Enthalpy Calculations

11.6 Other Methods of Caculating Enthalpy

By Lan Vu

Methods of calculating Enthalpy, ΔH

• ​Calorimetry

• Hess's Law

• ​Energy of Formation

• Bond Enthalpies​

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Hess's Law

​The heat of any reaction, ΔH, for a specific reaction is equal to the sum of the heats of reaction for any set of reactions which in sum are equivalent to the overall reaction

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Explain Hess's Law

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​Whole outfit $90

​$20

​$30

​$40

​BOUGHT SEPARATELY

=$90

Some Rules for Using Hess's Law

You can

• add coefficient to the reactants and products ​and must mulptiply the ΔH by the same amount

• you can flip the reaction, Δ​H is flip also

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Multiple Choice

Using the equations below:

C(s) + O₂(g) → CO₂(g) ∆H = –390 kJ

Mn(s) + O₂(g) → MnO₂(s) ∆H = –520 kJ

what is ∆H (in kJ) for the following reaction?

MnO₂(s) + C(s) → Mn_(s) + CO₂(g)

• 910

• 130

• -130

• -910

Multiple Choice

Consider the following equations.

Mg(s) + O₂(g) → MgO(s) ∆H = –602 kJ

H₂(g) + O₂(g) → H₂O(g) ∆H = –242 kJ

What is the ∆H value (in kJ) for the following reaction?

MgO(s) + H₂(g₎ → Mg(s) + H₂O(g)

• -844

• -360

• +360

• +844

Multiple Choice

The standard enthalpy change of formation values of two oxides of phosphorus are:

P₄(s) + 3O₂(g₎ → P₄O₆(s) ΔH_f = –1600 kJ mol^–1

P₄(s) + 5O₂(g) → P₄O₁₀(s) ΔH_f = –3000 kJ mol^–1

What is the enthalpy change, in kJ mol^–1, for the reaction below?

P₄O_6(s) + 2O₂(g) → P₄O₁₀(s)

• +4600

• +1400

• –1400

• –4600