Thermochemistry branch of chem dealing with the relationship between chemical action

and heat.

Applications of Heat and Energy

Energy

• All energy can be classified as either potential or kinetic.

• Potential energy is any type of energy that is stored.• Examples: batteries (stored chemicalconverts to electrical)• Top of a roller coaster hill (gravitational)

• Candy Bar (stored energy in its chemical bonds)• Kinetic energy – any energy from the movement of

matter.

Which has more heat - Lake Erie in December of a drop of boiling oil?

1 2

0%0%

1. Frozen Lake Erie2. Drop of boiling oil

Which has a higher temperature- Lake Erie in December of a drop of boiling oil?

1 2

0%0%

1. Frozen Lake Erie2. Drop of boiling oil

Heat vs. Temperature

• Heat is the amount of energy that flows from a hotter object to a colder one. – It is the sum energy of all of the molecules in a

substance

• Temperature is the average kinetic energy of an object.

Heat Transfer• Heat always flows from an object with more

heat (hotter) to an object with less heat.

• Note: Something that is cold just lacks heat. There is no unit for “cold”.

• When you feel cold, it is because you are losing heat (not gaining “cold”)

True or False: When you place ice cream in the freezer, heat is transferred from the ice cream to the freezer.

1 2

0%0%

1. True2. False

True or False: When we open the window, heat is transferred from your body to the air outside.

1 2

0%0%

1. True2. False

Units of Heat

• Heat is measure in Joules (J).• (The joule is the SI unit for all types of energy.)

• Example: when you heat a cup of tea, you use about 75,000J (or 75 kilojoules) of heat.

Joule is pronounced jewel

Other units of heat

• Heat is also measured in calories (cal). • 1000 calories equals a kilocalorie (Cal).• A calorie is the amount of heat needed to raise

the temperature of 1 gram of water by 1 degree Celsius.

• Also, 1 calorie = 4.184 Joules• And … 1 Cal = 4.184 kJ

What is the boiling point of water?

1 2 3 4 5

0% 0% 0%0%0%

1. 100oF2. 273K3. 32oF4. 212oC5. 100oC

What is the freezing point of water?

1 2 3 4 5

0% 0% 0%0%0%

1. 0oF2. 32K3. 100oF4. 273K5. 32oC

Units of Temperature

Units of Temperature

TF = 1.8TC + 32

TK = TC + 273

A solid becoming a liquid is called:

1 2 3 4 5 6

0% 0% 0%0%0%0%

1. Freezing2. Melting3. Evaporation4. Sublimation5. Condensation6. Deposition

A gas becoming a liquid is called:

1 2 3 4 5 6

0% 0% 0%0%0%0%

1. Freezing2. Melting3. Evaporation4. Sublimation5. Condensation6. Deposition

A solid becoming a gas is called:

1 2 3 4 5 6

0% 0% 0%0%0%0%

1. Freezing2. Melting3. Evaporation4. Sublimation5. Condensation6. Deposition

A liquid becoming a gas is called:

1 2 3 4 5 6

0% 0% 0%0%0%0%

1. Freezing2. Melting3. Evaporation4. Sublimation5. Condensation6. Deposition

Attractive Forces

• In gases, these attractive forces are minimal.• In solids and liquids, the forces are strong

enough to keep the materials from scattering everywhere.

• These attractive forces also determine the melting point and boiling point of different compounds. (ex. NaCl melts at 801o C)

Changes in State• Materials experience a change in state when

enough heat energy is applied to break apart (or form) the attractions between molecules.

• When intermolecular bonds are broken, heat is absorbed from the surroundings; when the bonds are formed, heat is taken from the system and released to the surroundings.

Changes in State

• Different states of matter (solids, liquids, and gases) have very different properties due to attractive forces that exist between atoms.

• To change from a solid to a liquid, for example, these attractive forces in solids must be broken so that the liquid molecules have more freedom to move. In gases, the molecules have even more mobility.

When a solid melts, heat is removed from the surrounding

environment to break those intermolecular forces.

True

False

0%0%

1. True 2. False

When a gas condenses, heat is removed from the surrounding

environment?

True

False

0%0%

1. True 2. False

What happens at B?

Solid

starts

to m

elt

Liquid st

arts t

o freeze

Gas s

tarts

to co

ndense

Liquid st

arts t

o evapora

te

0% 0%0%0%

1. Solid starts to melt2. Liquid starts to

freeze3. Gas starts to

condense4. Liquid starts to

evaporate

What happens at D?

Solid

starts

t...

Liquid st

arts .

..

Gas s

tarts

to ...

