Thermochemistry

Chapter 6Study of Energy Transfer in

Physical and Chemical Processes

Energy

• Every chemical change has an accompanying change of energy.• Combustion of fuels• Discharge of a battery• Metabolism of foods

• Every physical change has an accompanying change of energy.• Phase changes• Solution formation

Thermochemistry

• The study of energy and its transformations is known as THERMODYNAMICS.

• It studies the relationships between heat, work and the energy content of a system.

• In this chapter we will examine an aspect of thermodynamics that involves the relationships between chemical reactions and energy changes involving heat. This portion of thermodynamics is called THERMOCHEMISTRY.

The Nature of Energy

• The capacity to do work or to transfer heat.• Conserved.• A state function.• independent of the path, or how you get from

point A to B.• Work is energy used to cause an object with

mass to move.• Work = Force x distance• Heat is energy used to cause the temperature

of an object to increase.• The energy transferred as a result of

temperature difference.

Kinetic and Potential Energy

• Kinetic Energy = Energy of motionEk= ½ mv2

Mass (m)Velocity (v)

• Potential Energy = Energy of positionEp= mghMass (m)Gravitational constant = 9.8 m/s2 (g)Height of the object relative to some reference height (h)

Potential Energy in Chemistry

Gravity is an important kind of force for large objects, but not in chemistry. (you will use it all the time in physics, though)

Gravitational force is negligible in extremely small particles such as atoms and molecules.

More important are the forces that arise from electrical charges.

Electrostatic Potential Energy –the interactions between charged particles.

Electrostatic Potential EnergykQ1Q2

• Eel = d

Q1and Q2represent the electrical charges on the two interacting objects, typically the charge on an electron 1.60 x 10 1.60 x 10 -19-19 Coulombs Coulombs

d is the distance separating them

k is a proportionality constant, 8.99 x 109J-m/C2

Joule-meter per Coulomb squared

Electrostatic Potential Energy

• When Q1and Q2 have the same sign (both positive, or negative), the two charges repel one another, pushing them apart, and Eel is positive.

• When they have opposite signs, they attract one another, pulling them toward each other, and Eel is negative.

• The lower the energy of a system, the more stable it is.

• Thus, the more strongly opposite charges interact, the more stable the system.

Chemistry and Energy

• The chemical energy of substances is due to the potential energy stored in the arrangements of the atoms of the substance.

• The energy possessed by a substance because of its temperature is associated with kinetic energy of the molecules.

Units of Energy

• The SI unit for energy is the joule, J

in honor of James Joule, a British scientist who investigated work and heat.

• Joule = 1 kg-m2/s2

• A mass of 2 kg moving at a speed of 1 m/s possesses a kinetic energy of 1 joule

• Remember, Ek= ½ mv2

• A small unit, so kilojoules (kJ) is often used when discussing the energies associated with chemical reactions.

Calories

• A more familiar unit to us.

• The amount of energy needed to raise the temperature of 1 grams of water from 14.5 C to 15.5 C.

• 1 calorie = 4.184 J (exactly)

• 1 Calorie = 1000 calories = 1 kcal.

The Universe

• is divided into two halves.

the system and the surroundings.

• The system is the part you are concerned with.

• The surroundings are the rest.

• Exothermic reactions release energy to the surroundings. (ΔH negative)

• Endothermic reactions absorb energy from the surroundings. (ΔH positive)

DirectionEvery energy measurement has three parts.

• 1. A unit ( Joules of calories).

• 2. A number how many.

• 3. A sign to tell direction.

negative -exothermic

positive-endothermic

Heat Flow

Reaction Progress

Same Rules for Heat and Work

• Heat given off is negative.

• Heat absorbed is positive.

• Work done by system on surroundings is positive.

• Work done on system by surroundings is negative.

• Thermodynamics-The study of energy and the changes it undergoes.

First Law of Thermodynamics

• The energy of the universe is constant.

• Energy may be transferred between the system and its surroundings.

• Law of conservation of energy

Internal Energy

• The sum of all the kinetic and potential energy of all its components.

Represented by E

Generally don’t know the actual numerical value

We can measure the change in internal energy,

Delta E, DE= E final–E initial

Remember the three parts of DE,

• 1. A number

• 2. A unit (together these 2 give the magnitude of change)

• 3. A sign that gives the direction

Signs of ΔE

• A positive value of ΔE results when E final > E initial, indicating the system has gained energy from its surroundings.

• ENDOTHERMIC

• A negative value of ΔE is obtained when E final < E initial, indicating the system has lost energy to its surroundings.

• EXOTHERMIC

Relating ΔE to heat and work

• q = heat• w = work• ΔE = q + w• First Law of Thermodynamics in algebraic

terms

• It takes the system’s point of view to decide signs.

• When heat is added to a system or work is done on a system, its internal energy increases.

Sign Conventions for q, w & ΔE

State Functions

• Reminder, Internal Energy is a state function

• A property of a system that is determined by specifying the system's condition, or state (in terms of temperature, pressure, location, etc.)

• The value of a state function depends only on the present state of the system, not on the path the system took to reach that state.

• ΔE only depends on the initial and final states of the system, not on how the change occurs.

Analogy

• Suppose you are traveling between Chicago and Denver. Chicago is 596 ft above sea level; Denver is 5280 ft above sea level.

• No matter what route you take, the altitude change will be 4684 ft. The distance you travel, however, will depend on your route.

• Altitude is a state function

• Distance is not a state function