Energy• The ability to do work or produce heat

• Exists in 2 basic forms: – Potential: energy due to position or composition– Kinetic: energy of motion

• Chemical systems contain both PE & KE

(KE-from the random motion; PE-from the number & type of atoms, how they’re bonded and the way the atoms are arranged)

• Most of the energy we use comes from a chemical rxn (respiration & food)

Law of Conservation of Energy

• Similar to the conservation of mass

• States that in any chemical rxn or physical process, energy can be converted from one form to another but it is neither created nor destroyed.

• Einstein put a new wrinkle in both of these laws with his theory E = mc2

Chemical Potential Energy• Energy stored in a substance because of its

composition

• Affected by type of atom, type of bonds, number of bonds, and arrangement of atoms

• Much of all chemical PE is released as heat (q)

• Even a well-tuned car is only about 25% efficient with the rest being lost to heat

Heat• Energy that is in the process of flowing from a

warmer object to a cooler object (so really no such thing as cold)

• Units for heat:

calorie = amount of heat required to raise the temperature one gram of pure water by 1o C

joule = measures mechanical energy (kg.m2/s) but converts to calories (1 cal = 4.12 J)

BTU = energy required to raise 1# of water by 1o F, from 63o to 64o Farenheit

Units of heat• Unfortunately calories, Calories, joules and

kilojoules are all commonly used so need to be able to convert (Table 16.1 p. 491)

Examples

A fruit and oatmeal bar contains 142 Calories. Convert this energy to calories.

An endothermic process absorbs 138kJ. How many calories of heat are absorbed?

Specific Heat• The amount of heat required to raise the

temperature of one gram of that substance by 1o C.

• Each substance has its own specific heat.

• Metals tend to heat quickly (& cool quickly) -they have lower specific heat (c) but water, glass, ceramic all have higher c values

• Helps to explain why dark green grass is cooler than a white sidewalk on a hot day and the moderating effect of lakes, etc.

Calculating c• In the construction of bridges and

skyscrapers, gaps must be left between beams for heating & cooling. Specific heat is an important part of calculating this expansion & contraction.

• If a sample of iron with a mass of 10.0 g changed from 50.4o C to 25.0o C and released 114 J of heat, what is iron’s specific heat?

114 J = 10.0 g x (50.4o – 25.0o) x CC = .449 J/g.Co

Try It

• If the temperature of 34.4 g of ethanol increases from 25.0o C to 78.8o C, how much heat has been absorbed by the ethanol?

• 4516 J or 4.5 kJ