Unit 10 Thermochemistry

First Law of Thermodynamics

Energy can be converted from one form to another, but cannot be created or destroyed (Law of Conservation of Energy)

Heat Capacity (C)

Energy required to raise the temperature of an object by 1°C

Specific Heat (c)

Energy required to raise the temperature of 1 g of substance by 1°C

Fuel Value

Energy released when 1 g of substance is completely combusted

Fuel Values for Common Nutrients

Fat 9.0 kcal/g

Protein 4.0 kcal/g

Carbohydrate 4.0 kcal/g

Alcohol 4.0 kcal/g

Latent Heat of Vaporization

Energy required to vaporize 1 g of a substance at its boiling point

Latent Heat of Fusion

Energy required to melt 1 g of a substance at its melting point

Why is latent heat of vaporization greater than latent heat of fusion?

Takes more energy to vaporize (completely break intermolecular forces) vs melt (just loosen them)

Phase Changes

Melting latent heat of fusion

Freezing —melting

Vaporization latent heat of vaporization

Condensation —vaporization

Sublimation latent heat of fusion + latent heat of vaporization

Deposition —sublimation

Does temperature change during phase changes?

Temperature does not change

Standard Enthalpy of Formation

Heat change that results when one mole of a compound is formed from elements at standard state

Standard State

1 atm partial pressure for a gas

1 M concentration for a solute

usually 25°C

Enthalpy of Combustion

Heat released when 1 mole of a substance is combusted

Enthalpy of Solution or Dilution

Heat change when 1 mole of a solute is dissolved or diluted

Enthalpy of Neutralization

Heat change when an acid and a base react to form 1 mole of water

Bond Energy

Energy required to break a chemical bond

Endothermic is … bonds

Breaking

Exothermic is … bonds

Making

Why is using bond energy to estimate standard enthalpy of a reaction less accurate than using standard heat of formation?

Bond energy values are averages from many different molecules

State Functions

Determined by the current state of the system, not the path taken to arrive at that state

Hess’s Law

Change in enthalpy is the same whether the reaction takes place in one step or in a series of steps

Spontaneous Process

Capable of proceeding in a given direction, without being driven by outside energy

Entropy (S)

Measure of the randomness or disorder of a system

Related to number of microstates (W) which is how many different ways a system can be arranged

Second Law of Thermodynamics

Entropy of the universe is always increasing

More gas particles mean … entropy

More

As temperature increases, the entropy of a system …

Increases

Enthalpy + Entropy

Gibbs Free Energy (G) energy available to do work

ΔG and Spontaneity

ΔG < 0 (exergonic) spontaneous

ΔG > 0 (endergonic) non spontaneous

Signs of ΔH ΔS ΔG

ΔH + ΔS +

• ΔG spontaneous at high temperatures

• T > ΔH/ΔS

ΔH + ΔS -

• Never spontaneous

ΔH - ΔS +

• Always spontaneous at all temperatures

ΔH - ΔS -

• ΔG spontaneous at low temperatures

• T < ΔH/ΔS