From the enthalpies of reaction 2 H2(g)+O2(g) ⟶2 H2 O(g) ΔH=-483.6 kJ 3 O2(g) -⋯⟶ 2 O3(g) Δ

step 1 So I find it helpful to start by writing the equation I'm trying to find, which is 3H2 plus ozon

From the enthalpies of reaction 2 H2(g)+O2(g) ⟶2 H2 O(g) ...

From the enthalpies of reaction $2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g) \quad \Delta H=-483.6 \mathrm{~kJ}$ $3 \mathrm{O}_{2}(g) \stackrel{-\cdots}{\longrightarrow} 2 \mathrm{O}_{3}(g) \quad \Delta H=+284.6 \mathrm{~kJ}$ calculate the heat of the reaction $$ 3 \mathrm{H}_{2}(g)+\mathrm{O}_{3}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(g) $$

From the enthalpies of reaction
$2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g) \quad \Delta H=-483.6 \mathrm{~kJ}$
$3 \mathrm{O}_{2}(g) \stackrel{-\cdots}{\longrightarrow} 2 \mathrm{O}_{3}(g) \quad \Delta H=+284.6 \mathrm{~kJ}$
calculate the heat of the reaction
$$
3 \mathrm{H}_{2}(g)+\mathrm{O}_{3}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(g)
$$

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Key Concepts

Hess's Law

Hess's Law is a principle stating that the total enthalpy change of a chemical reaction is the same regardless of the path taken, because enthalpy is a state function. This means that if a target reaction can be expressed as a combination of other reactions with known enthalpy changes, the overall enthalpy change can be calculated by appropriately adding or subtracting those individual changes.

Enthalpy (?H)

Enthalpy is a thermodynamic property that represents the total heat content of a system at constant pressure. It is used to determine whether a reaction is endothermic or exothermic, with negative values indicating exothermic reactions and positive values indicating endothermic reactions.

Reaction Manipulation in Thermochemistry

Reaction manipulation involves altering chemical equations—such as reversing or scaling them—while keeping track of how their enthalpy changes accordingly. When a reaction is reversed, the sign of its enthalpy change also reverses, and if the equation is multiplied by a factor, the enthalpy change must be multiplied by the same factor. This technique is essential for applying Hess's Law to calculate unknown reaction enthalpies.

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