Chemistry Practice Problems Hess's Law Practice Problems Solution: Calculate ΔH for the reaction N2H4 (l) + O2 (g) → ...

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Solution: Calculate ΔH for the reaction N2H4 (l) + O2 (g) → N2 (g) + 2H2O (l)given the following data2 NH3 (g) + 3N2O (g) → 4N2(g) + 3H2O (l)      ΔH = -1010 kJN2O (g) + 3 H2 (g) → N2H4 (l) + H2O (l)           ΔH = -317 kJ2NH3 (g) + 1/2 O2 (g) → N2H4 (l) + H2O (l)       ΔH = -143 kJH2 (g) + 1/2 O2 (g) → H2O (l)                            ΔH  = -286 kJ

Problem

Calculate ΔH for the reaction 

N2H(l) + O2 (g) → N2 (g) + 2H2O (l)

given the following data

2 NH3 (g) + 3N2O (g) → 4N2(g) + 3H2O (l)      ΔH = -1010 kJ

N2O (g) + 3 H(g) → N2H4 (l) + H2O (l)           ΔH = -317 kJ

2NH3 (g) + 1/2 O(g) → N2H4 (l) + H2O (l)       ΔH = -143 kJ

H(g) + 1/2 O(g) → H2O (l)                            ΔH  = -286 kJ

Solution

We’re being asked to determine the enthalpy change (ΔHrxn) for the chemical reaction:


N2H(l) + O2 (g) → N2 (g) + 2 H2O (l)


We can use Hess’s Law to determine the enthalpy change of the overall reaction from the given reactions:


1. 2 NH3 (g) + 3 N2O (g)  4 N2(g) + 3 H2O (l)      ΔH = -1010 kJ

2. N2O (g) + 3 H(g) → N2H4 (l) + H2O (l)              ΔH = -317 kJ

3. 2 NH3 (g) + 1/2 O(g) → N2H4 (l) + H2O (l)        ΔH = -143 kJ

4. H(g) + 1/2 O(g)  H2O (l)                               ΔH  = -286 kJ


We now need to find a combination of reactions that when added up, gives us the overall reaction.


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