Liquid st

arts .

..

0% 0%0%0%

1. Solid starts to melt2. Liquid starts to

freeze3. Gas starts to

condense4. Liquid starts to boil

What happens at E moving from right to left in the graph?

1 2 3 4

0%

18%

64%

18%

1. Solid starts to melt2. Liquid starts to

freeze3. Gas starts to

condense4. Liquid starts to

evaporate

What happens at C moving from right to left in the graph?

Solid

starts

to m

elt

Liquid st

arts t

o freeze

Gas s

tarts

to co

ndense

Liquid st

arts t

o evapora

te

0% 0%0%0%

1. Solid starts to melt2. Liquid starts to

freeze3. Gas starts to

condense4. Liquid starts to

evaporate

Heating Curves

Melting Point / Freezing Point

• The melting point (same temperature as freezing point) is the temperature at which a solid turns to a liquid.

• Latent Heat of Fusion – the amount of heat needed to freeze one gram of a substance (or the heat released when one gram of a substance melts.)

• Note: units in J/g

Latent Heat of Fusion

Q = m x Hfus

Heat = mass x Heat of Fusion

****For ice to water: Hfus = 334 J/g

(every substance has a difference heat of fusion)

Boiling Point / Condensation Point

• The boiling point (same temperature as condensation point) is the temperature at which a liquid turns to a gas.

• Latent Heat of Vaporization – the amount of heat needed to vaporize one gram of a substance (or the heat release when one gram of a substance condenses.)

• Note: units in J/g

Latent Heat of Vaporization

Q = m x Hvap

Heat = mass x Heat of Vaporization

****For water to steam: Hvap = 2260 J/g

(every substance has a difference heat of vaporization)

Heat Graph calculations

• When the graph is flat, use latent heat equations because of change of state.

• For melting: Q = m x Hfus

• For evaporating: Q = m x Hvap

Specific Heat

• Different substances absorb (and lose) heat at different rates.

• Specific heat (SH) is the amount of heat (q) needed to raise the temperature of 1 gram of a substance by 1 degree Celsius.

Heat Graph calculations

• When the graph is sloped, use specific heat equations because of change of temperature

• Q = SH x m x Temp. Change or

Measuring Heat Changes

• A calorimeter is an instrumentused to measure heat changes. By placing an object in an insulatedcontainer, the heat loss can be measured by the temperature gain of the water.Heat lost by object = Heat gained by water -qobject = qwater

Why do chemical reactions occur between some substances and not in others?

•Chemical reactions occur so that the atoms in the elements involved attain a more stable state of being.

Collision Theory

• Collision theory – molecules must collide with the proper orientation and sufficient energy to react.

Activation Energy

• The activation energy is the amount of energy required to break the bonds between the atoms of the reactants.

Condition Necessary for Reactions to Occur

1) Collision: Reactants must collide.

2) Orientation: The reactants must align properly to react.

3) Energy: The activation energy must be attained to react.

Energy in Chemical Reactions

• Many chemical reactions also produce energy changes.

• Definitions:• System – the reactants and products in the

reaction• Surroundings – everything else around the

reaction (eg air in the room, reaction flask)

Heat of Reaction

• Heat of Reaction (ΔH) – the amount of heat lost or gained in a reaction

• Heat of Reaction: ΔH = Hproducts – Hreactants

Hair products

Exothermic Reactions

• Exothermic Reactions – energy is produced by a reaction; energy flows from the system to the surroundings

• ΔH is negative because the reaction loses heat.

Exothermic Reaction Graph

Endothermic Reactions

• Endothermic Reactions – energy is gained by a reaction; energy flows from the surroundings into the system

• ΔH is positive because the reaction gains heat.

Endothermic Reaction Graph

Classify the reaction:2H2O + 572kJ -> 2H2+ O2

1 2

33%

67%1. Exothermic2. Endothermic

ΔH of the reaction = -560kJ

1 2

27%

73%1. Exothermic2. Endothermic

After the reaction, your hand gets burnt from the heat. The reaction must be:

1 2

27%

73%1. Exothermic2. Endothermic

Heat Values in Chemical Reactions

• Heat of Reaction is a stoichiometric value and is proportional to the coefficients of the reactants and products.

• 2H2O + 572kJ -> 2H2+ O2

• Therefore, for every 2 moles of water that react, 572kJ of energy are required.

Presence of a Catalyst – a substance that increases the rate without being permanently changed- lowers activation energy

Also used:inhibitors – “tie up” a reaction so that it does not occur (opposite of a catalyst)- preservatives - anti-rust agents