THERMO-CHEMISTRY

Thermochemistry

Study of the heat changes that occur during a chemical

reaction

EnergyThe capacity to do work or generate

heat

In chemistry, energy is stored

in chemical bonds

When bonds are made, energy is released. When

bonds are broken, energy is absorbed.

The main form of energy in

chemistry is HEAT!

Heat (q)Energy that is

transferred between objects that have a

temperature difference

Heat always moves from a

higher temp to a lower temp

Terms to know: universesystem

surroundings

Law of Conservation of Energy

In any process, energy is neither

created nor destroyed

Some reactions absorb energy

(heat) and some give off energy

(heat).

Exothermic Reaction

A reaction that releases heat into the surroundings

(heat exits)

Endothermic Reaction

A reaction that absorbs heat from the surroundings

(heat goes en)

Enthalpy Change (H)

Amount of heat lost or gained in a

reaction(unit = kJ)

Sign of H

Process Heat

Positive Endo-thermic

Absorbed

Negative Exo-thermic

Released

Temperature (T)

A measure of the average kinetic energy

of the particles in a substance

Heat (Q)Energy that is

transferred between objects that have a

temperature difference

Heat (Q)Is measured in Joules (J)

(or kJ)

1 kJ = 1000 J

So…how do we measure heat?

We can only measure changes in heat

(absolute heat doesn’t exist)

Specific HeatAmount of heat needed to

raise the temp of 1g of a substance 1C

gC

J

Unit =

The Heat EquationQ = mCpT

Q = heat (joules-J)m = mass (grams)

Cp = specific heat (J/g C)T = change in temp (C)

How much heat is needed to raise the temperature of 350

grams of water from 0.0°C to 25°C?

37000 Joules

A 450 gram sample of water cools from

55°C to 10.0°C. How much heat is

lost?

85000 Joules

A 15.8-gram piece of zinc increases in temperature

from 5.5°C to 6.9°C when 8.5 Joules of heat are added. What is the

specific heat of zinc?

0.38 J/g°C

calorie (cal)Quantity of heat

needed to raise the temp of 1g of pure

water 1C

How much heat is needed to increase the

temperature of 10g H2O from 35C to

45C?100 cal

1000 calories = 1 Calorie

(Calorie is the kind on food labels, etc)

1 calorie = 4.184 Joules

Calorimeter

An insulated chamber used to measure the

amount of heat absorbed or released

during a reaction

When heat energy is added, the molecules move faster and

faster, increasing the temperature

What happens when the phase is not changing:

When heat energy is added, instead of increasing the temperature (increasing the kinetic energy of the

molecules), the intermolecular bonds are broken,

changing the phase of the substance.

What happens during a phase change:

PHASES OF WATER

Heat of Fusion (Hf)

Amount of heat necessary to convert a given

amount of a solid into a liquid

Q = mHf

Heat of Vaporization (Hv)Amount of heat necessary

to vaporize a given amount of liquid at its

boiling point

Q = mHv