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Energy
Kinetic energy (KE): energy of motion
KE = ½ mV2 V: velocity
Potential energy (PE): stored energy
Law of conservation of energy
(Position & Composition)
Kinetic and Potential Energy
A
BB
A
Kinetic energy Potential energy
Change in potential energy for ball A (change in level)
Work: change the position of ball B
Heat: friction between surface & ball
Frictional heating (heat is a form of energy).T of hill increases
Heat and Temperature
Temperature: measure of random motions of the components of substance.
T ↑ move fasterKinetic energy ↑
Energy is transferred from high T to low T.
Heat: Flow of energy due to a T difference.
Heat and Temperature
ColdWater
HotWater
Tfinal =Thot initial + Tcold initial
2
Heat
units of heat: calorie (cal) English system
joule (J) SI system
1 cal = 4.184 J
Joule: Energy (heat) required to raise T of one gram of water by 1C.
Amount of heat = specific heat × mass × change in temperature
Amount of heat = SH × m × (T2 – T1)
T2 = final temperature
T1 = initial temperature
SH = Specific heat capacity (cal/g °C)
Heat
Heat of reaction
C3H8(s) + 5O2(g) 3CO2(g)+ 4H2O(l) + heat (energy)
2HgO(s) + heat (energy) 2Hg(l) + O2(g)
Endothermic reaction
Exothermic reaction
All combustion reactions are exothermic.
Heat of reaction
Surroundings Surroundings
System System
Energy Energy
Exothermic Endothermic
(PE)
Reactant
Product
Energy released to the surroundings as heat
Surrounding
Exothermic (burning)
Thermodynamics
Thermodynamics: study of energy
Law of conservation of energy: energy of the universe is constant.
Internal energy (E): sum of the kinetic and potential energies.
E = q + w “delta”: change
WorkHeat
The first law of thermodynamics:
SurroundingsSurroundings
SystemSystem
EnergyEnergy
ExothermicEndothermic
E 0E 0
Thermodynamics
E = q + w
Energy flows into system via heat (endothermic): q = +x
Energy flows out of system via heat (exothermic): q = -x
Enthalpy
For a reaction in constant pressure,the change of enthalpy is equal to energy that flows as heat.
Hp = heat
Constant pressure
“-” heat or Hp: exothermic: heat flows out of the system.
“+” heat or Hp: endothermic: heat flows into the system.
Enthalpy (Thermochemistry): heat of chemical reactions.
Calorimetry
Calorimeter:
A device to measure the heat energy released or absorbed by a reaction.
T H
Hess’s Law
State function: a property of system that changes independently of its pathways.
Enthalpy is a state function.
In a chemical reaction, change of enthalpy is the same whether the reaction takes place in one step or in a series of steps.
N2(g) + 2O2(g) 2NO2(g) H1 = 68 kJ
N2(g) + O2(g) 2NO(g) H2 = 180 kJ
2NO(g) + O2(g) 2NO2(g) H3 = -112 kJ
N2(g) + 2O2(g) 2NO2(g) H2 + H3 = 68 kJ
1 Step
2 Steps
Two rules about enthalpy
1. If a reaction is reversed, the sign of H is also reversed.
N2(g) + 2O2(g) 2NO2(g) H1 = 68 kJ
2NO2(g) N2(g) + 2O2(g) H1 = -68 kJ
2. If the coefficients in a balanced reaction are multiplied by an integer, the value of H is also multiplied by the same integer.
N2(g) + 2O2(g) 2NO2(g) H1 = 68 kJ
2N2(g) + 4O2(g) 4NO2(g) H1 = 2 68 kJ = 136 kJ 2
Law of conservation of energy
Quality-Quantity of Energy
Why are we concerned about energy?
Gasoline + O2 CO2 + H2O + energy
Spread in universe
Concentrated energy Spread energyUse of energy to do work
Quantity Quality Heat death
Energy and Our World
Sun
Woody plants
Coal
Natural gas
Petroleum
Photosynthesis Source of
energy
6CO2 + 6H2O + energy of sun C6H12O6 + 6O2
Photosynthesis
glucose
1. Natural gas – 90 to 95 percent methane. – 5 to 10 percent ethane, and a mixture of other low-boiling alkanes.
2. Petroleum– A thick liquid mixture of thousands of compounds, most of them
hydrocarbons.
Energy and Our World
Fossil Fuels: formed from the decomposition of marine plants and animals.
(C5-C10)
(C10-C18)
(C1-C4)
(C15-C25)
(C25)
3. Coal
– Was formed from the remains of plants that were buried (under high P and T).
– 20% of our energy.
– Expensive, dangerous, and produces pollution (CO & SO2).
Energy and Our World
Driving forces
Energy spread: concentrated energy is dispersed widely.
(Exothermic process)
Matter spread: molecules of a substance are spread out and occupy a larger volume.
Dissolving is endothermic process,but because of matter spread, it occurs.
heat
Entropy (S)
A measure of disorder or randomness.
Energy spread Faster random motions of the molecules in surroundings.
Matter spread Components of matter are dispersed (occupy a larger volume).
A Spontaneous process is one that happens in nature on its own.(because of increasing entropy)
Dissolving
The second law of thermodynamics:
The entropy (S) of the universe is always increasing.
We run towards a disorder (heat death of universe).