Thermochemistry Branch of chemistry 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?

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1. Frozen Lake Erie2. Drop of boiling oil

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

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1. Frozen Lake Erie2. Drop of boiling oil

Heat vs. Temperature

Heat vs. Temperature

• Heat is the amount of energy that is transferred from one substance to another due to the difference in temperature.

• 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.

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1. True2. False

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

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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

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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

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.

SPECIFIC HEAT

SPECIFIC HEAT: The quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius (or one Kelvin).

q = s x m x DTq = thermal energy (J)

sh = specific heat (J/goC)m = mass (g)

DT = change in temperature (Tfinal – Tinitial) (oC)

SPECIFIC HEAT• Determine the energy (in kJ) required to raise

the temperature of 100.0 g of water from 20.0 oC to 85.0 oC?

m = 100.0 g DT = Tf -Ti = 85.0 - 20.0 oC = 65.0 oC

q = m x s x DT s (H2O) = 4.184 J/ g - oC

q = (100.0 g) x (4.184 J/g-oC) x (65.0oC) q = 27196 J (1 kJ / 1000J) = 27.2 kJ

• Determine the specific heat of an unknown metal that required 2.56 kcal of heat to raise the temperature of 150.00 g from 15.0 oC to 200.0 oC?

S = 0.0923 cal /g -oC

LAW OF CONSERVATION OF ENERGY

• The law of conservation of energy (the first law of thermodynamics), when related to heat transfer between two objects, can be stated as:

The heat lost by the hot object = the heat gained by the cold object

-qhot = qcold

-mh x sh x DTh = mc x sc x DTc

where DT = Tfinal - Tinitial

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

PRACTICE PROBLEM #71. Iron metal has a specific heat of 0.449 J/goC. How much heat is transferred to a

5.00 g piece of iron, initially at 20.0 oC, when it is placed in a beaker of boiling water at 1 atm?

2. How many calories of energy are given off to lower the temperature of 100.0 g of iron from 150.0 oC to 35.0 oC?

3. If 3.47 kJ were absorbed by 75.0 g H2O at 20.0 oC, what would be the final temperature of the water?

4. A 100. g sample of water at 25.3 oC was placed in a calorimeter. 45.0 g of lead shots (at 100 oC) was added to the calorimeter and the final temperature of the mixture was 34.4 oC. What is the specific heat of lead?

5. A 17.9 g sample of unknown metal was heated to 48.31 oC. It was then added to 28.05 g of water in an insulted cup. The water temperature rose from 21.04 oC to 23.98oC. What is the specific heat of the metal in J/goC?

0.792 J/goC

180. J

1.28 J/g oC

1.23 x 103 cal

31.1 oC

LAW OF CONSERVATION OF ENERGY• Assuming no heat is lost, what mass of cold water

at 0.00oC is needed to cool 100.0 g of water at 97.6oC to 12.0 oC?

-mh x sh x DTh = mc x sc x DTc

- (100.0g) (1 cal/goC) (12.0-97.6oC) = m (1 cal/goC) (12.0 - 0.0 oC)

8560 cal = m (12.0 cal/g)m = 8560 cal / (12.0 cal/g)

m = 713 g

• Calculate the specific heat of an unknown metal if a 92.00 g piece at 100.0oC is dropped into 175.0 mL of water at 17.8 oC. The final temperature of the mixture was 39.4oC.

s (metal) = 0.678 cal/g oC

GROUP STUDY PROBLEM #7• _____1. A 250.0 g metal bar requires 5.866 kJ to change its temperature

from 22.0oC to 100.0oC. What is the specific heat of the metal in J/goC?

• _____2. How many joules are required to lower the temperature of 100.0 g of iron from 75.0 oC to 25.0 oC?

• _____ 3. If 40.0 kJ were absorbed by 500.0 g H2O at 10.0 oC, what would be the final temperature of the water?

• _____ 4. A 250 g of water at 376.3 oC is mixed with 350.0 mL of water at 5.0 oC. Calculate the final temperature of the mixture.

• _____5. A 400 g piece of gold at 500.0oC is dropped into 15.0 L of water at 22.0oC. The specific heat of gold is 0.131 J/goC or 0.0312 cal/goC. Calculate the final temperature of the mixture assuming no heat is lost to the surroundings.

A solid becoming a liquid is called:

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

A gas becoming a liquid is called:

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

A solid becoming a gas is called:

1 2 3 4 5 6

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

A liquid becoming a gas is called:

1 2 3 4 5 6

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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

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1. True 2. False

When a gas condenses, heat is removed from the surrounding

environment?

True

False

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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?

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 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

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

Exoth

ermic

Endoth

ermic

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1. Exothermic2. Endothermic

ΔH of the reaction = -560kJ

Exoth

ermic

Endoth

ermic

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1. Exothermic2. Endothermic

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

Exoth

ermic

Endoth

ermic

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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