Thermochemistry

Thermochemistry

Thermodynamics = study of energy and its transformations

Thermochemistry = study of chemical reactions involving changes in heat energy

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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

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Major Types of EnergyPotential 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-m2/s2

A mass of 2 kg moving at a speed of one meter per second possesses a kinetic energy of 1 Joule.

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12KE = m v 2

Thermochemistry

Potential Energy

m = mass in kilograms (kg)

g = acceleration due to gravity (9.8 m/s2)

h = height (m)

PE = potential energy in joules (J)

1 Joule = 1 kg-m2/s2

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PE = m g h

Thermochemistry

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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 = 4.184 J

• 1000 calories = one nutritional Calorie

Thermochemistry

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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

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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

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Internal EnergyThe 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 = Efinal − Einitial (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

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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)

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Thermochemistry

ExampleAs 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 = - 790 J

What does it mean?The system gave off 790 J of energy to its surroundings

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Thermochemistry

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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

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Recall: Endothermic• When heat is absorbed (taken in) by the system from the surroundings, the process

is endothermic.

H = Hfinal − Hinitial

H = Hproducts − Hreactants

H = positive value for endothermic

Thermochemistry

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Recall: Exothermic When heat is released (given off) by the system into the surroundings, the process is

exothermic.

H = Hfinal − Hinitial

H = Hproducts − Hreactants

H = negative value for exothermic