Chapter 8: Energy

Energy, Work, and Heat

• Thermodynamics - the study of energy and temperature changes

Energy

• Energy - the ability to do work

• Potential energy - stored energy

• Kinetic energy - energy of motion

Types of Energy

• Heat - the transfer of kinetic energy

• Work - the transfer of energy from one form to another

Units of Energy

• 1 joule (J) = 1 kg.m²/s²

• 1 calorie ( c ) = 4.184 J

• 1,000 calories = 1 kcal = 1 Calorie

Change in Energy

• $\Delta E=q+w$

• $\Delta E$ = Change in energy

  • $\Delta$ = final - initial

• q = heat released

• w = work done

Endothermic and Exothermic Changes

• Exothermic change

  • Releases heat energy

  • Heat/energy is a product

  • Feels hot

• Endothermic change

  • Absorbs energy

  • Energy/heat is a reactant

  • Feels cold

Systems and Surroundings

• System - the part of the universe being studied (reaction)

• Surroundings - the rest of the universe (everything else)

• Ice example

  • Ice is the system

  • Ice absorbs heat from the surroundings

Direction of Energy and Work Changes

• EXOTHERMIC CHANGE

  • Releases heat energy

  • “loses” (-)

  • -q

• ENDOTHERMIC CHANGE

  • Absorbs heat energy

  • “gains” (+)

  • +q

• SYSTEM DOES WORK on surroundings

  • Releases energy

  • “loses”

  • -w

• SURROUNDINGS DOES WORK on system

  • Absorbs energy

  • “gains”

  • +w

The Law of Conservation

• Energy cannot be created or destroyed.

• It can change forms and be transferred.

• $\Delta E$ _system = -$\Delta E$ _surroundings

  • Equal in magnitude, opposite in sign

Heat Energy and Temperature

• Heat - the total kinetic energy transferred from on substance to another

• Temperature (T) - the average kinetic energy of the particles in a substance

• Specific Heat (s) - the amount of heat required to raise the temperature of 1 gram of material by 1 degrees Celsius

  • Equation:

    • specific heat = heat/(mass)x(change in temperature)

    • $s=\dfrac{q}{m\Delta T}$

    • s = specific heat

    • q = heat

    • m = mass

    • $\Delta T$ = change in temperature

• Heat Capacity (C) - the amount of heat required to raise an object by 1 degree Celsius

  • DOES NOT CONSIDER MASS

  • Equation:

    • heat capacity = heat/change in temperature

    • $C=\dfrac{q}{\Delta T}$

    • C = heat capacity

    • q = heat

    • $\Delta T$ = change in temperature

Calorimetry

• Calorimetry experiments - measure the flow of heat.

  • 1. Coffee cup calorimetry

  • 2. Bomb calorimetry

• Taking hot metal and putting it into water.

• Bomb Calorimetry - measures energy content (heats of reaction) in food and fuel.

  • Equation:

    • $q=C\Delta T$

    • q = heat

    • C = heat capacity

Coffee-Cup Calorimetry

Heat Energy and Chemical Reactions

• Chemical reactions involve changes in energy.

• The energy of a reaction is an extensive property.

  • Relies on how much matter is involved/ amount of substance.

Fuel Value

• Fuel value - the amount of energy that can be produced by the combustion of a material (combustion reaction).

Reaction Enthalpy $\Delta H_{rxn}$

• Reaction enthalpy - the amount of heat energy absorbed or released in a chemical reaction at constant pressure.