ThermochemistryChapter 8

THERMOCHEMISTRY

Thermochemistry is branch of chemistry concerned with the heat changes that occur during chemical reactions

THERMODYNAMICS

(from the Greek word therme, meaning “heat” and  dynamics,  meaning “moving”)

Thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables suchas temperature, volume and pressure. 

Energy is the capacity to do work or supply heat• Kinetic energy is the energy because of

movement of a bodyEK

= 1/2mv2

• Potential energy is the energy available by virtue of an object’s position

• Thermal energy is the kinetic energy associated with the random motion of atoms and molecules

• Chemical energy is the energy stored within the bonds of chemical substances

Heat is the amount of thermal energy transferred between two bodies because of temperature difference.(depend on amount of substance)

Heat alone cannot be measured Only changes caused by heat can be detectedHeat always flows from warmer to cooler

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.Temperature = Thermal Energy

900C400C

greater thermal energyLow temperature

Energy & ChemistryEnergy & Chemistry

All of thermodynamics depends on All of thermodynamics depends on the law of the law of

CONSERVATION OF ENERGYCONSERVATION OF ENERGY..

• The total energy is unchanged in The total energy is unchanged in a chemical reaction.a chemical reaction.

• If E of products is less than If E of products is less than reactants, the difference must be reactants, the difference must be released as E.released as E.

UNITS OF ENERGYUNITS OF ENERGYUNITS OF ENERGYUNITS OF ENERGYEK

= 1/2mv2 =SI units (kg m2)/s2

= J = J (JOULE (JOULE common scientific units)common scientific units)

Mostly expressed in terms of kJMostly expressed in terms of kJ

Some scientists use calorie where Some scientists use calorie where

1 cal= heat required to raise temp. of 1 g of 1 cal= heat required to raise temp. of 1 g of HH22O by 1O by 1ooC.C.

1 cal = 4.184 joules1 cal = 4.184 joules

Nutritionists used Calorie (for food energy)Nutritionists used Calorie (for food energy)

1Cal=1000 cal = 1 kcal1Cal=1000 cal = 1 kcal

James JouleJames Joule1818-18891818-1889

Thermochemistry is the study of heat change in chemical reactions.The system is the specific part of the universe that is of interest in the study.

open

mass & energyExchange:

closed

energy

isolated

nothing

SYSTEMSURROUNDINGS

Internal Energy (E)Internal Energy (E)

• PE + KE = Internal energy (E)PE + KE = Internal energy (E)

• Int. E of a chemical system depends onInt. E of a chemical system depends on

• number of particlesnumber of particles

• type of particlestype of particles

• temperaturetemperature

• The higher the T the higher the The higher the T the higher the internal energyinternal energy

• So, use changes in T (∆T) to So, use changes in T (∆T) to monitor changes in E (∆E).monitor changes in E (∆E).

FIRST LAW OF THERMODYNAMICS

• The total internal energy of an isolated system is constant.

OR

• The change in the internal energy of a closed thermodynamic system is equal to the sum of the amount of heat energy supplied to or removed from the system and the work done on or by the system.

ΔE = Efinal − Einitial

Directionality of Heat TransferDirectionality of Heat Transfer• Heat always transfer from hotter

object to cooler one.

• EXOthermic: heat transfers from SYSTEM to SURROUNDINGS.

T(system) goes downT(system) goes downT(surr) goes upT(surr) goes up

Directionality of Heat TransferDirectionality of Heat Transfer• Heat always transfer from hotter object

to cooler one.

• ENDOthermic: heat transfers from SURROUNDINGS to the SYSTEM.

T(system) goes upT(system) goes upT (surr) goes downT (surr) goes down

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundingsΔE = -ve.

Endothermic process is any process in which heat has to be supplied to the system from the surroundings.ΔE = +ve.

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

energy + H2O (s) H2O (l)

State functions (written in capital letters) are properties that are determined by the present state of the system, regardless of path that condition was achieved. changes in state functions are Reversible.

