CHAPTER 10 ENERGY
NATURE OF ENERGY
Energy – the ability to do work
May be potential or kinetic
Potential energy is due to position or
composition
Kinetic energy is due to motion depends on
mass and velocity KE=1/2 mv2
TEMPERATURE AND HEAT
Temperature is a measure of the average
kinetic energy of the random motions (thermal
energy) of the components of a substance.
Heat is the flow of energy due to temperature
differences – flows from high to low
Exothermic – energy released
Endothermic – energy absorbed
THERMODYNAMICS
The study of energy
Law of conservation of energy is often
referred to as the First Law of
Thermodynamics
Thermodynamic quantities have 3 parts: a
number indicating magnitude of change, a
sign ( + or -) indicating the direction of
change, and a unit (J, kJ, cal, kcal)
ENERGY CONVERSIONS
Energy (heat) may be expressed in joules,
kilojoules, calories or kilocalories.
1 calorie (cal) = 4.184 joules (J)
How many joules in 60.1 calories?
HOW MANY CALORIES IN 28.4 J?
SPECIFIC HEAT CAPACITY
Defined as the amount of energy required
to change the temperature of 1 gram of a
substance by 1oC
Different substances have different
specific heat capacities. Metals tend to
have a low specific heat capacity compared
to water – they tend to heat quickly and
cool quickly, while water does not.
CALCULATING ENERGY CHANGES
How much energy is required to heat 25 g of
liquid water from 25oC to 100.oC? Specific heat
capacity of liquid water is 4.18 J/goC.
Q = mcΔT
Q (energy) = m(mass) x c(specific heat) x ΔT
(change in temperature = Tf – Ti)
Q = (25 g)(4.18 J/goC)(100.o – 25oC)
If ΔT is positive, Q will be positive. If ΔT is
negative, Q will be negative.
How many joules of heat are given off when 5.0
g of water cools from 75oC to 25oc? (Specific
heat of water = 4.18 J/goC)
HEATING CURVE OF WATER
ENERGY DURING PHASE CHANGE
Energy is increasing/decreasing during phase
changes even though temp. remains constant.
Q = mass(m) x heat of fusion (Hf) for melting
and freezing
Q = mass(m) x heat of vaporization (Hv) for
boiling and condensing
Values will be positive for melting and boiling
Values will be negative for freezing and
condensing
How many calories are given off when 85 g of
steam condense to liquid water? (Hv= 539.4
cal/g)
How many joules does it take to melt 35g of ice
at 0oC? (Hf = 333 J/g)
How many joules are required to convert 10.0g
of ice at -10.0oC to steam at 150. oC? (specific
heat of ice = 2.06 J/goC and specific heat of
steam = 2.03 J/goC) A five step problem!
YOUR ASSIGNMENT:
Practice problems are in the yellow boxes.
Page 329 Practice Problem 10.2
Page 330 Practice Problem 10.3
Page 332 Practice Problem 10.4
Page 333 Section 10.2 Review Questions
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