CHEM 1A03 LECTURE 11 – THERMOCHEMISTRY
October 31 2012
Quiz 2 Content: Test 1, Chapter 15, 5 and 20.
Test 2 Content: Fund. Skills, chapter 8,9,10 (6 marks)
Chapter 15(6) 5(6) 20(6), 7(4)[up to slide 25) and Lab 2+3
Complex sequence of exothermic chemical reactions between a fuel and an oxidant (typically
• Rapid combustion: accompanied by the production of heat or both heat and light in the
form of either a glow or flames.
• Slow Combustion: takes place at low temperatures. Respiration is an example of slow
The higher the heat of combustion, the better the fuel.
Heat of Combustion: Energy release by combusting materials.
Photosynthesis is an endothermic process. Sunlight is used to build glucose and other simple
sugars. ∆H = +2.8x10 kJ/mol
Energy is extracted by combustion of the plant material
As the forward process is endothermic, the reverse reaction (respiration) is exothermic.
Issues with Burning Fossil Fuels
Non-renewable (took >300 million years to develop)
Contributes to Global Warming – Net CO in2rease in atmosphere
System versus Surroundings
Open system: material and energy flow
Closed system: only energy exchange
Isolated system: neither material nor energy exchange
Energy: the capacity to do work Potential Energy, Kinetic Energy, Thermal Energy (KE of molecules, atoms)
Heat (q): energy transferred between a system and its surroundings as a result of a temperature
Heat Capacity (C): The amount of heat required to change the temperature of a system by 1
degree (J/ C)
Thermal energy is expressed as a molecule’s internal motions. Molecular-level complexity is
correlated to heat capacity. Energy available from these internal degrees of freedom (rotational,
translational, vibrational) contributes to a substance’s specific heat capacity.
Capacity to Store Heat
q = mc∆T = C∆T
Heat Capacity (C): The quantity of heat (q) required to change the temperature of a system by
one degree. Mass times specific heat capacity compounded.
Specific heat capacity: system is 1 g of material
Molar heat capacity: system is 1mol of material.
Sample Problem: A 100.0 g copper sample (specific heat = 0.385J.g-1.0C-1) at 100.0 C is
added to 50.0 g water, at 26.5oC. What is the final temperature?
Heat goes out of one system and into another.
qwater -q copper
mc(T -T) = -mc(T -T) NOTE :=> T will be equal for both copper
F I water F I copper F
Rearrange for T F.
Heat of Reaction: q rxn
The quantity of heat exchanged between a system and its surroundings when a chemical
reaction occurs within the system at constant temperature.
qrxn< 0 exothermic reaction (heat produced)
qrxn0 endothermic reaction (heat required)
qsystem 0 isolated system: thermal energy transferred between components of the system.
Endothermicity does not necessitate nonspontaneity exothermic reactions are not indicative of
Bomb calorimetry (Constant Volume)
qcalC cal ∆T
qcal= amount of heat transferred to surroundings by system. This is equal to amount of heat
given off by reaction. qsystem 0 in bomb calorimetry; in this case, the surroundings is everything OUTSIDE of the
Coffee Cup Calorimetry – Constant Pressure
Rather than gauging temperature change with a thermometer, we will use the phase change of
ice instead. Ice, in llbm with water is at 0 C as long as ice is still present. Therefore, temperature
does not begin to change until ice is completely gone.
Enthalpy Change, ∆H and Phase Change
When water boils or ice melts, what is the temperatur