PHYSICAL CHEMISTRYThermo-chemistry11-12 Heat of formation, heat of reaction and heat of solution, Hess’s law, calorimetry

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THERMOCHEMISTRY Thermochemistry is the

branch of chemistry which deals with the thermal changes accompanying chemical and physical transformations. The aim of thermochemistry is not only the determination of energy emitted or absorbed but also to develop methods for calculating these thermal readjustments without recourse to experiment. 2

THERMOCHEMISTRY

For practical purpose it is essential to know whether the heat is absorbed or evolved and how much of it, because if the heat is evolved it must be removed to effect appropriate reaction and if it is absorbed, required amount of heat must be supplied.

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CHEMICAL REACTIONS A reaction always begins

with the breaking a few bonds and ends with the formation of a few new bonds.

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Since the reaction begins with breaking of bonds, hence it invariably needs energy to start. This energy is called energy of activation.

THERMAL CHANGES IN CHEMICAL REACTIONS Energy of activation is

required to start a reaction irrespective of the fact whether the reaction is exothermic or endothermic.

The energy of activation is actually the barrier between the reactants and the products and must be crossed by providing energy to start the reaction

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EXOTHERMIC REACTIONS

In exothermic reactions the total energy of the products is less than the total energy of the reactants. Hence, the difference of energy is released usually as heat.

Since more energy is released than activation energy, the energy released is more than enough to activate more reactant molecules. Hence once activated the reaction goes on by itself and no more heating is required. 6

EXOTHERMIC REACTIONS

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If the difference of activation energy and the total energy released is less, the reaction is less exothermic

If the difference of activation energy and the total energy released is more, the reaction is more exothermic

ENDOTHERMIC REACTIONS

In endothermic reactions the total energy of the products is greater than the total energy of the reactants. Hence, the difference of energy is absorbed by the system.

Since the energy of products is greater than the reactants, the energy released after activation is less than the activation energy and is not sufficient to activate more reactant molecules. That is why endothermic reactions require continuous supply of energy (heat) for propagation of reaction and the reaction stops as and when the supply of heat is interrupted.

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ENDOTHERMIC REACTIONS

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If the difference of activation energy and the total energy release is less, the reaction is less endothermic

If the difference of activation energy and the total energy release is more, the reaction is more endothermic

HEAT OF FORMATION The heat of formation is the amount of heat absorbed

or evolved when a mole of a substance is formed from it’s elements. For example if 2 grams of hydrogen are burned in oxygen to form liquid water, 68.320 Kilo Calories of heat are evolved.

H2 + ½ O2 = H2O ΔH = -68.320 Kcal

The heat associated with chemical reaction not only depends on whether the reaction is carried out at constant pressure but also the amounts of substances. Hence if the amount of hydrogen in the above reaction is doubled (i.e. 4 grams) the amount of heat evolved will also be doubled, i.e. 68.320 x 2 = 136.640

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HEAT OF FORMATION

H2 + ½ O2 = H2O In the above reaction:

Whether the work will be done or not? If work is done, will it be negative (-) or positive

(+)?

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HEAT OF REACTION All chemical reactions are accompanied by

absorption or evolution of heat. This thermal change is called heat of reaction.

Tables of standard enthalpies of a large number of substances are available and simply substituting the values of enthalpies in a reaction, the heat of reaction can be calculated.

For example sodium carbonate reacts with HCl, producing NaCl, CO2 and H2O; the heat of this reaction can be calculated by the use of standard enthalpies as follows:

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HEAT OF REACTION

By subtracting the total energy of reactants from the total energy of products the heat of reaction is obtained.

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HEAT OF REACTION AT CONSTANT PRESSURE

If a reaction is carried out at constant pressure the heat of reaction will be:

qp = ΔE + PΔV = (Ep-Er) + P(Vp-Vr)

= (Ep+PVp) – (Er+PVr)

=Hp - Hr

= ΔH ΔH = HH2O (l) – [HH2 (g) +H1/2 O2 (g)] 14

HEAT OF REACTION AT CONSTANT VOLUME

At constant volume ΔV = 0, hence, heat of reaction will be:

qv = ΔE = Ep - Er

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HEAT OF SOLUTION Solution of one substance in another is

accompanied by absorption or evolution of heat and this thermal effect is called heat of solution of the substance.

Per mole of dissolved substance, the heat of solution at any given temperature and pressure depends upon the amount of solvent in which solution takes place. The grater the dilution the greater the enthalpy of solution. Hence for heat of solution it is essential to specify the number of moles of solvent per mole of solute.

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HEAT OF SOLUTION Sulphuric acid dissolves

in water with evolution of heat. If one mole of H2SO4 is dissolved in water the heat evolved depends on the number of moles of water. It is evident from results that, the greater the dilution, the greater is the ΔH.

