Chemistry please help I don’t understand anything

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ΔH₁₊₂₊₃ = -74.6 Kj

Video Lecture I did on these type problems... See attached

Explanation:

The vid starts with some simple Hess Law additions and progresses through to more complex systems. The problem given in your example is a bit complex as a starting point when learning about these systems, so I suggest watching the above video lecture that develops the concept from less complex systems to the more complex.

Never the less, here's your problem, but I suggest you go through the video recommended above to start with some less complex problems.

Target Rxn => C(s) + 2H₂(g) => CH₄(g)

Given:

Rxn 1 => C(s) + O₂(g) => CO₂(g)                                      ΔH₁ = -393.5 Kj

Rxn 2 => CH₄(g) + 2O₂(g) => CO₂(g) + 2H₂O(g)            ΔH₂ = -890.8 Kj

Rxn 3 => H₂(g) + 1/2O₂(g) => H₂O(l)                               ΔH₃ = -285.8 Kj

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Start by adding Rxns 1 & 2, but reverse Rxn 2, change sign of   ΔH₂ to +890.8Kj

Rxn 1 => C(s) + O₂(g) => CO₂(g)                          ΔH₁ = -393.5 Kj

Rxn 2 => CO₂(g) + 2H₂O(l) => CH₄(g) + 2O₂(g)  ΔH₂ = +890.8 Kj

Rxn 1,2 => C(s) + 2H₂O(l) => CH₄(g) + O₂(g)       ΔH₁₊₂ = +497.3 Kj

Now, Double Rxn 3 and add to Rxn 1,2 => Target Rxn

Rxn 1,2 => C(s) + 2H₂O(l) => CH₄(g) + O₂(g)        ΔH₁₊₂ = +497.3 Kj

Rxn 3   => 2H₂(g) + O₂(g) => 2H₂O(l)                   ΔH₃ = 2(-285.8) Kj = -571.6 Kj

Rxn 1,2,3 => C(s) + 2H₂(g) => CH₄(g)

ΔH₁₊₂₊₃ = (+497.3 Kj) + (-571.6 Kj) = -74.3 Kj.