Thermochemistry Thermodynamics = study of energy and its transformations Thermochemistry = study of chemical reactions involving changes in heat energy © 2009, Prentice-Hall, Inc.

Thermochemistry © 2009, Prentice-Hall, Inc. Energy Energy = the ability to do work or transfer heat energy. Work = energy used to cause an object with mass to move (w = f x d) Heat = energy used to cause the temperature of an object to increase

Thermochemistry © 2009, Prentice-Hall, Inc. Major Types of Energy Potential energy = energy an object possesses by virtue of its position or chemical composition. Kinetic energy = energy an object possesses by virtue of its motion.

Thermochemistry Kinetic Energy m = mass in kilograms (kg) v = velocity in meters per second (m/s) KE = kinetic energy in joules (J) 1 Joule = 1 kg-m 2 /s 2 A mass of 2 kg moving at a speed of one meter per second possesses a kinetic energy of 1 Joule. © 2009, Prentice-Hall, Inc. 1 2 KE =  m v 2

Thermochemistry Potential Energy m = mass in kilograms (kg) g = acceleration due to gravity (9.8 m/s 2 ) h = height (m) PE = potential energy in joules (J) 1 Joule = 1 kg-m 2 /s 2 © 2009, Prentice-Hall, Inc. PE = m g h

Thermochemistry © 2009, Prentice-Hall, Inc. Units of Energy The SI unit of energy is the joule (J). An older, non-SI unit is still in widespread use: the calorie (cal). 1 cal = J 1000 calories = one nutritional Calorie

Thermochemistry © 2009, Prentice-Hall, Inc. First Law of Thermodynamics Energy is neither created nor destroyed, but it can undergo a transformation from one type to another. (Law of Conservation of Energy) The total energy of the universe is a constant. The energy lost by a system must equal the energy gained by its surroundings, and vice versa.

Thermochemistry © 2009, Prentice-Hall, Inc. System and Surroundings System = the molecules we want to study (here, the hydrogen and oxygen molecules). Surroundings = everything else (here, the cylinder and piston).

Thermochemistry © 2009, Prentice-Hall, Inc. Internal Energy The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E.  E = E final − E initial (It’s a state function) If  E is positive, the system absorbed energy from the surroundings. If  E is negative, the system released energy to the surroundings.

Thermochemistry © 2009, Prentice-Hall, Inc.  E = q + w When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w). That is,  E = q + w.

Thermochemistry q, w, and their signs + q = system gains or takes in heat -q = system loses or gives off heat + w = work is done on the system by the surroundings (piston pushed in) - w = work is done by the system on its surroundings (piston moves out) © 2009, Prentice-Hall, Inc.

Thermochemistry Example As hydrogen and oxygen gas are ignited in a cylinder, the system loses 550 J of heat to its surroundings. The expanding gases move a pistion to do 240 J of work on its surroundings.  E for system = ? Answer:  E = q + w  E = (-550 J) + (-240 J)  E = J What does it mean? The system gave off 790 J of energy to its surroundings © 2009, Prentice-Hall, Inc.

Thermochemistry © 2009, Prentice-Hall, Inc. Enthalpy &  H The symbol for enthalpy is H. Enthalpy is the internal energy plus the product of pressure and volume: At constant pressure:  H =  E = q So at constant pressure,  heat lost or gained by the system. H = E + PV

Thermochemistry © 2009, Prentice-Hall, Inc. Recall: Endothermic When heat is absorbed (taken in) by the system from the surroundings, the process is endothermic.  H = H final − H initial  H = H products − H reactants   H = positive value  for endothermic

Thermochemistry © 2009, Prentice-Hall, Inc. Recall: Exothermic  When heat is released (given off) by the system into the surroundings, the process is exothermic.  H = H final − H initial  H = H products − H reactants   H = negative value  for exothermic