Use average bond energies to calculate delta H for the reaction of H2 and O2 to form water.

ΔHrxn = - 3294.8 KJ/mol

Explanation:

H - H                         H - O - H

H - H   +   O = O   →  H - O - H  

broken bond:

⇒ 2 bond H - H + 1 bond O = O

bond formed:

⇒ 4 bond H - O

bond energies:

∴ H - H: 436.4 KJ/mol

∴ O = O: 494 KJ/mol

∴ H - O: 482 KJ/mol

broken bond:

⇒ bond energie = 2(436.4) + 494 = 1366.8 KJ/mol

bond formed:

⇒ bond energie = 4(- 482) = - 1928 KJ/mol

⇒ ΔHrxn = - 1928 KJ/mol - (1366.8 KJ/mol)

⇒ ΔHrxn = - 3294.8 KJ/mol

air

in a tire you fill a tire with air

then a couple days later you check the air pressure in the tire

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