Chapter 6 Thermochemistry General Chemistry: An Integrated Approach Hill, Petrucci, 4 th Edition Mark P. Heitz State University of New York at Brockport

Chapter 6Thermochemistry

General Chemistry: An Integrated Approach

Hill, Petrucci, 4th Edition

Mark P. HeitzState University of New York at Brockport

© 2005, Prentice Hall, Inc.

Chapter 6: Thermochemistry 2

EnergyLiterally means “work within,” however no object contains work

Energy refers to the capacity to do work– that is, to move or displace matter

EOS

2 basic types of energy: – Potential (possibility of doing work because of composition or position) – Kinetic (moving objects doing work)


Energy

Potential Energy – in a gravitational field (= position)

Kinetic Energy – energy of motion

PE = mghm = mass (kg)g = gravity constant (m s–2)h = height (m)

KE = 1/2mv2

m = mass (kg)v = velocity (m s–1)v2 = (m2 s–2)

EOS

units are kg m2 s–2 J units are kg m2 s–2 J


Work

Work is the product of the force in the direction of motion and the distance the object is moved

Work = force × distance energy (J)

Collisions in the real world are not perfectly elastic

EOS

Energy transfer occurs as a result of inelastic collisions e.g., the ball loses height


Thermochemistry

EOS

Thermochemistry is the study of energy changes that occur during chemical reactions

Universe Focus is on heat and matter transfer between the system ...

System

Surroundings

Su

rrou

nd

ingsS

urrou

nd

ings and the

surroundings


Thermochemistry

Types of systems one can study:

OPEN

MatterMatter

EnergyEnergy

CLOSED

Matter

EnergyEnergy

Matter

EOS

ISOLATED

MatterMatter

EnergyEnergy


Internal EnergyInternal Energy (U) is the total energy contained within the system, partly as kinetic energy and partly as potential energy

EOS

Kinetic involves three types of molecular motion ...


Internal EnergyInternal Energy (U) is the total energy contained within the system, partly as kinetic energy and partly as potential energy

Potential energy involves intramolecular interactions ...

EOS

and intermolecular interactions ...


Heat (q)

Heat is energy transfer resulting from thermal differences between the system and surroundings

“flows” spontaneously from higher T lower T

EOS

“flow” ceases at thermal equilibrium


Heat Transfer Mechanism Illustrated

EOS

Inelastic molecular collisions are responsible for heat transfer


Heat Transfer Illustrated


Work (w)

Work is an energy transfer between a system and its surroundings

Recall from gas laws … the product PV = energy

EOS

Pressure–volume work is the work of compression (or expansion) of a gas


Calculating Work (w)

PV work is calculated as follows:

w = –PV

Sign conventions: think from the perspective of the system

SYSTEM WORK

EOS

If work is done by the system, the system loses energy equal to –w



SYSTEM WORK

Expansion is an example of work done by the system—the weight above the gas is lifted

EOS

compression (or expansion) of a gas

ExpansionWork



SYSTEM WORKIf work is done on the system, the system gains energy equal to +w

EOS

compression (or expansion) of a gas


States of a System

The state of a system refers to its exact condition, determined by the kinds and amounts of matter present, the structure of this matter at the molecular level, and the prevailing pressure and temperature

Example: internal energy (U) is a function of the state of the system ...

EOS


State Functions

A state function is a property that has a unique value that depends only on the present state of a system and not on how the state was reached, nor on the history of the system

EOS

U = Uf – Ui


First Law of ThermodynamicsThe Law of Conservation of Energy states that in a physical or chemical change, energy can be exchanged between a system and its surroundings, but no energy can be created or destroyed

The change in U is related to the energy exchanges that occur as heat (q) and work (w)

EOS

The First Law:U = q + w


First Law – Sign Conventions

EOS

Energy entering a system carries a positive sign: if heat is absorbed by the system, q > 0. If work is done on a system, w > 0

