AP Chemistry - Thermochemistry 1

definitions and equations

Thermochemistry

Thermochemistry

the study of energy and its interconversions chemical reactions and physical processes.

Energy

Capacity to do work or produce heat

Kinetic Energy Of Motion

½ mass velocity squared; the energy an object possesses due to its motion.

KE = 3RT/2

Potential Energy

Energy that can be converted to useful work or energy die to the composition toits position or state.

ON EARTH potential energy

PE = (mass)(height of object)(g from a ref.pt)

Acceleration due to gravity - 9.81 meter/sec squared.

Heat

Involves the transfer of energy between 2 objects

Due to a temperature difference

• Represented by q

• Energy measures in joules ( 1 calorie = 4.184 joules)

• J = kg x m²/s²

• 1 kJ = 1000 J

work

force x distance something is moved

State Function

a property independent of pathway

• sometimes called intensive property

• this property does not depend on amount of substance.

Examples of State Functions

• energy change of a system

• specific of a substance

• temperature

• density

Extensive Property

depends directly on the amount of substance present

Example of Extensive Property

• Mass

• heat

• work

• length

Universe

made of a system and the surroundings

System

the reaction we are focusing on

Surroundings

everything else in the universe not in the system. In a solution, the solvent is part of the surrounding.

Exothermic

energy in the form of heat flows OUT of the system

Energy is released to the surrounding as heat

Endothermic

energy as heat flows INTO a system

Energy is absorbed from the surroundings as heat

specific heat

The energy required to raise the temperature of one gram of a substance 1 degree Celsius

ex. specific heat water = 4.184 J/CxG

Calorie

amount of heat required to raise 1 gram of pure water 1 C

• (note : one nutritional Calorie is kcal and begins with a capital C)

• 1 calorie = 4.284 J

Molar Heat Capacity

the energy required to raise the temperature of one mole of substance by 1 degree Celsius

Enthaply

• heat content of a system at constant pressure

• State function

• Most cases enthalpy is equal to q at constant pressure

Work

force x distance something moves

• Distance is in meters

In a cylinder

• Work done by the piston and cylinder = (P)(volume of the cylinder)

• (P)(V) = (F)(D) - both measure energy because both are equal to work

***** FOR AN IDEAL GAS*******

• work for closed system = -P V at constant pressure

Gas may expand

If a system expands = positive work on surrounding and negative work of the system.

The system Is releasing (exothermic) heat energy and the surrounding Is absorbing (endothermic) the heat energy.

• -w = P V or w = -P V

A gas may contract

• negative work on surroundings and positive work on the system.

• w = -P V

the surrounding is releasing (exothermic) heat energy and the system is absorbing (endothermic) the heat energy.

Sign convention

In thermodynamics you must have a sign + a number and a unit.

• the sign represents the system pov

Internal Energy of System

• This is the sum of the kinetic and potential energies of all the particles of the system

  •  E = q + w

  • change in energy = heat + work

The First Law Of Thermodynamics

the energy of the universe is constant