2.6 The Senses 52
2.6.3 Sound 58
2.0 Introduction
The word spontaneous means voluntary or occurring without external influence or assistance. A spontaneous reaction can therefore be defined as a reaction that occurs without external aid or incitement. For example sodium metal reacts explosively with water. This is why sodium metal is kept in oil to avoid contact with water. Calcium carbide commonly called carbide by welders is kept away from water. This is also to stop the spontaneous reaction of carbide with water.
In unit 1 you learnt the different forms of energy and the energy changes in physical and chemical processes.
The concept of heat content (enthalpy) was introduced and chemical changes were classified as exothermic or endothermic processes depending on whether heat evolution or absorption accompanied the reactions.
Now you will look at some examples of process that have been studied.
H2(8)+1/2
02(g) F1200
AH = –285.6 kJmot' AH = –285.6 kJ mot' NH4 NO3(4) + H2 00) —0 NH44(4.0 +NO3-0q)AH = 26. 5 kJ mot'
The first process, the reaction of hydrogen with oxygen is an exothermic reaction but yet the reaction of hydrogen with oxygen is not a spontaneous reaction. Recall that for a reaction to occur in a jar of hydrogen gas, a spark must be introduced i.e reaction is aided to occur. Another example is the reaction of nitrogen and oxygen which is also an exothermic reaction. These two gases N 2 and
02
are present in the air and no reaction occurs. The reaction of nitrogen and oxygen will not occur at ordinary temperature.The second process, the dissolution of ammonium trioxonitrate (v) is an endothermic reaction. When the solid dissolves, the beaker cools down appreciably. This process, though endothermic occurs spontaneously.
Ice melts when removed from the freezer compartment and placed outside spontaneously but it is an endothermic process. The above examples and many others suggest that enthalpy data alone are not sufficient to predict whether a change will be spontaneous or not.
In this unit a new concept is introduced for reactions which when considered together with the enthalpy change will enable us predict more accurately the direction of a spontaneous process.
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2.1 Objectives
By the end of this unit you should be able to:
• Define entropy and entropy change.
• Calculate entropy change from suitable data.
• Predict qualitatively the entropy changes for reactions and processes.
• Explain the concept of free energy.
• Predict the direction of a spontaneous change from values of entropy and enthalpy change and temperature.
• Recall the equation AG = AH — TAS 2.2 Entropy and Entropy Change
Entropy is a measure of the disorder of a system. The greater the disorder the greater the entropy. The entropy change is the measure of the change in disorder that accompanies a chemical of physical process.
The entropy and entropy change are given the symbols S and AS respectively. Similar to enthalpy.
AS reaction Sr — Sit and for many reactants and products ASreaction =ES —ESR
For a process that is accompanied by greater disorder the entropy increases and AS is positive. When a process is accompanied by less disorder (more order) the entropy decreases and AS is negative. The change in entropy is a driving force behind reactions and processes. It is a measure of the capacity for a spontaneous change. Entropy is measured in units of energy. The SI unit is the joules.
Entropy is affected by many factors
(i) The physical state: The solid state with its ordered structure has the least entropy while the gas state has the highest. The liquid is intermediate.
Ice water AS + ve
Water * Steam AS + ye
Solid — liquid, liquid — gas and solid — vapour transitions are accompanied by increased entropy.
(ii) Temperature: The higher the temperature, the higher the average kinetic energy .of the particles, the more the random motion and collisions leading to greater disorder. Entropy increases with increasing temperature. For a gas sample when there is a change of temperature and pressure the entropy change depends on the volume change. Entropy increases with increasing volume of a gas sample.
(iii) Change in the number of gas molecules: There is more random motion in a gas than in a liquid or solid.
The more the number of gas molecules resulting from a reaction the greater the entropy of such a reaction.
e.g. N10, (s) 2NO2(g) AS + ve
Ca CO3(s) CaO + CO2(g)
AS +ve H2 ± IA 0 2(g) -0 H2 0(n AS —ve
Na„ ) + H 20th Na0H(aq) + H2(s) AS +ve
(iv) Mixing: There is more disorder in a mixture of samples than in each of the pure samples. When a solid dissolves in a liquid entropy increases (AS + ye). When two gases mix, there is also increase in entropy,
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2.3 Free Energy and the Free Energy Change
Free energy is defined as the energy that is available to do work. Free energy is related to the enthalpy and entropy.
G = H —
TS where G
is the free energyand H, S and T are enthalpy, entropy and the absolute temperature for a change, physical or chemical at a constant temperature.
AG = AH — TAS. For the standard state AG' = AH° — TAS° where T = 298K
The free energy determines whether a reaction will occur spontaneously or not.
For a spontaneous change AG is less than zero
i.e AG — ve. When AG is greater than zero (AG+ve) the process is not spontaneous.
Such reactions will occur by supplying a driving force e.g. by heating. At equilibrium AG = 0 Note the TAS term which is a measure of the total entropy change at the temperature of the reaction.