Energy leaving a system carries a negative sign: if heat is given off by the system, q < 0. If work is done by a system, w < 0


Heats of ReactionThe heat of reaction (qrxn) is the quantity of heat exchanged between the system and its surroundings

Examples – for exothermic reactions, in isolated systems, system T in non-isolated systems, heat is given off to

the surroundings, i.e., q < 0

EOS

– endothermic reactions,in isolated systems, system T in non-isolated systems, heat is absorbed

from the surroundings, i.e., q > 0


Conceptualizing an Exothermic ReactionSurroundings are at 25 °C

Hypothetical situation: all heat is instantly released to the surroundings. Heat = qrxn

Typical situation: some heat is released to the surroundings,

some heat is absorbed by the solution.

In an isolated system, all heat is absorbed by the solution.

Maximum temperature rise.

25 °C

32.2 °C 35.4 °C


Internal Energy Changes

w = –PV and U = q + w

All the thermal energy produced by conversion from chemical energy is released as heat

EOS

Because the reaction is exothermic, both qV and U are negative

For systems where the reaction is carried out at constant volume, V = 0 and U = qV


Internal Energy Changes

w = –PV and U = q + w

Most of the thermal energy is released as heat, but some is work used to expand the system against the surroundings

EOS

The quantity of heat liberated is somewhat less than in the constant-volume case

For systems where the reaction is carried out at constant pressure, U = qP – PV or qP = U + PV


Example 6.2

The internal energy of a fixed quantity of an ideal gas depends only on its temperature. If a sample of an ideal gas is allowed to expand against a constant pressure at a constant temperature, (a) what is ∆U for the gas? (b) Does the gas do work? (c) Is any heat exchanged with the surroundings?

Analysis and Conclusions(a) Because the expansion occurs at a constant temperature, the expanded gas (state 2)

is at a lower pressure than the compressed gas (state 1) but the temperature is unchanged. Because the internal energy of the ideal gas depends only on the temperature, U2 = U1 and ∆U = U2 – U1 = 0.

(b) The gas does work in expanding against the confining pressure, P. The pressure–volume work is w = –P∆V, as was illustrated in Figure 6.8. The work is negative because it is done by the system.

(c) The work done by the gas represents energy leaving the system. If this were the only energy exchange between the system and its surroundings, the internal energy of the system would decrease, and so would the temperature. However, because the temperature remains constant, the internal energy does not change. This means that the gas must absorb enough heat from the surroundings to compensate for the work that it does in expanding: q = –w. And, according to the first law of thermodynamics, ∆U = q + w = –w + w = 0.


Example 6.2 continued

In an adiabatic process, a system is thermally insulated from its surroundings so that there is no exchange of heat (q = 0). If an ideal gas undergoes an adiabatic expansion against a constant pressure, (a) does the gas do work? (b) Does the internal energy of the gas increase, decrease, or remain unchanged? (c) What happens to the temperature?

Exercise 6.2A

a) yes w = - PΔV

b) ΔU = q + w = 0 + w = w = - PΔV since ΔV >0, ΔU< 0

c) For an ideal gas U ~ T so if U decreases, T decreases


Enthalpy

Most heats of reaction are measured at constant pressure … it is useful to have a function equal to qP

Enthalpy (H) is the sum of the internal energy and the pressure–volume product of a system

qP = H = U + PV

Enthalpy is an extensive property (depends on how much of the substance is present)

EOS

Enthalpy is a state function. U, P, and V are all state functions, therefore H must be a state function also


Enthalpy Diagrams

EOS


• H changes sign when a process is reversed.

• Therefore, a cyclic process has the value H = 0.

Reversing a Reaction

Same magnitude; different signs.


Using H

Values are measured experimentally

Negative values indicate exothermic reactions

Positive values indicate endothermic reactions

Changes sign when a process is reversed. Therefore, a cyclic process has the value H = 0

EOS

For problem-solving, one can view heat being absorbed in an endothermic reaction as being like a reactant and heat being evolved in an exothermic reaction as being like a product


• We measure heat flow using calorimetry.

