Standard Heats of Formation

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Presentation transcript:

Standard Heats of Formation Chapter 5 part 4

Standard Heat of Formation ΔH°f The change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances at their standard state.

ΔH°f The symbol ° means the process was carried out at standard conditions. Review: What are standard conditions? Pressure is 1 atm & temperature is 25° C or 298 Kelvin. Also: if in solution, the concentration is 1 M.

Standard States Elements in their standard states at 1 atm and 298 Kelvin. Liquid mercury, solid iron, gaseous oxygen. In solutions at standard state, the concentration is 1 M. The standard heat of formation of all elements is zero.

How are the ΔH°f ‘s found?

(Did I mention?) The ΔH°f of all elements is zero. Compounds store energy in their bonds. In any chemical reaction: Energy is needed to break open the bonds of the reactants. Energy is stored in the bonds of the products.

The ΔH°reaction can be calculated The difference between the energy in the bonds of the products and the energy in the bonds of the reactants is the energy of the reaction. ΔH° rxn = npΔH°f (products) -  nrΔH°f (reactants) Remember to multiply the enthalpies by their corresponding integer found in the balanced reaction equation.

Keep in Mind When a reaction is reversed the ΔH is equal and opposite. When a balanced reaction is multiplied by an integer, the ΔH for that reaction is also multiplied. ΔH° rxn = npΔH°f (products) -  nrΔH°f (reactants) Elements in their standard state have a ΔH°f of Zero

Example: 4NH3 (g) + 7O2 (g)  4NO2 (g) + 6H2O (l) 2Al (s) + Fe2O3 (s) Al2O3 (s) + 2 Fe (s) 2CH3OH (l) + 3O2 (g)  2CO2 (g) + 4H2O (l)

Answers -1396 kj -850 kj -1454 kj

In 1995, the United States consumed 9 In 1995, the United States consumed 9.2 x 1019J of energy, most of it by burning things. (http://itl.chem.ufl.edu/2045_s00/lectures/lec_8.html)

Consider the following spontaneous reaction of A3 molecules: (a) Write a balanced equation for the reaction. (b) What are the signs of ∆H, ∆S, and ∆G for the reaction? Explain.