:06pm
Add equation 1 to the reverse of equation 2 to equation 3. That will give you twice the equation you want and you can take half of it later.

Add kJ/mole for equation to the negative of equation 2 to the kJ for equation 3. All of that together gives twice what you are looking for so divide the total by 2 and divide the equation by 2.

- chemistry -
**mohd**, Sunday, April 11, 2010 at 4:12pm
in front of last equation i forgat to wright the delta H .it is -527.5 KJ

- chemistry -
**DrBob222**, Sunday, April 11, 2010 at 4:14pm
That doesn't change anything. When you reverse an equation change the sign of delta H, otherwise leave it.

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