If H2( g)+Cl2( g) ⟶2 HCl ; ΔH^∘=-44 k cal 2Na(s) 12 IICl(g) ⟶2 NaCl(s)+II2( g) ΔH=152 kcal Then,

step 1 If hydrogen gas plus chlorine gas forms 2 HL and delta H is equal to minus 44 kilo cal, two mole

If H2( g)+Cl2( g) ⟶2 HCl ; ΔH^∘=-44 k cal 2Na(s) 12 IICl(g)...

If $\mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{HCl} ; \Delta \mathrm{H}^{\circ}=-44 \mathrm{k} \mathrm{cal}$ 2Na(s) $12 \mathrm{IICl}(\mathrm{g}) \longrightarrow 2 \mathrm{NaCl}(\mathrm{s})+\mathrm{II}_{2}(\mathrm{~g})$ $\Delta H=152 \mathrm{kcal}$ Then, $\mathrm{Na}(\mathrm{s})+0.5 \mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow \mathrm{NaCl}(\mathrm{s}) ; \Delta H^{\circ}=$ ? (1) $108 \mathrm{kcal}$ (2) 196 kcal (3) $-98 \mathrm{kcal}$ (4) $54 \mathrm{kcal}$

If $\mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{HCl} ; \Delta \mathrm{H}^{\circ}=-44 \mathrm{k} \mathrm{cal}$
2Na(s) $12 \mathrm{IICl}(\mathrm{g}) \longrightarrow 2 \mathrm{NaCl}(\mathrm{s})+\mathrm{II}_{2}(\mathrm{~g})$
$\Delta H=152 \mathrm{kcal}$
Then, $\mathrm{Na}(\mathrm{s})+0.5 \mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow \mathrm{NaCl}(\mathrm{s}) ; \Delta H^{\circ}=$ ?
(1) $108 \mathrm{kcal}$
(2) 196 kcal
(3) $-98 \mathrm{kcal}$
(4) $54 \mathrm{kcal}$

Key Concepts

Reaction Manipulation

Reaction manipulation involves reversing, scaling, or combining different chemical equations to derive a new reaction whose enthalpy change is determined from the known reactions. This process, which is a direct application of Hess’s Law, allows one to calculate the ?H for a desired reaction using other reactions with known enthalpies.

Stoichiometry in Thermochemistry

Stoichiometry in thermochemistry involves using the mole ratios from balanced chemical equations to appropriately scale the enthalpy changes. This ensures that the energy change calculated corresponds accurately to the chemical amounts used or produced in the target reaction.

Hess’s Law

Hess’s Law asserts that the overall enthalpy change for a reaction is independent of the reaction pathway and is the sum of the enthalpy changes of individual steps that lead to the final reaction. This principle enables the calculation of unknown reaction enthalpies by algebraically combining other reactions with known enthalpy changes.

Standard Enthalpy of Formation

The standard enthalpy of formation is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states under standard conditions. It is a foundational concept used to determine the energy changes associated with forming compounds from their constituent elements.

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