H is called the enthalpy The enthalpy change

H is called the enthalpy. The enthalpy change for a system at constant pressure equals the heat absorbed or released. Are H and E similar ( i. e. are qv, qp similar ) ? ( p. V ) << E H ≈ E For reactions where only liquids and solids are involved, and where reaction is carried out at constant pressure (usually in the presence of the atmosphere) V and therefore ( p. V ) = p V is usually negligible.

N 2 + O 2 2 NO n = 0 ( p. V ) = 0 at const T, H= E N 2 + 3 H 2 2 NH 3 n = -2 (p. V) = -2 RT H = E + n. RT for reactions of ideal gases H = 2 Hfo ( NH 3 ) = 2 ( -46, 110 ) = -92, 220 joules

Example : Convert 1 mole of liquid H 2 O at 100 o C into 1 mole of H 2 O vapor at 100 o. C and p = 1 atm. H = 44, 013. 6 joules/mole What is E? H = E + ( p. V ) = E + p V (const p) 1 mole H 2 O = 18 gm; volume 1 gm of H 2 O at 100 o. C is 1. 04 m. L/gm vi = 18 ( 1. 04 ) = 18. 72 m. L vf = n. RT / p assume ideal gas

v = 30, 586 - 19 = 30, 567 m. L = 30. 57 liters. 082 L-atm = 8. 314 joules 101. 4 joules/L-atm E = H - p V E = 44013. 6 joules/mole - 3099. 5 joules/mole

General Definitions of heat capacities (for 1 mole of gas): (sub P means take derivative at constant P) (sub V means take derivative at constant V) Substitute d. Q = d. E + pd. V= d. E - dw (1 st Law) Remember (1 mole gas) = (3/2)R + R = (5/2)R

Bonus * Bonus * Bonus

Thermochemistry: Can measure H directly by making a laboratory determination of qp. Want to set up a table of enthalpy changes for chemical reactions Standard Enthalpy Change H for a system Standard State of a Substance That form of the substance which K is most stable at a pressure of 1 atm and T = 298�

Examples Carbon : at 1 atm, 25 C, Stand. St. is graphite (not diamond or coal) Bromine : 1 atm, 25 C Std. St. is liquid Bromine For a reaction: C ( graphite ) + O 2 ( g ) CO 2 ( g ) measure qp in lab (Heat released when burn graphite at const p) This means evolve 393. 52 kjoules of heat in converting 1 mole C graphite, 1 mole O 2 gas, into 1 mole CO 2 gas at 1 atm and 298 K. Can also burn CO in lab to produce CO 2:

Hess' Law of Constant Heat Summation Heat evolved or absorbed at constant pressure for any chemical change is the same regardless of the path by which the change occurs.

C (graphite) + O 2 (g) CO (g) + (1/2) O 2 (g) Can’t measure in the lab. (Always get a little CO 2)

Enthalpy's of Formation Standard Enthalpy of formation is ∆H for a reaction where a pure compound is formed from its elements with all substances in their standard states ( 25 C ) H 2 (g) + 1/2 O 2 (g) = H 2 O(l) ∆Hf ( H 2 O ) = -285. 8 kjoules Enthalpy of formation of elements in their standard state is defined to be zero: Why are ∆Hf useful?

Suppose we want to know ∆H for the reaction: 1 2 CH 3 COOH = 2 C (s) + 2 H 2 (g) + O 2 (g) = CH 4 (g) + CO 2 2 : ∆H = + ∆Hf ( CH 4 ) + ∆Hf ( CO 2 ) because ∆H is independent of path. In principle can always accomplish a chemical transformation by following a path which 1 st decomposes reactants into elements then reforms product.