2.4. Applications of the Free Energy Equation Five cases will be considered.
2.4.1 An exothermic reaction accompanied by an increase in entropy.
For the above type of reaction
All is — ve and AS is + ve
e.g. 2Na 00+2H2OM-10 2NaOH (aq) +H 2(g) Ca C2(g) + H20® --+Ca0(s) +C2H263)*
The reactions above are exothermic reactions and are accompanied by greater disorder. The free energy change for such reactions at any temperature is less then zero i.e even at absolute zero temperature AG is still less than zero. The reactions are always spontaneous. The only way to prevent such reactions would be to keep the reactants apart. This is why carbide and sodium are kept away from water.
24.2 An endothermic reaction accompanied by a decrease in entropy
Here All is + ve and AS is — ve. It follows that AG must be positive. The reaction is therefore unlikely to occur. The reverse of the reactions will serve as
NaOH(, +H2(8)
Nat, + 714200
CaO (s) + C 2H240 CaC2(s) +H20(0
For the above
reactions even at absolute zero, AG is greater than zero. They are not spontaneous.2.43 An exothermic reaction accompanied by a decrease in entropy For the above group of reactions.
Al-I is — ye and As is — ve water Ice Ali —ve
steam • water Ali—ve
In both processes All is — ye and AS is —ve. The disorder decreases as a result of the processes. Because AS is —ve.
— TAS is + ye and All is — ye. For such processes there is an equilibrium temperature when AG = 0.
This is case when TAS =
AH
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T = All Teq — equilibrium temperature AS
Above 'cc AG is + ye and the processes are non spontaneous. Below T e the AG is — ye and the processes are spontaneous. The tendency for a spontaneous process decreases with increasing temperature for the process. The equilibrium temperatures are the melting and boiling points respectively.
2.4.4. An endothermic reaction accompanied by an increase in entropy The AH is + ve and AS is + ve
NH 4NO3 (s) +H 20(I) NH4+(a4) + NO3—(aq) Ice water
.Because AS > 0
— TAS < 0 but AH is +ve. There is an equilibrium temperature (T.) as in the last example.
T(eq) = AH AS
Above Teq AG is —ve and the reactions are spontaneous. Below T the AG is +ve and the reactions are non-spontaneous reaction. The tendency for a non-spontaneous reaction increases with increase in temperature.
Solubility of ammonium trioxonitrate (v) increases with increasing temperature. For this reaction to have occurred spontaneous at laboratory temperature suggests that the equilibrium temperature is below the laboratory temperature. In the case of the ice -, water reaction the equilibrium temperature is the melting point which is about 0°C. Because the laboratory temperature is above 0°C, the melting process is spontaneous and occurs as soon as the ice is removed from the freezer and placed outside in the laboratory.
2.4.5 A reaction in which the enthalpy change is zero
For some reactions the enthalpy is very negligible. The driving force for such reactions is the entropy change alone. When the reaction is accompanied by an increase in entropy the reaction is spontaneous.
2.5. Conclusion
Enthalpy factor alone cannot be used to predict the spontaneity of a physical or chemical process. The entropy also contributes to the driving force of reactions. The free energy is a better and more reliable predictor of spontaneity of reactions. The free energy gives us the net energy that is available to do work. A knowledge of the free energy change of a reaction at a particular temperature can assist in creating favourable condition for preventing reactions or making reactions occurs. Some chemicals are kept at very low temperatures in the laboratory and some are kept away from other chemicals or even moisture to prevent reactions that will lead to unpleasant results.
2.6 Summary
• The spontaneous reaction is defined and the concepts of entropy and free energy introduced.
• Factors that will affect the entropy and entropy change are enumerated.
• The concepts of free energy is used to explain the spontaneity or otherwise of reactions.
• The temperature effect on the spontaneity of reactions is also explained.
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2.7 Tutor-Marked Assignments
1. (a) Explain the terms entropy and entropy change
(b) Will the entropy change increase, decrease or remain the same for the listed reactions?
(i) C(4) + 01(g) CO2(g) (ii) 2C0(g) + 02(g) 2CO2(g) (iii) HCI(s) +NH3(g) ---' NH 4 Cl(,)
(iv) 2 HCL) +Na2 CO3(.1) ---,2NaC1( .0 + H2 0(0 :FCO2(ao (v) C2H4(g) H2(g) C2 H6(8)
In each case give a reason for your answer.
2. (a) Predict and explain the effect of temperature and pressure on the spontaneity of the following process/reaction.
(i) water ice
(ii) water stream
(iii) N2(g) +3H2(8) -P2NH3(g) AI-1= — 92kJ mol 2.8 References
Bajah, S. T., Teibo, B. 0., Onwu G. and Obikwere, A. (2002) Senior Secondary Chemistry Textbook 2.
Lagos. Longman Publishers.
Osei Yaw Ababio, (2002). New School Chemistry. Onitsha. Africana-FEP Publishers.
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