Thermochemistry deals with the energy changes that take place during chemical reactions.

Questions:

• How much energy does a rxn produce/consume?

• Is a reaction thermodynamically favorable?

The First Law of Thermodynamics

Energy:

1st Law of Thermodynamics:

Forms of Energy

1. Potential energy:

2. Kinetic energy:

chemical energy:

A + B ---> A-B + energy

or

weak bonds ---> strong bonds + energy

Temperature: a measure of the ___________________ of molecules in a substance. For a gas, E_k = 3/2 RT (per mole)

Transfer of Energy

1. heat:

2. work:

system: portion of universe we focus on (reactants and products).

surroundings: portion of the universe which interacts with the system (solvent, container).

internal energy (E):

DE = q + w (because of 1st Law)

DE = change in internal energy of system

q = heat gained by system

w = work done on system

q > 0 heat added to system

w > 0 mechanical energy added to system

Energy is a state function.

For a given compound,

E(T,P,state of matter,n).

so DE is independent of pathway

It depends only on the initial and final state.

Enthalpy

The enthalpy (H) of a system is defined as follows:

H = E + PV

(H is a state function)

DE = q + w

DE = E_prod - E_react = q - PDV

q = E + PDV

DH = DE + D(PV)

or DH = DE + PDV

(if P is constant)

so DH = q (if P is constant)

The change in enthalpy for a chemical reaction carried out at constant P is equal to the heat gained by the system.

Definition of enthalpy:

For a chemical rxn, A ---> B

DH = DH_B - DH_A = H_prod - H_react

DH is heat or enthalpy of reaction;

change in enthalpy of the system

Usually DH is about equal to DE.

Exothermic vs Endothermic

(see blackboard)

Relationships involving DH

1. DH is ____________ property.

Enthalpy change is directly proportional to the amounts of substances involved in a rxn.

N₂ + O₂ ---> 2 NO DH = 180.50 kJ

½ N₂ + ½ O₂ ---> NO DH = 90.25 kJ

2. DH changes sign when a rxn is ____________.

½ N₂ + ½ O₂ ---> NO DH = 90.25 kJ

(endothermic)

NO ---> ½ N₂ + ½ O₂ DH = -90.25 kJ

(exothermic)

3. Hess's Law: If a process can be considered to occur in steps (individual reactions), then the enthalpy change for the overall process is equal to the ________ of the enthalpy changes for the individual steps.

½ N₂ + O₂ ---> NO₂ DH = ?

½ N₂ + ½ O₂ ---> NO DH₁ = 90.25 kJ

NO + ½ O₂ ---> NO₂ DH₂ = -57.07 kJ

DH₃ = _________

**Definition: The enthalpy (or heat) of formation of a compound (DH_f(cpd)) is _________________________________

 ____________________________________________________________

C(graphite) + O₂(g) ---> CO₂(g)

DH^o_rxn = DH_f^o(CO₂(g)) = -393.5 kJ/mol

DH^o_rxn = H^o_final - H^o_initial

DH^o_rxn = Sn_prod DH_f^o(prod) - Sn_react DH_f^o(react)

(Know this equation!)

Standard pressure is 1 atm. and temperature is usually 25^oC.

DH_f^o(element) = 0 (by def.)