Thermodynamics

What is thermochemistry?

• The study of the changes in heat energy that accompany chemical reactions and physical changes

How do we measure these changes in heat energy?

• Both physical and chemical processes should be conducted in a calorimeter, a device used to measure the heat absorbed or released

• Calorimeters should have good isolative properties to prevent heat from entering or leaving when measuring the heat transfer of the physical or chemical process

What types of heat transfer will we measure?

• Endothermic --> when heat is absorbed during a chemical or physical change

Examples include:• fusion (melting) solid + ΔH liquid• sublimation solid + ΔH gas (dry ice)• vaporization liquid + ΔH gas• baking bread• photosynthesis

• Exothermic-->when heat is released during a chemical or physical change

Examples include:

Solidification (freezing) liquid solid + ΔH

Condensation gas liquid + ΔH

Deposition gas solid + ΔH (frost)

Rusting iron

Burning a candle

Mathematical equation used to measure energy absorbed or

released during a temperature change

Q = cpn∆T

Q = energy gained or lost

Cp = specific heat

n = amount (g)

∆T = change in temperature

How will this energy be measured?

• The SI unit of energy is the Joule (J)

• The calorie(cal) is more commonly used to measure food energy

• 1 cal = 4.18 J

What is specific heat (cp)?

• The amount of heat energy required to raise the temperature of 1 gram of a substance by 1 Celsius degree or Kelvin

• This value is dependant on:-the substance-the state the substance is in (solid, liquid

or gas)

Mathematical equation used to measure energy absorbed or

released during a phase change

Q = Hxn

Q = energy gained or lost

Hx = heat of fusion, vaporization, solidification or condensation

n = amount (grams or moles)

What is Hx?

• Heat of fusion (Hf) --> the amount of energy required to melt 1 mole (or gram depending on the units given) at it’s melting point

• Heat of solidification (Hs) --> same as Hf except a negative value because energy needs to be removed to go from a liquid to a solid

• Heat of vaporization (Hv) --> the amount of energy required to vaporize 1 mole (or gram) at it’s boiling point

• Heat of condensation (Hc) --> same as Hv except a negative value because energy needs to be removed to go from a gas to a liquid

Endo Versus Exo?

• An endothermic energy change will be denoted with a positive value

• Ex. 468 J

• An exothermic energy change will be denoted with a negative value

• Ex. -9548 cal

Simple Practice Problems

• How many joules of heat are given off when 5.0 g of water cool from 75.0°C to 25.0°C? (cp= 4.18 J/g°C)

• How many calories is this?

• How many joules of energy does it take to melt 35 g of ice at 0°C?

(Hf = 333 J/g)

• How many calories are given off when 85 g of steam condenses to liquid water? (Hv= 539.4 cal/g)

Practice Problem• An unknown compound has the following properties:• Hs = -5800 J/mol• Hv = 54200 J/mol• cp (liquid) = 185.3 J/mol•C• cp (gas) = 42.9 J/mol •C• mp = 5C• bp = 15CA. Be able to sketch a fp/bp graph for this substance, labeling the

axes and indicating the mp and bp)B. Suppose 65000 J of heat are added to a 1.08 mol solid sample

of this substance at 5C. When all the heat is absorbed by the substance, at what temperature and state(s) will the substance be?

1 cal = 4.18 J

What is Equilibrium?

• A dynamic condition in which 2 opposing changes occur at equal rates in a closed system.

• Ex. A pool is at maximum capacity with 50 people and at equilibrium. If 2 people enter the pool, how many people need to exit the pool to keep it at equilibrium?