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Hess’s Law Chapter 6 Section 3 & 4

Hess’s Law

Enthalpy is a state function.

Going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

Hess’s Law

Reaction 1: C (s) + ½ O2 (g) → CO (g) ΔH1 = -110.5 kJ Reaction 2: CO (g) + H2O (ℓ) → H2CO2 (g) ΔH2 = + 33.7 kJ Reaction 3: H2CO2 (g) → H2CO2 (ℓ) ΔH3 = - 62.9 kJ

Net reaction: C (s) + ½ O2 (g) + H2O (ℓ) → H2CO2 (ℓ)

Characteristics of Enthalpy Change, H

1. If a reaction is reversed, the sign of H is reversed.

2. The magnitude of H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of H is multiplied by the same integer. Enthalpy is an extensive property.

Xe + 2F2 XeF4 H = -251 kJ

XeF4 Xe + 2F2 What is the H?

2Xe + 4F2 2XeF4 What is the H? -502 kJ

+251 kJ

Hess’s Law I Sample Exercise 6.7 page 244

Two forms of carbon are graphite, the soft, black, slippery material used in “lead” pencils and as a lubricant for locks, and diamond, the brilliant, hard gemstone. Using the entalpies of combustion for graphite (-394 kJ/mol) and diamond (-396 kJ/mole), calculate H for the conversion of graphite to diamond.

Cgraphite (s) C diamond (s)

Hess’s Law II Sample Exercise 6.8 page 245

Diborane(B2H6) is a highly reactive boron hydride that was once considered as a possible rocket fuel for the US space program. Calculate the H for the synthesis of diborane from its elements, according to the equation

2B (s) + 3H2 (g) B2H6 (g)

Using the following data:

Reaction H

(a) 2B (s) + 3/2 O2 (g) B2O3 (s) -1273 kJ

(b) B2H6 (g) + 3O2 (g) B2O3 (s) + 3H2O(g) -2035 kJ

(c) H2 (g) + ½ O2 (g) H2O(l) -286 kJ

(d) H2O(l) H2O(g) 44 kJ

Standard Entalpies of Formation

Hf is the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states.

Indicates that the process has been carried out under standard conditions – the substance is in its standard state.

We cannot determine absolute values of enthalpy.

We can measure changes in enthalpy with heat-flow experiments.

Conventional Definitions of Standard States

For a Compound

The standard state of a gaseous substance is a pressure of exactly 1 atm

For a pure substance in a condensed state (liquid or solid), the standard state is the pure liquid or solid.

For a substance present in a solution, the standard state is a concentration of exactly 1 M.

For and Element

The standard state of an element is the form in which the element exists under condition of 1 atm and 25 C.

Standard Enthalpies of Formation Table p. 247

BACK

Combustion of Methane

CH4 (g) + O2 (g) CO2 (g) + H2O(g)

Combustion of Methane

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O(g) Hf -75kJ 0kJ -394kJ -286kJ

[np Hf ] (products) - [nr Hf ](reactants) = Hreaction

[(1• -394) + (2 • -286)] - [(1 • -75) + (2 • 0)] = Hreaction

= -891 kJ

Key Concepts When Doing Enthalpy Calculations

When a reaction is reversed, the magnitude of H remains the same, but its sign changes.

When the balanced equation for a reaction is multiplied by an integer, the value of H for that reaction must be multiplied by the same integer.

The change in enthalpy for a given reaction can be calculated from the enthalpies of formation of the reactants and products:

Hreaction = [np Hf ] (products) - [nr Hf ](reactants)

Elements in their standard states are not included in the Hreaction calculations. That is, Hf for an element in its standard state is zero.

Combustion of Ammonia

Enthalpies from Hf I Sample Exercise 6.9 page 249

Using the standard enthalpies of formation listed in Table 6.2, calculate the standard enthalpy change for the overall reaction that occurs when ammonia is burned in air to form nitrogen dioxide and water. This is the first step in the manufacture of nitric acid.

4NH3 (g) + 7O2 (g) 4NO2 (g) + 6H2O (l)

Enthalpies from Hf II Sample Exercise 6.10 page 251

Using the standard enthalpies of formation in Table 6.2, calculate the standard change in enthalpy for the thermite reaction:

2Al (s) + Fe2O3 (s) Al2O3 (s) + 2Fe (s) This reaction occurs when a mixture of powdered aluminum and iron (III) oxide is ignited with a magnesium fuse.

Enthalpies from Hf III Sample Exercise 6.11 page 252

Methanol is often used as a fuel in high-performance engines in race cars. Using the data in Table 6.2, compare the standard enthalpy of combustion per gram of methanol with that per gram of gasoline. Gasoline is actually a mixture of compounds but assume for this problem that gasoline is pure liquid octane.

2CH3OH (l) + 3O2 (g) 2CO2 (g) + 4H2O (l)