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### Energy

L

A tale of two types

Exothermic

Potential Energy

Kinetic Energy

Energy of motion

Measurement of Temperature

°C – relative

K - actual

Endothermic

- Stored energy
- Based on composition or location
- Measured with:
- Measured in:
- Calories (cal)
- Joules (J)

L

The Law of Conservation of Energy

- Energy can neither be created nor destroyed, it can only change forms
- Energy is the ability to do work or produce heat

Specific Heat (c)

- The amount of energy to raise 1g of a substance by 1°C
- Unique to a substance
q = mcDT

q = heat (J or cal)

m = mass (g)

c = specific heat (J/(g°C) –or-- cal/(g°C))

DT = change in temperature (°C)

Specific Heat continued

The temperature of a sample of iron with a mass of 10.0g changed from 50.4°C to 25.0°C with the release of 114 J heat. What is the specific heat of iron?

q = mcDT

114 J = 10.0g (c) (50.4°C – 25.0°C)

114 J = 254 g°C (c)

0.449 j/g °C

Practice

- If the temperature of 34.4g of ethanol increases from 25.0°C to 78.8°C how much heat will be absorbed by ethanol? (cethanol = 2.44 j/g°C)
- A 4.50g nugget of pure gold absorbed 276J of heat. What was the final temperature of the gold if the initial temperature was 25.0°C? (cgold = 0.129J/g°C)

- 4520 J
- 500 °C

Vocabulary

If this is a reaction “you are part of the system”

- System – where the reaction is taking place and the elements involved
- Surrounding – everything not the system

Types of Reactions

Endothermic

Exothermic

Releases energy

System heat decreases

Surrounding heat increases

Bond formation is always exothermic

Therefore energy is a product

Conversion of PE to KE

- Absorbs energy
- System heat increases
- Surrounding heat decreases

- Bond breaking is always endothermic
- Therefore energy is a reactant

- Conversion of KE to PE

In order for a reaction to proceed it must overcome the AE

Heat of Products

Heat of reaction

Heat of Reactants

- ΔHreaction = Hproducts - Hreactants
- H = heat (J)
- ΔH is the change in the heat of the system under constant pressure.

- Exothermic: –ΔH.
- Makes kinetic energy

- Endothermic: +ΔH.
- Uses kinetic energy

Endothermic

- Potential Energy

- Kinetic Energy

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