AP* Chemistry
THERMOCHEMISTRY
Terms for you to learn that will make this unit understandable:
Energy (E) – the ability to do work or produce heat ; the sum of all potential and kinetic energy in
a system is known as the internal energy of the system
Potential energy – in chemistry this is usually the energy stored in bonds (i.e., when
gasoline burns there are differences in the attractive forces between the nuclei and the
electrons in the reactants and the products)
Kinetic energy – energy of motion, usually of particles, proportional to Kelvin temperature;
kinetic energy depends on the mass and the velocity of the object: KE = ½ mv2
Law of Conservation of Energy – energy never created nor destroyed
AKA energy of the universe is constant
AKA First Law of Thermodynamics
Heat (q) – transfer of energy in a process (flows from a warmer object to a cooler one – heat
transfers because of temperature difference but, remember, temperature is not a measure of
energy—it just reflects the motion of particles)
Enthalpy (H)– heat content at constant pressure
¾ Enthalpy of reaction (ΔHrxn) – heat absorbed or released by a chemical reaction
¾ Enthalpy of combustion (ΔHcomb) -- heat absorbed or released by burning (usually with O2)
¾ Enthalpy of formation (ΔHf) – heat absorbed or released when ONE mole of compound is
formed from elements in their standard states
¾ Enthalpy of fusion (ΔHfus) -- heat absorbed to melt 1 mole of solid to liquid @MP
¾ Enthalpy of vaporization (ΔHvap) -- heat absorbed to change 1 mole liquid to gas @BP
System – area of the universe we are focusing on (i.e., the experiment)
Surroundings – everything outside of the system
Endothermic – net absorption of energy (heat) by the system; energy is a reactant; (i.e., baking
soda and vinegar when mixed get very cold to the touch)
Exothermic – net release of energy (heat) by the system; energy is a product; (i.e., burning
methane gas in the lab burner produces heat; light sticks give off light which is also energy)
State Function – A property independent of past or future behavior ; (it does not matter which
road brought you to school today–you started at your house and ended here –there are probably
lots of ways for that to happen)
Entropy (S) – measure of disorder in the system (measure of chaos)
Gibb’s Free Energy (G)– criteria for spontaneity and amount of free energy to do work
Thermodynamics – study of energy and its interconversions
Work – force acting over distance
Standard Conditions—you already know about STP, but recall that the T is STP is 0°C and
humans are not happy lab workers when it is that cold! So, think of standard conditions as
standard lab conditions which are 1 atm of pressure, 25°C (much more comfy!) and if solutions are
involved, their concentration is 1.0 M. All of this information is communicated by adding the
symbol ° to G, H or S. So, if you see ΔH°, then you automatically know the pressure, temperature
and conditions that apply to that value!
There has recently been a change in how enthalpy, entropy and free energy units are
expressed. For example, you may see ΔH° values expressed as kJ in older printed material.
Currently, they should be expressed in kJ/mol where the “mol” is “moles of reaction”. See
Jim Spencer’s article on AP Central for additional information.
*AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this product.
© 2008 by René McCormick. All rights reserved.
, ENERGY AND WORK
¾ See definition of energy.
¾ ΔE = q(heat) + w(work)
¾ Signs of q
+q if heat absorbed
–q if heat released
¾ Signs of w (commonly related to work done by or to gases)
♦ + w if work done on the system (i.e., compression)
♦ −w if work done by the system (i.e., expansion)
¾ When related to gases, work is a function of pressure
¾ (pressure is force per unit of area) and Δvolume
w = −PΔV
NOTE: Energy is a state function. (Work and heat are not.)
Exercise 1 Internal Energy
Calculate ∆E for a system undergoing an endothermic process in which 15.6 kJ of heat flows and
where 1.4 kJ of work is done on the system.
17.0 kJ
Exercise 2 PV Work
Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external
pressure of 15 atm.
