Transcript Chapter 10 - Bakersfield College

Chemistry B2A
Chapter 10
Energy
Energy
Universe
Matter
Energy
Empty space
Energy: ability to do work or produce heat.
Energy
Kinetic energy (KE): energy of motion
KE = ½ mV2
V: velocity
Potential energy (PE): stored energy
(Position & Composition)
Law of conservation of energy
Kinetic and Potential Energy
Kinetic energy
Potential energy
A
B
A
Change in potential energy for ball A
(change in level)
T of hill increases
B
Work: change the position of ball B
Heat: friction between surface & ball
Frictional heating (heat is a form of energy).
Heat and Temperature
Temperature: measure of random motions of the components of substance.
T↑
move faster
Kinetic energy ↑
Heat and Temperature
Cold
Water
Hot
Water
Energy is transferred from high T to low T.
Heat: Flow of energy due to a T difference.
Tfinal =
Thot initial + Tcold initial
2
Heat
units of heat:
calorie (cal)
joule (J)
English system
SI system
Joule: Energy (heat) required to raise T of one gram of water by 1C.
1 cal = 4.184 J
Heat
Amount of heat = specific heat × mass × change in temperature
Amount of heat = SH × m × (T2 – T1)
SH = Specific heat capacity (cal/g °C)
T2 = final temperature
T1 = initial temperature
Heat of reaction
2HgO(s) + heat (energy)  2Hg(l) + O2(g)
Endothermic reaction
C3H8(s) + 5O2(g)  3CO2(g)+ 4H2O(l) + heat (energy)
Exothermic reaction
All combustion reactions are exothermic.
Heat of reaction
Surroundings
Surroundings
Energy
Energy
System
System
Exothermic
Endothermic
Exothermic (burning)
Surrounding
Reactant
(PE)
Energy released to the surroundings as heat
Product
Thermodynamics
Thermodynamics: study of energy
The first law of thermodynamics:
Law of conservation of energy: energy of the universe is constant.
Internal energy (E): sum of the kinetic and potential energies.
 “delta”: change
E = q + w
Heat
Work
Thermodynamics
E = q + w
Energy flows into system via heat (endothermic): q = +x
Energy flows out of system via heat (exothermic): q = -x
Surroundings
Surroundings
Energy
Energy
System
System
E  0
E  0
Endothermic
Exothermic
Enthalpy
Enthalpy (Thermochemistry): heat of chemical reactions.
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.
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.
1 Step
N2(g) + 2O2(g)  2NO2(g)
H1 = 68 kJ
2 Steps
N2(g) + O2(g)  2NO(g)
2NO(g) + O2(g)  2NO2(g)
H2 = 180 kJ
H3 = -112 kJ
N2(g) + 2O2(g)  2NO2(g)
H2 + H3 = 68 kJ
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.
2
N2(g) + 2O2(g)  2NO2(g)
H1 = 68 kJ
2N2(g) + 4O2(g)  4NO2(g)
H1 = 2  68 kJ = 136 kJ
Quality-Quantity of Energy
Law of conservation of energy
Why are we concerned about energy?
Gasoline + O2  CO2 + H2O + energy
Spread in universe
Concentrated energy
Quantity
Use of energy to do work
Quality
Spread energy
Heat death
Energy and Our World
Woody plants
Photosynthesis
Sun
Source
of
energy
Coal
Natural gas
Petroleum
6CO2 + 6H2O + energy of sun
Photosynthesis
C6H12O6 + 6O2
glucose
Energy and Our World
Fossil Fuels: formed from the decomposition of marine plants and animals.
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.
(C1-C4)
(C5-C10)
(C10-C18)
(C15-C25)
(C25)
Energy and Our World
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).
Greenhouse Effect
Driving forces
Energy spread: concentrated energy is dispersed widely.
(Exothermic process)
heat
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.
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).
The second law of thermodynamics:
The entropy (S) of the universe is always increasing.
We run towards a disorder (heat death of universe).
A Spontaneous process is one that happens in nature on its own.
(because of increasing entropy)
Dissolving