Thermochemistry-Honors Chem @ DFA

Thermochemistry

Study of the energy and heat associated with chemical reactions and/or physical transformations.

Thermochemistry focuses on..

energy changes, particularly on the system’s energy exchange with its surroundings.

Thermal energy

the portion of internal energy that is responsible for a system’s temperature.

temperature

it is the measure of a total amount of kinetic energy (energy of motion) in the system.
→ it increases with mass.

Heat

thermal energy that is transferred between systems at different temperatures.

Heat flows..

It flows spontaneously from systems at high TEMP to systems low TEMP.

Thermal Equilibrium

Once the temperatures of the systems are the same, heat no longer flows

Entropy(ΔS)

measure of disorder in a system.

Note #1 (Entropy)

In general, the universe (and everything in it) naturally becomes MORE disorderly. Changes are more thermodynamically favorable (spontaneous) when the system has an increase in entropy.

Enthalpy (ΔH)

is the amount of heat content used or released in a system at constant pressure.

Note #2 (Enthalpy)

Changes are more thermodynamically favorable (spontaneous) when the system has a decrease in enthalpy.

ENDOTHERMIC processes have

+ΔH

EXOTHERMIC processes have

-ΔH

Spontaneous changes have..

POSITIVE entropy and NEGATIVE enthalpy

spontaneous process example

Burning is a spontaneous process because The process is EXOTHERMIC and The particles are INCREASING IN ENTROPY as they vaporize

Gibbs Free Energy:
So what happens if one of the driving forces is favorable while the other is not (i.e. -ΔS while -ΔH)?

ΔG = ΔH - TΔS
Spontaneous reactions have -ΔG

More about enthalpy… #1

The energy used to form a compound from its elements is called enthalpy (or latent heat) of formation (ΔHf) ΔHf C6H12O6 = -1273 kJ/mol

More about enthalpy… #2

The energy released when a compound is burned in oxygen is called enthalpy (or latent heat) of combustion (ΔHc) ΔHc C6H12O6 = -2801 kJ/mol

Heat of reaction (ΔHrxn)

is the amount of heat absorbed or released in the transformation of reactants into products under the same conditions. (Photosynthesis)

Enthalpy (Heat) of Reaction : Example

6 CO2 + 6 H2O à C6H12O6 + 6 O2 ΔHrxn = +2801 kJ/mol CH4(g) + 2 O2(g) à CO2 + 2 H2O ΔHrxn = -891 kJ/mol

Hess’s Law (single rxn)

ΔHrxn = ΣΔHf products - ΣΔHf reactants

Calculate the ΔHrxn for the following: C3H6O(ℓ) + 4O2(g) --->

3CO2(g) + 3H2O(ℓ)
ΔH°f, C3H6O: -248 kJ/mol
ΔH°f, O2: 0 kJ/mol
ΔH°f CO2: -393.5 kJ/mol
ΔH°f, H2O: -285.83 kJ/mol
ΔHrxn = [3 (-393.5) + 3 (-285.83)] - [ (-248) + 4 (0) ]
= -1790 kJ

Hess’s Law (multiple rxns)

During discharge of a lead-acid storage battery, the following chemical reaction takes place: Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O.

Phase Changes, Revisited

Remember that as a substance is undergoing a phase change, its temperature does NOT change

Why not? (Phase change)

Energy is being used to separate the particles and is being stored as potential energy, so there is no heat available to be measured.

enthalpy (or latent heat) of fusion (ΔHfus)

The heat used to melt a substance

enthalpy (or latent heat) of vaporization (ΔHvap)

The heat used to vaporize a substance.

enthalpy (or latent heat) of combustion (ΔHc)

the energy released when a compound is burned in oxygen

enthalpy (or latent heat) of combustion (ΔHc)

The energy released when a compound is burned in oxygen