In the previous section, we saw details about endothermic or exothermic nature of enthalpy of solution. In this section, we will see spontaneity
• Basics of spontaneity can be written in 15 steps:
1. Consider the open vessel in fig.6.20(a) below. It contains some water at room temperature 25 ०C
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| Fig.6.20 |
2. We know that, a mass of water is nothing but a collection of H2O molecules
• All those molecules are in random motion
• So all those molecules have kinetic energies
• The temperature of the water mass is the average kinetic energy of the molecules
3. Due to the random motion, the molecules are constantly colliding with each other
• Imagine that, due to the collisions, some molecules near the surface of the water, looses considerable kinetic energies
♦ Then the temperature of those molecules will fall
• Let the temperature fall to such a low level that, those molecules freeze and become ice
4. Remember that, loss of kinetic energy is due to collisions. The kinetic energies lost by the molecules will be gained by the surrounding molecules
• So, when the molecules freeze and become ice, the surrounding molecules become warm
• We will get some ice at the center of the vessel and that ice will be surrounded by warm water. This is shown in fig.6.20(b)
5. Formation of ice in this manner, does not defy the law of conservation of energy because,
♦ there is no destruction of energy
♦ also, no energy is created
• The energy lost by some molecules is gained by some other molecules
6. Even though the law of conservation is obeyed, we never see such a spontaneous formation of ice at room temperature
• A Spontaneous process is the one which do not require external source of energy to proceed
• For a process to be called spontaneous, it is not necessary that it occurs at a high speed
• A slow process can also be called spontaneous if it does not require external supply of energy
• The reaction between hydrogen and oxygen is an example of a slow spontaneous process
♦ A mixture of H2 and O2 can be left undisturbed in a container for many years
♦ There will not be any noticeable effects
♦ But the reaction will be taking place all the time, with out any aid of external energy
7. The spontaneous formation of ice in this manner requires heat to flow (without external help) from a cold object to a hot object
◼ We never observe such a flow. What we observe is the flow of heat from hot object to cold object
• We can say:
♦ The spontaneous process proceeds only in one direction: Ice → Water
♦ We never see the spontaneous process: Water → Ice
✰ For this process, we have to supply energy through a refrigerator
◼ In fact, all naturally occurring processes (physical or chemical) will tend to proceed in one direction only
8. Let us see another example:
• If a canister containing some gas is opened, the gas molecules will spontaneously spread out into the whole volume of the room
• We never see the gas molecules in the room to enter back spontaneously into the canister
9. One more example:
• Consider the burning of carbon
• During the process, carbon combines with oxygen to give carbon dioxide
♦ That is: C + O2 → CO2
♦ This is a spontaneous process
♦ Once the carbon is ignited, no external energy is required to keep the process going
• In the reverse process, carbon and oxygen is obtained from carbon dioxide
♦ That is: CO2 → C + O2
♦ This reverse process is not spontaneous
♦ Energy is required to accomplish this reverse process
10. We see that, all spontaneous processes proceed in one direction only
• We want to answer this question:
Why all spontaneous processes proceed only in one direction?
11. To find the answer, we consider some common phenomena that we see in our day to day life
(i) Flowing of water
• The flow of water starts from top of hill and ends at the ground level
• During the flow, the potential energy stored in the water is continuously released in the form of kinetic energy
• When the water reaches the ground level, it’s potential energy will be zero. This is because, all the potential energy is released (in the form of kinetic energy) into the surroundings
(ii) Stone falling from a height
• The fall of stone starts from a higher level and ends at the ground level
• During the fall, the potential energy stored in the stone is continuously released in the form of kinetic energy
• When the stone reaches the ground level, it’s potential energy will be zero. This is because, all the potential energy is released (in the form of kinetic energy) into the surroundings
12. The flow of water and fall of stone are spontaneous processes. They do not require any aid of external energies
• So we are inclined to think that:
All processes in which there is a ‘release of stored energy’ will be spontaneous
13. We know that, in exothermic reactions, the stored chemical energy is released as heat energy
• So we are inclined to think that:
♦ All exothermic reactions are spontaneous
♦ The reverse of an exothermic reaction is endothermic
✰ It involves absorbtion of energy
♦ So a spontaneous reaction proceeds in the exothermic direction only
• Let us examine whether this is true
14. Some thermochemical equations are given below:
(i) 1/2N2(g) + 3/2H2(g) → NH3(g); ΔH⊖r = – 46.1 kJ mol-1
(ii) 1/2H2(g) + 1/2Cl2(g) → HCl(g); ΔH⊖r = – 92.32 kJ mol-1
(iii) H2 + 1/2O2(g) → H2O(l); ΔH⊖r = –285.8 kJ mol-1
• The above three reactions are spontaneous
• The negative sign of ΔH⊖r shows that, they are exothermic reactions
◼ So we become even more inclined to think that:
♦ All exothermic reactions are spontaneous
♦ Being exothermic is the only criterion for spontaneity
15. But before making a decision, let us see two more thermochemical equations:
(i) 1/2N2(g) + O2(g) → NO2 (g); ΔH⊖r = +33.2 kJ mol-1
(ii) C(graphite, s) + 2S(l) → CS2(l); ΔH⊖r = +128.5 kJ mol-1
• The above two reactions are spontaneous
• The positive sign of ΔH⊖r shows that, they are endothermic reactions
◼ So it is impossible to conclude that:
Being exothermic is the only criterion for a reaction to be spontaneous
• Scientists became convinced that, there are some other factors also playing major roles
• Researches in this direction lead to the discovery of entropy and the second law of thermodynamics
• We will see them in the next section
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