Energy

Section 3-3 (somewhat)

Energy

• Physical and chemical changes are always accompanied by energy changes.– Released (Exothermic)– Absorbed (Endothermic)

Energy Transfer

• The most common form is HEAT– The energy transferred as a result of temperature

difference– Represented by q or ΔH– If a system is undisturbed, energy will continue to

transfer until all temperatures are equal.

Energy Units

• Energy is measured in joules (J)– Derived SI Unit

James Prescott Joule

Energy Units

• Sometimes energy is measured in calories (c)– One calorie = 4.184 joules– The “calories” in food are actually kilocalories

(upper case “C”)

A whole bunch of Calories

Energy Units

• Calories can easily be converted to joules: • 1cal = 4.184 joules• Convert 10 calories to joules:• (10 cal) x (4.184 J/cal) = 41.84 J

Practice:

1. Convert 4 calories to joules

2. Convert 9,000 calories to joules.

3. If 4 Thin Mint Girlscout Cookies contain 160 Calories, how many joules of energy do they contain?

Big C!!!

Energy and Chemical Change

• Chemical changes are always accompanied by a change in energy.

• Endothermic Reaction – energy is absorbed by the reaction

• Exothermic Reaction – energy is released by the reaction

• Rule of Thumb: Nature tends to run downhill… Exothermic reactions tend to take place spontaneously (without outside help).

Activation Energy

• All reactions need energy to start reacting – called activation energy– The energy that pushes a reaction to happen

Measuring Energy Changes

Section 3-3Pages 65-69

Calorimeter

• Used to measure the energy given off or absorbed during a chemical or physical change

Specific Heat

• To change the temperature of a substance, heat must be added or removed.– Some things need more heat to change than

others– Water requires 4.184 joules to change 1 degree

celsius– Aluminum requires 0.902 joules– BIG difference!!!

Specific Heat

• Specific Heat (Cp) – The heat needed to raise the temperature of one gram of a substance by one Celsius degree– (Cp of water is 4.184 J/gC°)

• Every substance has its own specific heat– Use Appendix Tables A-3 and A-5 in back of book

Law of Conservation of Energy

• Energy cannot be created nor destroyed– Must be transferred from one substance to

another substance• Heat lost by one thing is gained by another• Two things of different temperatures will

continue to exchange energy until they are both the same temperature

Law of CoE continued

• Here is the relationship:

(heat gained or lost) = (mass in grams)(change in temperature) (specific heat)

q = (m) (ΔT) (Cp)

– To calculate change in temperature: ΔT = Tf – Ti

In a calorimeter

• We add water and measure the change in the temperature of the water to determine the amount of heat released.

• (m)(ΔT)(4.184 J/gC°) = (m)(ΔT)(Cp)

• Must assume that the calorimeter does not absorb or release energy.

Water Other Substance