• A calorimeter is a device used to make this measurement.

• A “coffee cup” calorimeter may be used for measuring heat involving solutions.

A “bomb” calorimeter is used to find heat of combustion; the “bomb” contains oxygen and a sample of the material to be burned.

Calorimetry


Calorimetry Relationships

The heat capacity (C) of a system is the quantity of heat required to change the temperature of the system by 1 oC

calculated from C = q/T units of J oC–1 or J K–1

Specific heat is the heat capacity of a one-gram sample

EOS

Specific heat = C/m = q/mT units of J g–1 oC–1 or J g–1 K–1


Specific Heats

Molar heat capacity is the product of specific heat times the molar mass of a substance

units are J mol–1 K–1

A useful form of the specific heat equation is:q = m CT

If T > 0, then q > 0 and heat is gained by the system

EOS

If T < 0, then q < 0 and heat is lost by the system


Specific Heats

EOS


Hess’s Law ofConstant Heat Summation

The heat of a reaction is constant, regardless of the number of steps in the process

Hoverall = H’s of individual reactions

When it is necessary to reverse a chemical equation, change the sign of H for that reaction

EOS

When multiplying equation coefficients, multiply values of H for that reaction


An Enthalpy Diagram

EOS


Standard State Conditions

The standard state of a solid or liquid substance is the pure element or compound at 1 atm pressure and the temperature of interest

Gaseous standard state is the “ideal gas” at 1 atm pressure and the temperature of interest

EOS

e.g., at 1 atm, 25 oC standard state for Hg is liquid, C is solid, water is liquid, He is gas


Standard Enthalpies

The standard enthalpy of reaction (Ho) is the enthalpy change for a reaction in which the reactants in their standard states yield products in their standard states

The standard enthalpy of formation (Hof) of a

substance is the enthalpy change that occurs in the formation of 1 mol of the substance from its elements when both products and reactants are in their standard states

EOS


Standard Enthalpiesof Formation at 25 oC

EOS


Calculations Based onStandard Enthalpies of Formation

Ho = p × Hof (products) – r × Ho

f (reactants)General Expression:

Each coefficient is multiplied by the standard enthalpy of formation for that substance

The sum of numbers for the reactants is subtracted from the sum of numbers for the products

EOS

With organic compounds, the measured Hof is

often the standard enthalpy of combustion Hocomb


Standard Enthalpies of Formationof Ions in Aqueous Solution


Combustion Fuels

A fuel is a substance that burns with the release of heat

EOS

These fossil fuels were formed over a period of millions of years from organic matter that became buried and compressed under mud and water

Fossil Fuels: Coal, Natural Gas, and Petroleum


Respiration Foods

EOS

1 Food Calorie (Cal) is equal to 1000 cal(or 1 kcal)

The three principal classes of foods are fats, proteins, and carbohydrates

Foods: Fuels for the Body


Summary of Concepts

• Thermochemistry concerns energy changes in physical processes or chemical reactions

• Thermochemical ideas include the notion of a system and its surroundings; the concepts of kinetic energy, potential energy, and internal energy; and the distinction between two types of energy exchanges: heat (q) and work (w)

EOS

• Internal energy (U) is a function of state


Summary of Concepts

• Enthalpy (H) is a function based on internal energy, but modified for use with constant-pressure processes

• The first law of thermodynamics relates the heat and work exchanged between a system and its surroundings to changes in the internal energy of a system

EOS

• A calorimeter is used to measure quantities of heat


Summary of Concepts

• The concepts of standard state, a standard enthalpy change, and a standard enthalpy of formation are important in thermochemical calculations

• Some practical applications of thermochemistry deal with the heats of combustion of fossil fuels and the energy content of foods

EOS

Documents

Chapter 6 Thermochemistry General Chemistry: An Integrated Approach Hill, Petrucci, 4 th Edition Mark P. Heitz State University of New York at Brockport