CHM 432 Chapter 1

CHM 432: FUNDAMENTAL PHYSICAL CHEMISTRY
CHAPTER 1: THERMOCHEMISTRY
Thermodynamics is the study of heat and its transformations.

Thermochemistry is a branch of thermodynamics that deals with the heat involved with
chemical and physical changes.
THERMOCHEMISTRY
Thermochemistry is the study of heat changes taking place during chemical reactions.
BASIC TERMS
system and
Enthalpy, H Heat,q
surrounding
TYPES OF REACTIONS
Exothermic Endothermic
reaction reaction
STANDARD ENTHALPY OF REACTIONS
Types of standard enthalpy Determination

Definition
reactions of H
system and surrounding

Systems: the specific part of the universe that is of interest in the study.
Surroundings: the rest of the universe.
TYPES OF SYSTEM
Open Systems
• energy and matter can be exchanged with the surroundings.
• Water in tumbler placed in open area
Closed Systems
• energy can be exchanged with the surroundings, matter cannot.
• Hot water in steel box
Isolated Systems
• neither energy nor matter can be exchanged with the surroundings.
• Hot coffee in thermos flask
Concept of Enthalpy
 The heat content of the system is known as the enthalpy of the system.
 The total heat absorbed or liberated in a chemical reaction is called the heat of
reaction.
 We cannot measure the absolute value of the enthalpy of the system, but we can
measure the change in enthalpy.
 The experimental technique for measuring enthalpy changes is called calorimetry.
 Calorimeter is a device for measuring quantities of heat
SIMPLE CALORIMETER
Figure: A simple calorimeter can be constructed from two polystyrene cups. A thermometer
and stirrer extend through the cover into the reaction mixture.
COMMERCIAL SOLUTION CALORIMETERS
Figure: Commercial solution calorimeters range from (a) simple, inexpensive models for
student use to (b) expensive, more accurate models for industry and research.
The symbol for the change in enthalpy is ∆H.
 Enthalphy (H) is measured in the units of kJ.
For a reaction A + B C + D,
∆H = Total enthalpy of the products – Total enthalpy of the reactants
= H2 – H1
Where H2 = (HC + HD) and H1 = (HA + HB)
ENTHALPY (H)
The energy possessed by a system is called the enthalpy or heat content of the
system and is given the symbol H
•The change of the heat content is given by ΔH,
ΔH = H final – H initial
ΔH = H product – H reactant
Endothermic vs Exothermic Reactions
ENDOTHERMIC REACTION
 Dissolving ammonium chloride in water
 Photosynthesis
 Evaporating liquid water
 Melting ice Cracking alkanes
EXOTHERMIC PROCESS
 Combustion of a fuel
 Corrosion of metal (an oxidation reaction)
 Respiration
 Nuclear fission
Energy Profile
The activation energy (Ea) is the energy barrier which the reactant molecules must
overcome if they are react successfully to form the product molecules.
Endothermic Reaction
An endothermic reaction absorbs heat from the surroundings and is shown by a fall in
temperature.
The enthalpy changes of the reaction, ∆H, has a positive value because the heat content of
the products is higher than the heat content of the reactants.
Exothermic Reaction
 An exothermic reaction releases heat to the surroundings and is shown by a rise in
temperature.
 The enthalpy changes of the reaction, ∆H has a negative value because the heat
content of the products is less than the heat content of the reactants.
Exothermic and Endothermic Reactions

To measure energy changes of the reaction:
H= H (product) – H (reactant) = H (final) – H (initial)
 If H (final) < H (initial) : exothermic reactions ( has a negative H)
e.g. : combustion, neutralisation, dissolving NaOH pellets
 If H (final > H (initial) : endothermic reaction ( has a positive H)
e.g.: mixing ammonium thiocyanate and barium hydroxide crystals
Standard Conditions for calculating Enthalpy Changes

The numerical value of enthalpy change is influenced by 5 factors:
(a) The temperature at which the experiment is carried out
(b) The physical states (solid, liquid, gas) of the reactants – for example, the reaction
between solid iodine and hydrogen is endothermic, but the reaction between
gaseous iodine and hydrogen is exothermic.
(c) The allotropic forms of the reactants
(d) The pressure of the gaseous reactants
(e) The concentration of the reactants.
The standard conditions for measuring enthalpies are:

1. Temperature of 25 oC (298 K)
2. Pressure of 1 atm (or 101 kPa)
3. Solutions having a concentration of 1.0 mol dm-3
STANDARD ENTHALPY OF REACTION ΔHθ
The enthalpy changes for a reaction in which the reactant yield products in their
standard states.
STANDARD STATE / CONDITIONS