Moles of water ΔH (cal)

0.5 -3810

1 -6820

2 -9960

3 -11890

4 -13120

6 -14740

10 -16240

3200 -20050

infinite -2299017

HEAT OF SOLUTION

The dilution of H2SO4 with water given by equation:

H2SO4 (l) + aq = H2SO4

(aq)The heat of solution, ΔH, for this process is given by:

ΔH = H – (n1H1 + n2H2)Where H1 and H2 are the enthalpies of two pure solution constituents and H is the enthalpy of solution.

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HESS’S LAW OF HEAT SUMMATION As we have seen that E and H are functions

of the state of system and consequently ΔE and ΔH must be true quantities, independent of path.

It follows from this that the heat absorbed or evolved in a chemical reaction is independent of the particular manner in which the reaction is carried out. This generalization of the statement is called Hess’s law of heat summation.

This principle makes it possible to calculate the heats of many reactions which cannot be directly measured. 19

HESS’S LAW OF HEAT SUMMATION

If we intend to determine the enthalpy ΔH of the following reaction:

2C (s)+ 2H2 (g) + O2 (g) = CH3COOH ΔH25 ºC = ?

It is not possible to determine the enthalpy of this reaction as the reaction does not occur in this manner. However we can determine the enthalpy of this reaction by a different way using Hess’s law.

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HESS’S LAW OF HEAT SUMMATION Using the available calorimetric data for the

following reactions and Hess’s law we can calculate ΔH of acetic acid.

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HESS’S LAW OF HEAT SUMMATION

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MEASUREMENT OF THERMAL CHANGES -CALORIMETRY Heat changes involved in a reaction, are

determined using the instrument named calorimeter. There are different types of calorimeters, however, all essentially consists of an insulated chamber filled with definite amount of water. The actual reaction is carried out in a separate chamber which is immersed in water. The thermal changes occurring in the reaction chamber directly bring about thermal changes in the water, which are then measured and calculated.

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CALORIMETRY The calorimeter is essentially an

insulated chamber filled with definite amount of water in which reaction chamber is immersed. The water in the insulated chamber

In an exothermic reaction, the heat evolved is transferred to water and the rise in temperature of water is measured. The data then collected is used to calculate the amount of heat evolved in the reaction.

In case of an endothermic reaction, the heat is absorbed and the temperature of water is lowered, which is measured and corresponding heat change is calculated.

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CALORIMETRY Various types of calorimeters are

used to meet different requirements. The one shown in the figure is called bomb calorimeter. It consists of an outer insulated housing and an inner container containing pure water. The reaction chamber, called bomb, is a sealed vessel immersed in water. The water is circulated using the stirrer to keep the temperature homogenous though out its body. The sample resting on a boat in the reaction chamber is ignited electrically and the heat changes thus occurring are read from the changes occurring in water.

Since the process takes place at constant volume, the reaction vessel is specially constructed to withstand high pressure.

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CALORIMETRY The simple calorimeter

used in student laboratories is the styrofoam coffee cup. The coffee cup is covered by the lid and a thermometer inserted through the lid reads the temperature changes.

Styrofoam is a good thermal insulator over a short time and provides good results for students experimantal work.

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CALORIMETRY – CALCULATING THERMAL DATA

Heat flow is calculated by using the equation:

q = C x m x ΔTWhere “q” is the heat flowing

through system boundary, “C” is the specific heat, “m” is the mass of system and “ΔT” is rise or fall of temperature.

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PROBLEM 1

consider a chemical reaction which occurs in 200 grams of water with an initial temperature of 25.0°C. The reaction is allowed to proceed in the coffee cup calorimeter. As a result of the reaction, the temperature of the water changes to 31.0°C. Calculate the enthalpy change (ΔH) of this reaction in Joules.N.B. The specific heat of water is = 4.18 J/g,˚C

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PROBLEM 1 - SOLUTION The change in temperature of water is used to calculate the

heat evolved or absorbed in the reaction. Use is made of the following equation:

q = C x m x ΔTwhile q = ΔH

where C is the specific heat capacity, m is the mass and ΔT is the change in temperature of water.

C = 4.18 J/g ˚CM = 200 gΔT = (31.0 – 25.0) = 6.0 ˚C

qwater = 4.18 J/g,°C x 200 g x (31.0°C - 25.0°C) = 5.016 kJ

Since we know that the reaction is exothermic hence the sign for ΔH will be negative, therefore ΔH for the reaction will be:

ΔH = - 5.016 kJ

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PROBLEM 2

When 7.1 g of NH4NO3 was added to 100 g of water at 18.2 ºC, the temperature of solution dropped to 12.8 ºC. Calculate the enthalpy change, ΔH of solution for 1 mole of NH4NO3.

(specific heat capacity of water = 4.184 J g-1K-1)

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