Chemistry 101
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
T final
T final
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
Food energy is measured in Calories (note the capital C).
1 Cal = 1 kcal = 1000 cal
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
m = mass (g)
T1 = initial temperature
Heat
• Specific heat capacity is
the energy required to
change the temperature of
a mass of one gram of a
substance by one Celsius
degree.
Note: ALEKS uses “c” instead of “SH” to stand for specific
heat capacity.
Heat
Practice 1:
• Calculate the amount of heat energy (in joules)
needed to raise the temperature of 6.25 g
of water from 21.0°C to 39.0°C.
• We are told the mass of water and the temperature increase. We
look up the specific heat capacity of water, 4.184 J/g°C.
Q = SH x m x T
Q = (4.184 J/g°C) x (6.25 g) x (39.0°C – 21.0°C)
Q = 471 J
Heat
Practice 2:
• A silver-gray metal weighing 15.0 g requires 133.5 J to
raise the temperature by 10.°C. Find the heat capacity.
Q = SH x m x T
(133.5 J) = SH x (15.0 g) x (10.°C)
SH = 0.89 J/g°C
Al
Can you determine the
identity of the metal using
Table 10.1?
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
Exothermic = exit!
Heat of reaction
Practice:
• Classify each process as exothermic or endothermic. Explain why.
(The system is underlined.)
Exo
a)
Your hand gets cold when you touch ice.
b)
The ice gets warmer when you touch it.
Endo
c)
Water boils in a stove-top kettle.
Endo
d)
Water becomes ice in the freezer.
Exo
e)
Water vapor condenses on a cold pipe.
Exo
f)
Ice cream melts.
Endo
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
System does work on surroundings: w = -x
Surroundings do work on the system: w = +x
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
Enthalpy
Practice:
S(s) + O2(g)  SO2(g)
ΔH = –296 kJ
• Calculate the quantity of heat released when 2.10 g of sulfur is
burned in oxygen at constant pressure.
2.10 g S x
1 mol S
32.26 g S
0.0655 mol S x
= 0.0655 mol S
– 296 kJ
1 mol S
= – 19.4 kJ
Use the H value like a conversion factor.
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
Midterm 2
• Bring a 2B pencil.
• One 30423 Scantron form (available in Runner Bookstore).
• A calculator (scientific, not graphing).