Temperature • 25 oC or 298 K
Pressure • 1 atm or 101 kPa
Concentration of solution •1M
Physical states of reactants and products are specified (s, l, g, aq)
• H2(g) + 1⁄2 O2(g) H2O(g) ΔHθ = -241 kJ/mol
• H2(g) + 1⁄2 O2(g) H2O(l) ΔHθ = -286 kJ/mol
1. Direct method
( calorimeter exp )
HOW TO
DETERMINE
H
3. Using Hess 2. From data of
Law enthalpy of formation
1. DIRECT METHOD (CALORIMETER EXPERIMENTAL)
Calorimetry is a technique used to measure heat exchange in chemical
reactions.
Heat absorb or evolve is measured using a calorimeter.
Coffee-cup Bomb
Calorimeter Calorimeter
2 TYPES
CALORIMETER
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.
How does a calorimeter measure q

 Heat evolved in a reaction is absorbed by the calorimeter and its contents (water
or solution).
 In a calorimeter the temperature change of water or a solution is measured to
determine the heat absorbed or evolved by a reaction.
Calorimeter
Any device used to measure temperature changes resulting from enthalpy
changes in chemical reactions or physical changes is called a calorimeter.
The experimental technique of using a calorimeter to determine the enthalpy
change of a reaction is called calorimetry.
SALIN PAGES 30
MEASURING ENERGY CHANGES,

 Using bomb calorimeter
 Constant volume calorimeter
 At constant volume,q
MEASURING ENTHALPY CHANGES, H

o Heats of Reaction, q can be measured in the laboratory using a sample
calorimeter made up of a polystyrene cup.
o Constant-Pressure-Calorimeter
BOMB CALORIMETER
A bomb calorimeter is a type of

constant-volume calorimeter used
in measuring the heat of
combustion of a particular
reaction.
BOMB CALORIMETER
- For reactions involving gas
- Higher temperature
- -q reaction = +q calorimeter
- q reaction = C cal X T ( after bomb calorimeter have been calibrated)
HEAT CAPACITY
Molar Heat Capacity

Specific Heat Capacity
 heat required to raise the
 heat required to raise
temperature of 1 mole of
the temperature of 1
a substance by 1 K .
gram of a substance
by 1 K .
- Specific Heat Capacity - Molar Heat Capacity of

of Water Water
4.184 J /(g K) 75.4 J /(mol K)
Heat Capacity
 When heat is supplied to a substance, the temperature of the substance increase.
 The magnitude of the increase depends on the heat capacity of the substance.
 The heat capacity (C) is the amount of heat needed to change the temperature of
substance by 1 oC. It is measured in J oC-1 or J K-1
 The specific heat capacity (c) of a substance is the energy required to raise the
temperature of one gram (1 g) of the substance by 1 oC. The unit is J g-1 oC-1
Heat capacity = specific heat capacity x mass

J oC-1 = (J g-1 oC-1) x g
The formula used to calculate heat absorbed or evolved during a reaction is as follows.
q = mcθ or q = Cθ
q = heat change (absorbed or liberated)
c = specific heat capacity of the substance C = heat capacity of the substance
θ = change in the temperature m = mass
To Calculate heat (q)
q = mass x specific heat capacity x ∆T
q = m C ∆T
∆T is positive (temperature increases): q is positive
heat is gained by the system.
∆T is negative (temperature decreases): • q is negative.

heat is lost by the system.
2. FROM DATA OF STANDARD ENTHALPY OF FORMATION

The enthalpy of a reaction is equal to the total formation energy of the products
minus that of the reactants.
In general, when ALL enthalpies of formation are known:
∆Ho rxn = Σ n∆Hfo (products) - Σ m∆Hfo (reactants)

Remember that ∆ always = final – initial
– Σ : “the sum of”
– m and n : coefficients of the substances
Stoichiometry Of Thermochemical Equations
 A thermochemical equation is a balanced equation that includes the heat of reaction,
H .
 H value shown refers to the amounts (moles) of substances and their states of
matter in that specific equations.
 Sign: The sign of H depends on whether the reaction is exothermic (-) or
endothermic (+). A forward reaction has the opposite sign of the reverse reaction.
 Magnitude: The magnitude of H is proportional to the amount of substances
reacting.

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CHM 432: FUNDAMENTAL PHYSICAL CHEMISTRY

Thermodynamics is the study of heat and its transformations.

STANDARD ENTHALPY OF REACTIONS

Types of standard enthalpy Determination

system and surrounding

COMMERCIAL SOLUTION CALORIMETERS

∆H = Total enthalpy of the products – Total enthalpy of the reactants

Where H2 = (HC + HD) and H1 = (HA + HB)

Endothermic vs Exothermic Reactions

Exothermic and Endothermic Reactions

Standard Conditions for calculating Enthalpy Changes

The standard conditions for measuring enthalpies are:

STANDARD STATE / CONDITIONS

A “bomb” calorimeter is used to

How does a calorimeter measure q

MEASURING ENERGY CHANGES,

MEASURING ENTHALPY CHANGES, H

A bomb calorimeter is a type of

Molar Heat Capacity

- Specific Heat Capacity - Molar Heat Capacity of

Heat capacity = specific heat capacity x mass

∆T is negative (temperature decreases): • q is negative.

2. FROM DATA OF STANDARD ENTHALPY OF FORMATION

In general, when ALL enthalpies of formation are known:

∆Ho rxn = Σ n∆Hfo (products) - Σ m∆Hfo (reactants)

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