If the heat of formation of H2O(l) is –286 kJ/mol, which of the following thermochemical equations is correct?
(a) 2 H(g) + O(g) H2O(1)
AH = -286 kJ
(b) 2 H2(g) + O2(g) 2 H2O(1)
AH = -286 kJ
(c) H2(g) + 3 02(8) H2O(0
AH = -286 kJ
(d) H2(g) + O(g) →→ H2O(g)
AH = -286 kJ
(e) H2O(1) H2(g) + 2 02(8)
AH = -286 kJ
Practice Exercise 2

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answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

explanation :

let us write down the given equations

equation 1 :

equation 2 :

equation 3 :

let us use hess's law to find out which 2 equations can be added to get the third equation.

let us reverse equation 2 and multiply it by 2. the new equation that we get is,

equation 4 :

let us add equation 1 & 4

the above equation is similar to equation 3 but has different value for dhrxn.

therefore third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

the objects have the same charge.