Module 4
Use of free energy in chemical equilibria
Question Bank
- Define enthalpy, entropy and free energy?
Enthalpy:
Enthalpy a process or activity takes place at constant pressure, the heat released or absorbed is equal to the Enthalpy change. Enthalpy is occasionally called or known as “heat content”, “enthalpy” is derived from the Greek word which means “warming”. Enthalpy(H) is the sum of the internal energy(U) and the result of pressure(P) and volume(V).
Enthalpy H is written as,
H = U + PVm
Where, H = is the Enthalpy of the system
U = is the Internal energy of the system and is entirely dependent on the state functions T, p and U.
Enthalpy can also be written as
ΔH=ΔU+ΔPV.
P = is the Pressure of the system
V = Volume of the system Enthalpy is not directly measured, but the change in enthalpy (ΔH) is measured, which is the heat lost or added by the system,
Entropy:
It was Introduced by the German Physicist Rudolf Clausius in 1850, and is a highlight of 19th century Physics. The word“entropy” is derived from the Greek word, which means “turning”. It was derived to provide a quantitative measure for the spontaneous changes and Clausius introduced the concept of entropy as a precise way of expressing the Second Law of Thermodynamics, Clausius form of the second law states that spontaneous change for an irreversible process in an isolated system
Is a measure of randomness or irregularity or disorder of the system?
The more or the randomness, higher is the entropy.
Solid state has the lowest or least entropy, the gaseous state has the highest entropy and the liquid state has the entropy that lies between the two.
Entropy is a state function. The changes in its value during any process, is called the entropy change.
ΔS = S2 -S1 = ∑S products – ∑S reactants
Gibbs Free Energy is a measure of the potential for reversible or maximum work that may be done by a system at constant temperature and pressure. It is a thermodynamic function that was discovered in 1876 by Josiah Willard Gibbs to assume if a process will occur spontaneously at constant temperature and pressure. Gibbs free energy G is defined as
G = H - TS
Where H, T, and S are the Enthalpy, temperature, and entropy. The SI unit for Gibbs energy is the kilojoule.
2. Write the equations that interrelate enthalpy, entropy and free energy?
Gibbs free energy combines both the enthalpy and entropy into a single value.
Gibbs free energy is the energy t with a that associates itself with a chemical reaction. It equals the enthalpy minus the temperature of the product and the entropy of the system.
G=H-TS
At constant temperature
ΔG = ΔH– TΔS
ΔG predicts the direction of a chemical reaction. If ΔG value is negative, then the corresponding reaction is spontaneous. If ΔG value is positive then reaction is non-spontaneous.
ΔGº=ΔHº-TΔSº
Where
ΔGº = Gibbs free energy (J or KJ)
ΔHº=enthalpy
T=Temperature
ΔSº=Entropy
Gibbs free energy is the energy that is available to do quality work.
A reaction will spontaneously occur if ΔG<0 (exergonic reaction)
A reaction will not spontaneously occur if ΔG>0 (endergonic reaction).
If ΔG value is less than zero, there is a thermodynamic force for the reaction or it drives the process in the forward direction.
When ΔG is positive, then reactants are favoured, when ΔG=0 system is at equilibrium.
3. Explain E.M.F and its importance?
Electromotive force, or, as it is often written, e.m.f., is described as that source of energy which enables electrons movement around an electric circuit.
For any object to move from rest, there has to be some energy change. To ensure electrons movement round an electrical circuit, they should receive energy from a source of e.m.f. Which usually is a battery or a generator.
For every coulomb of electricity to move completely around an electrical circuit, a certain amount of electrical energy is needed, which depends on the particular circuit. The e.m.f. Is expressed in volts and is numerically the number of joules of energy given by the source of e.m.f. To each coulomb to enable movement around the circuit. The symbol for volt is the capital letter V.
Thus
Joules coulombs=volts.
