8. Section 6.1
The Nature of Energy
8
Energy
Potential energy – is energy that is stored in an object
or energy due to position or composition.
Kinetic energy – is energy of motion of the object and
depends on the mass of the object and its velocity.
Potential energy
Kinetic energy
thermal energy
is kinetic
9. Some Forms of Energy
• Electrical
kinetic energy associated with the flow of electrical charge
• Heat or Thermal Energy
kinetic energy associated with molecular motion
• Light or Radiant Energy
kinetic energy associated with energy transitions in an atom
• Nuclear
potential energy in the nucleus of atoms
• Chemical
potential energy in the attachment of atoms or because of
their position
10. Units of Energy
• Joule (J) is the amount of energy needed to move
a 1 kg mass a distance of 1 meter
1 J = 1 N∙m = 1 kg∙m2/s2
• calorie (cal) is the amount of energy needed to
raise one gram of water by 1°C
kcal = energy needed to raise 1000 g of water 1°C
food Calories = kcals
Energy Conversion Factors
1 calorie (cal) = 4.184 joules (J) (exact)
12. Section 6.1
The Nature of Energy
Thermal energy is the energy associated with the random
motion of atoms and molecules
13. Heat is the transfer of thermal energy between two bodies that
are at different temperatures.
Energy Changes in Chemical Reactions
Temperature is a measure of the thermal energy.
900C
400C
greater thermal energy
Temperature = Thermal Energy
17. Exothermic process is any process that gives off heat –
transfers thermal energy from the system to the surroundings.
Endothermic process is any process in which heat has to be
supplied to the system from the surroundings.
2H2 (g) + O2 (g) 2H2O (l) + energy
H2O (g) H2O (l) + energy
energy + 2HgO (s) 2Hg (l) + O2 (g)
energy + H2O (s) H2O (l)
22. State functions are properties that are determined by the state
of the system, regardless of how that condition was achieved.
Potential energy of hiker 1 and hiker 2
is the same even though they took
different paths.
energy, pressure, volume, temperature
DE = Efinal - Einitial
DP = Pfinal - Pinitial
DV = Vfinal - Vinitial
DT = Tfinal - Tinitial
24. Work is not a state function. Why?
w = F x d
F= Force
d= distance
Dw = wfinal - winitial
P x V = x d3 = F x d = w
F
d2
w = -P DV
different distances different works
Therefore work is path function not state function
27. Section 6.2
Enthalpy and Calorimetry
Heat = (mass) x (specific heat capacity) x (temp. change)
q = m.s.DT
q= Heat (J)
s = specific heat capacity (J/°C·g)
m = mass of solution (g)
ΔT = change in temperature (°C)
27
28. How much heat is given off when an 869 g iron bar cools
from 940C to 50C? s of Fe = 0.444 J/g • 0C
Dt = tfinal – tinitial = 50C – 940C = -89 0C
q = msDt = 869 g x 0.444 J/g • 0C x –890C = -34,000 J
29. How much heat is absorbed by a copper penny with mass
3.1 g whose temperature rises from -8 0C to 37 0C?
s of Cu = 0.385 J/g. 0C
Dt = tfinal – tinitial = 37 0C – (-8 0C) = 45 0C
q = msDt = 3.1 g x 0.385 J/g • 0C x 45 0C = 53.7 J
37. • if a reaction can be expressed as a series of steps, then
the ΔH for the overall reaction is the sum of the ΔHs of
reaction for each step.
1) A + 2B C ΔH1
2) C 2D ΔH2
A + 2B 2D ΔH3= ΔH1+ΔH2
41. Section 6.3
Hess’s Law
Example
Multiply reactions to give the correct numbers of
reactants and products:
4( ) 4( )
3( ) 3( )
Desired reaction:
2 2 2 32 N ( ) 6 H O( ) 3 O ( ) 4 NH ( ) g g g g
2 2 3
2 2 2
1 3
2 2
N ( ) H ( ) NH ( ) H = 46 kJ
2 H O( ) 2 H ( ) O ( ) H = +484 kJ
D
D
g g g
g g g
44. Section 6.4
Standard Enthalpies of Formation
Change in enthalpy that accompanies the formation of
one mole of a compound from its elements with all
substances in their standard states.
44
aA + bB cC + dD
DH0 dDH0 (D)
f
cDH0 (C)
f= [ + ] - bDH0 (B)
f
aDH0 (A)
f
[ + ]
DH0
nDH0 (products)
f= S mDH0 (reactants)
f
S-