Hess' Law

Enthalpy and Hess’ Law

Enthalpy change for any chemical reaction will be the same regardless of the ‘path’ taken as long as the starting conditions and final conditions, and reactions and products, are the same(Hess’ Law)

Rules for Enthalpy Changes

1) If you reverse a chemical reaction, you must reverse the sign of ΔH.,* direction of energy flow

2) The magnitude of ΔH is directly proportional to the number of moles of reactions and products in a reaction. If the coefficients are multiplied by a factor, the value of ΔH must be as well.

Standard Enthalpy changes of Reaction(Formation)

• measured under standard conditions, that is 298k and 1.00 × 10^5 Kpa

• standard enthalpy change of formation, △Hf ⍬ is the enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states.

• Give indication of stability of the substance and can be used to calculate enthalpy changes of ALL reactions (real and hypothetical)

Standard Enthalpy changes of Reaction(Combustion)

• ΔHf° of an element at ‘standard state’ (SATP) is ZERO!

  ΔHr° = (n x ΔHf°products)- (n x ΔHf° reactants)

• standard enthalpy change of combustion, △H ⍬ is the enthalpy change that occurs when one mole of the substance is burned completely in oxygen. products are carbon dioxide and water, can use fractions to balance!

  ΔHr° = (n x ΔHc°reactants)- (n x ΔHc °products)****