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Warm Up
Can you explain why the colder
block melts the ice faster?
Colder block is actually the same
temperature as warmer block …
both are at room temperature.
Block made of metal simply conducts
heat faster. Heat flows from your
hand faster into it and out of block
into ice faster as well.
The Flow of Energy – Heat
Chapter 8.1
Introduction to Thermochemistry
• Thermochemistry (or Thermodynamics) is
the study of heat, energy, and temperature
changes that occur during chemical reactions
(One of the main courses of study in Physical
Chemistry)
• Law of Conservation of Energy (1st Law
of Thermodynamics) – The law of
conservation of energy states that energy may
neither be created nor destroyed. Therefore
the sum of all the energies in the system is a
constant. (Whether in a reaction flask or the
universe)
James Joule (1818-1889)
Joule studied the nature of heat, and
discovered its relationship to mechanical
work. This led to the Law of conservation of
energy, and this led to the development of
the First law of thermodynamics. The SI
derived unit of energy, the joule, is named
for James Joule. He worked with Lord Kelvin
to develop the absolute scale of
temperature. Joule also made observations
of magnetostriction, and he found the
relationship between the current through a
resistor and the heat dissipated, which is
now called Joule's first law.
Temperature vs. Heat
• Temperature is the measure of the average
kinetic energy of the particles in a substance
– Measure using a thermometer
– Rises when heat is added
– Falls when heat is removed
• Heat (q) is the energy that is transferred between
two objects
– Cannot be measured directly
– Can only detect the changes caused by heat
Thermochemical Reactions
• System – reactants and products
• Surroundings – the reaction container
Exothermic Rxn
Endothermic Rxn
Heat/Energy flows out
of the system
Heat/Energy flows
into the system
Heat/Energy is a
product
Heat/Energy is a
reactant
q is negative
q is positive
Exothermic vs. Endothermic
Exothermic
Heat
Endothermic
Heat
State Properties
The state of a system is described by giving its …
-composition
-temperature
-pressure
-energy
-entropy
-volume
These above are some examples of state properties
State properties depend only on the state of a system,
not the way the system reached that state.
Measuring Heat
• The Joule (J) is the SI unit of heat and energy
• The calorie (cal) is also used to measure heat
capacity and is defined by the amount of heat
needed to raise the temperature of 1g of pure
water by 1C
• 1 J = 0.2390 cal
1 cal = 4.184 J
1 kcal = 4.184 kJ
• NOTE: The energy contained in food is measured in
Calories (Cal) = 1000 calories = 1 kcal
Hmmmm…..
Which of these pictures contains more heat?
Heat Capacity vs Specific Heat
• Heat capacity (C) is the amount of heat needed to
raise the temperature of a substance by 1C
q = C x Dt (Dt = tfinal – tinitial) (units J/C or J/K)
• Specific heat (c) is the amount of heat needed to
raise the temperature of one gram of a
substance by 1C
q = c x m x Dt (Dt = tfinal – tinitial) (units J/gC or J/gK)
Examples of specific heat capacities
Susbtance Specific heat
(cal/gc)
Water
1.00
Ice
0.50
Steam
0.40
Paper
0.33
Aluminum 0.21
Sand
0.19
Iron
0.11
Copper
0.09
Gold
0.03
Homework
Read 8.1 and 8.2
Problems 1, 3, and 5
Due tomorrow