Thermochemistry and Energy

Water freezing and boiling at the same time, really?!

O Thermochemistry is the study of energy changes that occur during chemical reactions and phase changes (changes of state)

The Nature of Energy

OEnergy is the ability to do work or produce heat

OHeat (q or Q) is energy; flows due to temperature differences (always hot to cold)

System System

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Law of Conservation of Energy

O Energy cannot be created nor destroyed; only converted into different types.

Kinetic vs. Potential Energy

OTwo main types of energy – kinetic and potentialO Kinetic – energy of motionO Potential – energy due to position or energy

stored in chemical bondsO Chemical potential energy - the energy stored in a

substance because of its composition O Example: gasoline

Kinetic vs. Potential Energy

OTwo main types of energy—kinetic and potentialO Kinetic—energy of motionO Potential—energy due to position or energy

stored in chemical bondsOChemical potential every—the energy stored

in a substance because of its compositionOExample: gasoline

Temperature vs. HeatOTemperature is a measure of the motion in

a sample

OTemperature is a measurement of heat.

OHeat is the total energy of molecular motion, dependent upon amount, size, and type of particles. Heat is energy.

Units of HeatO calorie - the amount of heat required to raise

the temperature of one gram of pure water by one degree Celsius

O Calorie – nutritional calorie;

O 1 Calorie = 1000 calories = 1 kilocalories (kcal)

O Joule – SI unit of heat

O 1 calorie = 4.184 J

Converting Energy Units

Calorie/calorie/kilocalorie calorie/Joule

1 C = 1000 cal = 1 kcal 1 cal = 4.184 J

Example 1A cereal has 155 nutritional Calories per serving.

How many calories, kilocalories and Joules is this?

Example 2A person on a diet consumed 1350 Calories in one day.

How many calories, kilocalories and Joules is this?

System and SurroundingsOUniverse = system + surroundings

OSystem – the specific part of the universe you wish to study. In chemistry this is your chemical reaction/physical process

OSurroundings – Everything else in the universe

OWhen heat is transferred it can flow in or out of the system

Endothermic vs. ExothermicOAn Exothermic process is one that

releases heat to its surroundings (feels warm)

OEnergyproducts < Energyreactants

OAn Endothermic process is one that absorbs heat from the surroundings (feels cold)

OEnergyproducts > Energyreactants

Exothermic Process

Potential

Eproducts < Ereactants

Endothermic Process

Potential

Eproducts > Ereactants

Q and heat flow

OExothermic process, heat is released, q is negative (-)

OEndothermic process, heat is absorbed, q is positive (+)

Specific Heat = Cs

O Specific heat of a substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius.

O Unit for specific heat is J/goC

O Each substance has a different specific heatO Water = 4.184 J/goCO Gold = 0.129 J/goCO Copper = 0.386 J/goC

O The lower the specific heat the lower the amount energy is required to raise its temperature.

Calculating Heat Released and Absorbed

O Q = mCs(T)

O Q = Heat (Joules or kilojoules)O m is mass (gram)

O T is temperature change (°C) Tfinal -Tinitial

O Cs is specific heat at a constant pressure (J/g°C)

Example 3If the temperature of 56.6g of ethanol increases from 45.0°C to

80.0°C, how much heat has been absorbed by the ethanol? Specific heat of ethanol =2.44J/g°C

Example 4A 4.00g sample of a substance was heated from 274K to 314K

and absorbed 32J of heat, what is the specific heat of the substance?

Example 5If 98000J of energy are added to 6200g of water, what will the

change in temperature of the water be? Specific heat of water is 4.184 J/g°C