Chem 12

Thermochemistry: An Intro

What is thermochemistry? The study of the

energy involved in chemical reactions

Law of Conservation of Energy: The total energy in the universe is constant

∆Euniverse = 0

But energy can be transfered from one substance to another

Kinetic Molecular Theory and Energy:

Kinetic molecular theory: substances are composed of particles that are continually moving and colliding with other particles.

Kinetic energy: energy of motion Potential energy: energy that is

stored

Ex: Think about a rock….

Heat is a form of energy

SI Units: joule (J)

Cont’d… When these collisions occur, a chemical

reaction may also occur resulting in the breaking and forming of chemical bonds

This requires energy!

The average kinetic energy of all the particles is proportional to the temperature

The higher the temperature the faster the particles move, the harder the collisions between particles.

But what is temperature???

Temperature (T ) A measure of the average kinetic

energy of the particles that make up a substance or system

A temperature change is explained as a change in kinetic energy

Temperature depends on the quantity of heat (q) flowing out or in of the substance.

Measured with a thermometer

Heat (q):Refers to the transfer of kinetic energy

between objects with diff temps

Units = joules (J)

1st law thermodynamics can be expressed as:

qsystem = - qsurroundings

First Law of Thermodynamics

Any change in the energy of a system is equivalent by an opposite change in the energy of the surroundings.

ΔEsystem = -ΔEsurroundings

According to this law, any energy released by a system is gained by the surroundings, and vice versa

So for heat, qsystem = -qsurroundings

A negative sign denotes heat lost (exothermic), while a positive sign denotes heat gained (endothermic)

Specific Heat Capacity: Amount of heat that can be transfered

depends on the type of substance SHC = the energy required to change 1

gram of the substance by 1°C. Units: J/g ∙ °C

Specific heat capacity formula:

q = m ∙ c ∙ ∆T

Remember:

∆T = Tf - Ti

Example: 15 g of ice was added to 60g of water. The

initial temp of water was 26.5 oC, the final temp of the mixture was 9.7 oC. How much heat was lost by the water?

q=mc ∆Tq=(60g) (4.18 J/g oC) (9.7-26.5 oC)q= - 4213.44 J

Heat Capacity (C):

Water in bathtub has higher heat capacity then water in tea cup due to greater mass

C = m x c C heat capacity KJ/°C m mass (kg) c SHC KJ/Kg ∙ °C

Heat Capacity cont… Can use heat capacity to determine heat

associated with temp change

q = C∆T

Try it :Read pages 626-635Practice Problems: (pg 634)1-4,(pg 636) 8, 11-14 (pg 637)