THERMOCHEMISTRY.

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Presentation transcript:

THERMOCHEMISTRY

Heating Curve of Water KE PE KE PE KE

THINGS TO REMEMBER ΔH for temperature changes uses the formula ΔH = c m ΔT and the table of specific heats. ΔH for phase changes uses the formula ΔH = m Hp and the table of phase changes ΔH for CHEMICAL REACTIONS uses the formula ΔH = Hproducts – Hreactants and the standard enthalpies of formation table Heat of formation for a free element is 0

ENVIRONMENT H is negative H is positive SYSTEM (chemical reaction)

∆H is negative ∆H

∆H is positive ∆H

Standard Heat of Formation (ΔHof) The amount of heat energy gained or lost when one mole of the substance is formed from its elements under standard conditions ( 25oC and 101.3 kPa)

IMPORTANT POINTS Balance the equation so that ONE MOLE OF THE REACTANT is used. Remember the diatomic molecules (H2, N2, O2, F2, Cl2, Br2 and I2) and write them correctly. The reactants must be elements, not polyatomic ions.

The formation reaction for liquid water is described by the following equation: H2 (g) + ½O2 (g) → H2O (l) + 285.8 kJ or H2 (g) + ½O2 (g) → H2O(l) ΔHof = -285.5 kJ

HESS’S LAW ΔH = ∑ΔHproducts – ∑ΔHreactants The heat of reaction can be determine by subtracting the sum of heats of formation of the reactants from the sum of the heats of formation of the products

C6H6 (l) + 15/2 O2 (g) → 6 CO2 (g) + 3 H2O(l) Example Calculate ΔH for the combustion of benzene, C6H6, as shown by the following reaction: C6H6 (l) + 15/2 O2 (g) → 6 CO2 (g) + 3 H2O(l)

Using the formula ΔH = ∑ΔHproducts – ∑ΔHreactants C6H6 (l) + 15/2 O2 (g) → 6 CO2 (g) + 3 H2O(l) (49.0) 15/2 x (0) 6 x (-393.5) 3 x (-285.5) 49.0 -3218.4 ∑ΔHreactants ∑ΔHproducts ΔH = -3218.4 kJ– (49.0kJ) = -3267.4 kJ