Examples

Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.

energy E, pressure P, volume V, temperature T

Parameters which are not state function are written in small letters

Exampleswork w, heat q, time t

EXPANSION WORK

6.7

w = F × d

F= - P × A - as it is against external pressure

w= - P × A × d

w = -PV

Enthalpy and the First Law of ThermodynamicsE = q + w = q – PVq= E + PV

At constant pressure, qP = E + PV =H Called Enthalpy

So E = H - PV Therefore H = E + PV AndH = E when V=0

At constant volume, qV = E as V=0

Thermodynamics

E = q + w

E is the change in internal energy of a system

q is the heat exchange between the system and the surroundings

w is the work done on (or by) the system

w = -PV when a gas expands against a constant external pressure

Enthalpy (H) (heat content) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.

H = H (products) – H (reactants)

Standard enthalpy (Ho)= Enthalpy change measured under standard conditions 1 atm pressure, 25oC temperature 1molar concentration

Thermochemical Equations

H2O (s) H2O (l) H = 6.01 kJ

Is H negative or positive?

System absorbs heat

Endothermic

H > 0

6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm.

Thermochemical Equations

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) H = -890.4 kJ

Is H negative or positive?

System gives off heat

Exothermic

H < 0

890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm.

H2O (s) H2O (l) H = 6.01 kJ

• The stoichiometric coefficients always refer to the number of moles of a substance

Thermochemical Equations

• If you reverse a reaction, the sign of H changes

H2O (l) H2O (s) H = -6.01 kJ

• If you multiply both sides of the equation by a factor n, then H must change by the same factor n.

2H2O (s) 2H2O (l) H = 2 x 6.01 = 12.0 kJ

H2O (s) H2O (l) H = 6.01 kJ

• The physical states of all reactants and products must be specified in thermochemical equations.

Thermochemical Equations

H2O (l) H2O (g) H = 44.0 kJ

How much heat is evolved when 266 g of white phosphorus (P4) burn in air?

P4 (s) + 5O2 (g) P4O10 (s) H = -3013 kJ

266 g P4

1 mol P4

123.9 g P4

x3013 kJ1 mol P4

x = 6470 kJ

Because there is no way to measure the absolute value of the enthalpy of a substance, must I measure the enthalpy change for every reaction of interest?Establish an arbitrary scale with the standard enthalpy of formation (H0) as a reference point for all enthalpy expressions.

f

Standard enthalpy of formation (H0) is the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm.

f

The standard enthalpy of formation of any element in its most stable form is zero.

aA + bB cC + dD

H0rxn dH0 (D)fcH0 (C)f= [ + ] - bH0 (B)faH0 (A)f[ + ]

H0rxn nH0 (products)f= mH0 (reactants)f-

Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

(Remember: Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end.)

Calculate the standard enthalpy of formation of CS2 (l) given that:C(graphite) + O2 (g) CO2 (g) H0 = -393.5 kJrxn

S(rhombic) + O2 (g) SO2 (g) H0 = -296.1 kJrxn

CS2(l) + 3O2 (g) CO2 (g) + 2SO2 (g) H0 = -1072 kJrxn

1. Write the enthalpy of formation reaction for CS2

C(graphite) + 2S(rhombic) CS2 (l)

2. Add the given rxns so that the result is the desired rxn.

rxnC(graphite) + O2 (g) CO2 (g) H0 = -393.5 kJ

2S(rhombic) + 2O2 (g) 2SO2 (g) H0 = -296.1x2 kJrxn

CO2(g) + 2SO2 (g) CS2 (l) + 3O2 (g) H0 = +1072 kJrxn+

C(graphite) + 2S(rhombic) CS2 (l)

H0 = -393.5 + (2x-296.1) + 1072 = 86.3 kJrxn

Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49.04 kJ/mol.

2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)

H0rxn nH0 (products)f= mH0 (reactants)f-

H0rxn 6H0 (H2O)f12H0 (CO2)f= [ + ] - 2H0 (C6H6)f[ ]

H0rxn = [ 12x–393.5 + 6x–187.6 ] – [ 2x49.04 ] = -5946 kJ

-5946 kJ2 mol

= - 2973 kJ/mol C6H6

Bond Dissociation energies