Ch. 5 Thermochemistry

Energy

•Energy is the ability to do work or transfer heat.

Thermodynamics

The study of energy and its transformations.

Thermochemistry

The study of chemical reactions and the energy changes that involve heat

•The most important form of potential energy in molecules is _____

electrostatic potential energy

Most common unit of energy

Joule

Electrostatic attraction

•Electrostatic attraction is seen between oppositely charged ions.

•Energy is released when…

•Energy is released when chemical bonds are formed;

Energy is consumed when…

Energy is consumed when chemical bonds are broken.

First Law of Thermodynamics

•Energy can be converted from one form to another, but it is neither created nor destroyed.

Ex: sunlight

System

Whatever is being studied

Surroundings

Controlled part of experiment that is not being studied

Open System:

A region of the universe being studied that can exchange heat and mass with its surroundings.

Closed System

A region of the universe being studied that can only exchange heat with its surroundings (not mass)

Isolated System

A region of the universe that can not exchange heat or mass with its surroundings

Internal Energy

The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we use E to represent it.

If the final amount of energy is greater than the initial…

The system absorbed energy from the surroundings

If the final amount of energy is less than the initial…

The system released energy to the surroundings

q

heat

w

work

Thermodynamic quantities

1) A number

2) A unit

3) A sign

Endothermic

When heat is absorbed by the system from the surroundings

Delta H is positive

Exothermic

When heat is released by the system into the surroundings

Delta H is negative

Enthalpy

The internal energy plus the product of pressure and volume

Enthalpy of reaction is also called the…

Heat of reaction

Three Statements About Enthalpy

1.Enthalpy is an extensive property.

2. The Enthalpy change for a reaction is equal in magnitude but opposite in sign to delta H for the reverse reaction

3. Enthalpy change for a reaction depends on the states of the reactants and products

Calorimetry

The measurement of heat flow

Calorimeter

•The instrument used to measure heat flow

Heat capacity

Amount of energy required to raise the temperature of a substance by 1 K (1 degree Celsius)

Specific heat

If the amount of energy required to raise the temperature of a substance by 1 K (1 degree Celsius) is one gram

Molar heat capactity

If the amount of energy required to raise the temperature of a substance by 1 K (1 degree Celsius) is one mole

Bomb Calorimetry

Reactions can be carried out in a sealed “bomb” such as this one.

The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction.

Hess’s Law

If a reaction is carried out in a series of steps, delta H for the overall reaction equals the sum of the enthalpy changes for the individual steps.

An enthalpy of formation

Enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.

Bond Enthalpy

-The enthalpy associated with breaking one mole of a particular bond in a gaseous substance.

-Always positive because energy is required to break chemical bonds

-Energy is always released when a bond forms between gaseous fragments.

-The greater the bond enthalpy, the stronger the bond.

Fuel value

The energy released when one gram of food is combusted is its fuel value.

The vast majority of the energy consumed in this country comes from ___.

Fossil fuels.

Renewable energy sources,

Renewable energy sources, like solar, wind, geothermal, hydroelectric, and biomass sources produce 9.9% of the U.S. energy needs.