‒270 L•atm
Thermochemistry 2
THERMOCHEMISTRY
Terms for you to learn that will make this unit understandable:
Energy (E) – the ability to do work or produce heat ; the sum of all potential and kinetic energy in
a system is known as the internal energy of the system
Potential energy – in chemistry this is usually the energy stored in bonds (i.e., when
gasoline burns there are differences in the attractive forces between the nuclei and the
electrons in the reactants and the products)
Kinetic energy – energy of motion, usually of particles, proportional to Kelvin temperature;
kinetic energy depends on the mass and the velocity of the object: KE = ½ mv2
Law of Conservation of Energy – energy never created nor destroyed
AKA energy of the universe is constant
AKA First Law of Thermodynamics
Heat (q) – transfer of energy in a process (flows from a warmer object to a cooler one – heat
transfers because of temperature difference but, remember, temperature is not a measure of
energy—it just reflects the motion of particles)
Enthalpy (H)– heat content at constant pressure
¾ Enthalpy of reaction (ΔHrxn) – heat absorbed or released by a chemical reaction
¾ Enthalpy of combustion (ΔHcomb) -- heat absorbed or released by burning (usually with O2)
¾ Enthalpy of formation (ΔHf) – heat absorbed or released when ONE mole of compound is
formed from elements in their standard states
¾ Enthalpy of fusion (ΔHfus) -- heat absorbed to melt 1 mole of solid to liquid @MP
¾ Enthalpy of vaporization (ΔHvap) -- heat absorbed to change 1 mole liquid to gas @BP
System – area of the universe we are focusing on (i.e., the experiment)
Surroundings – everything outside of the system
Endothermic – net absorption of energy (heat) by the system; energy is a reactant; (i.e., baking
soda and vinegar when mixed get very cold to the touch)
Exothermic – net release of energy (heat) by the system; energy is a product; (i.e., burning
methane gas in the lab burner produces heat; light sticks give off light which is also energy)
State Function – A property independent of past or future behavior ; (it does not matter which
road brought you to school today–you started at your house and ended here –there are probably
lots of ways for that to happen)
Entropy (S) – measure of disorder in the system (measure of chaos)
Gibb’s Free Energy (G)– criteria for spontaneity and amount of free energy to do work
Thermodynamics – study of energy and its interconversions
Work – force acting over distance
Standard Conditions—you already know about STP, but recall that the T is STP is 0°C and
humans are not happy lab workers when it is that cold! So, think of standard conditions as
standard lab conditions which are 1 atm of pressure, 25°C (much more comfy!) and if solutions are
involved, their concentration is 1.0 M. All of this information is communicated by adding the
symbol ° to G, H or S. So, if you see ΔH°, then you automatically know the pressure, temperature
and conditions that apply to that value!
There has recently been a change in how enthalpy, entropy and free energy units are
expressed. For example, you may see ΔH° values expressed as kJ in older printed material.
Currently, they should be expressed in kJ/mol where the “mol” is “moles of reaction”. See
Jim Spencer’s article on AP Central for additional information.
*AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this product.
© 2008 by René McCormick. All rights reserved.
, ENERGY AND WORK
¾ See definition of energy.
¾ ΔE = q(heat) + w(work)
¾ Signs of q
+q if heat absorbed
–q if heat released
¾ Signs of w (commonly related to work done by or to gases)
♦ + w if work done on the system (i.e., compression)
♦ −w if work done by the system (i.e., expansion)
¾ When related to gases, work is a function of pressure
¾ (pressure is force per unit of area) and Δvolume
w = −PΔV
NOTE: Energy is a state function. (Work and heat are not.)
Exercise 1 Internal Energy
Calculate ∆E for a system undergoing an endothermic process in which 15.6 kJ of heat flows and
where 1.4 kJ of work is done on the system.
17.0 kJ
Exercise 2 PV Work
Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external
pressure of 15 atm.
‒270 L•atm
Thermochemistry 2