4. Explain the Nernst equation?
Any change in the Gibbs free energy G directly correspond to changes in free energy for processes at constant temperature and pressure, change is the maximum non-expansion work obtainable under these conditions in a closed system; ΔG is negative for spontaneous process, positive for nonspontaneous process, and zero for processes at equilibrium.
It takes into consideration the values of the standard electrode potentials, temperature, activity and the reaction quotient for the calculation of cell potential. For any cell reaction, that occurs Gibbs free energy can be related to standard electrode potential as:
ΔG =-nFE
Where, n = number of electrons transferred in the reaction,ΔG= Gibbs free energy, E= cell potential F = Faradays constant (96,500 C/mol) and. Under standard conditions, the above equation can be written as,
ΔGo =-nFEo
5. How does corrosion effect metals and highlight its prevention?
Corrosion is the disintegration of a metal due to the chemical reactions between the metal and the surrounding environment. Both the types of metal and the environmental conditions, particularly gasses that come in contact with the metal, determine the form and rate of the corrosion.
All metals can corrode. Some metals, like pure iron, deteriorate very fast. Stainless steel, and, metals that combines with iron and other alloys, is slower to corrode and is therefore used more efficiently.
An effective prevention system begins in the design stage with a proper understanding of the environmental conditions and metal properties. Engineers who work with metallurgical experts should select the proper metal or alloy for every situation. They should also be aware of possible chemical interactions between metals used for surfaces, fittings, and fastenings. Some can be prevented from corrosion by adding alloys to a pure metal. Others can be avoided by carefully combining metals or management of the metal's environment.
6. State two differences between Hard and soft water?
7. Explain two parameters that affect the quality of water?
The parameters that affect the quality of water include:
Turbidity: is the cloudiness present in water. It is a measure of the ability of light to pass through water. It is caused by suspended materials such as silt, clay, plankton, organic material, and other particulate materials present in water.
Turbidity in drinking water is aesthetically unacceptable, which makes the water look unappetizing.
Temperature: Palatability, viscosity, solubility, odours, and chemical reactions are influenced by temperature. Thereby, the chlorination and sedimentation, processes and biological oxygen demand (BOD) are dependent on temperature.
8. Explain the Acid Base Reactions?
Acid Base reactions
The very basic and important compounds present in the world are the Acids and bases, they form the centre of all reactions, several theories are put forth regarding the Acid Base behaviour.
The Arrhenius theory
The Swedish physicist Svante Arrhenius had proposed this theory, according to him, in any aqueous solution the acid is the substance that is shown to increase the Hydronium ion (H3O+) concentration, on the other hand for a given aqueous solution a base is a substance that increases the concentration of hydroxide ion (OH−) concentration. Well-known acids examples include Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3), and acetic acid (CH3COOH). Examples of Bases includes such common substances as caustic soda (sodium hydroxide, NaOH) and slaked lime (calcium hydroxide, Ca (OH)2). Another common base which is ammonia (NH3), reacts with water to gives a solution that is basic in nature, when we consider the following balanced equation.NH3(aq) + H2O(l) → NH4+(aq) + OH−(aq) (This reaction occurrence is minimal; the hydroxide ion concentration is small but measurable.)
9. Define Redox Reactions?
Oxidation reduction or (redox reactions) these reactions involve one or more electrons from a reducing agent to an oxidizing agent. The Redox reactions has the effect of reducing the apparent or real electric charge on an atom with respect to the substance being reduced and increase the electric charge on an atom for the substance that is being oxidised. Simple redox reactions include the reactions of a substance or an element reacting with oxygen. For example, magnesium burns in oxygen to form magnesium oxide (MgO). The product formed is an ionic compound, made up of Mg2+ and O2− ions. In the reaction each oxygen atom accepts two electrons and gets reduced on the other hand each oxygen atom gives out two electrons and gets oxidised.
Another common redox reaction is one step in the rusting of iron in damp air. The reaction is as follows
2Fe(s) + 2H2O(l) + O2(g) → 2Fe (OH)2(s)Here iron metal is oxidized to iron dihydroxide (Fe (OH)2); elemental oxygen (O2) is the oxidizing agent.
