Under what condition, the heat evolved or absorbed in a reaction is equal to its free energy change?

Under what condition, the heat evolved or absorbed in a reaction is equal to its free energy change?
by

1 Answer

Answer :

Ans. As ΔG = ΔH – TΔS. Thus, ΔG = ΔH only when either the reaction is carried out at 0^0 K or the reaction is not accompanied by any entropy change, i.e., ΔS = 0
by

Related questions

Under what condition, the heat evolved/absorbed in a reaction is equal to its free energy change?

Description : Under what condition, the heat evolved/absorbed in a reaction is equal to its free energy change?

Last Answer : Ans. In ∆G = ∆H – T. ∆S, when reaction is carried out at 0 K or ∆S = 0, then ∆G = ∆H.

(i) Under what conditions is the heat absorbed by a system is equal to the work done by the system? (ii)In a process, 802 J of heat is absorbed by a system and 294 J of work is done by the system. What is the change in internal energy for the process?

Description : (i) Under what conditions is the heat absorbed by a system is equal to the work done by the system? (ii)In a process, 802 J of heat is absorbed by a system and 294 J of work is done by the system. What is the change in internal energy for the process?

Last Answer : (i) When ∆U=0 i.e, for isothermal process q = w (ii)ΔU = q + w ΔU = 802 + (-294) =508J

What is meant by entropy driven reaction? How can the reaction with positive change of enthalpy and entropy be made entropy driven?

Description : What is meant by entropy driven reaction? How can the reaction with positive change of enthalpy and entropy be made entropy driven?

Last Answer : Ans.The free energy change of a reaction is given by ΔG = ΔH - TΔS For a reaction to be spontaneous, ΔG should be -ve. If both ΔH and ΔS are positive ΔG can be -ve if TΔS>ΔH in magnitude. Thus, entropy ... is low, TΔS should be greater than ΔH (ii) If ΔS is small, T should be so large that TΔS>ΔH.

Is the bond energy of all the four C-H bonds in CH4 molecule equal? If not then why ? How is C-H bond energy then reported?

Description : Is the bond energy of all the four C-H bonds in CH4 molecule equal? If not then why ? How is C-H bond energy then reported?

Last Answer : Ans. No because after breaking of C-H bonds one by one, the electronic environments change. The reported value is the average of of the bond dissociation energies of the four C-H bonds.

Two ideal gases under same pressure and temperature are allowed to mix in an isolated system. What will be the sign of entropy change?

Description : Two ideal gases under same pressure and temperature are allowed to mix in an isolated system. What will be the sign of entropy change?

Last Answer : Ans. Entropy change (∆S) is positive. It is because disorder or degree of freedom increases on mixing.

State Hess’s law of constant heat summation.Give its two applications.

Description : State Hess’s law of constant heat summation.Give its two applications.

Last Answer : Ans. Hess's law states that enthalpy change for a reaction is same whether the reaction occurs in single step or several steps. OR If a reaction occurs in several steps then ,its standard enthalpy of ... the formation of the compound (ii) It is used to calculate Lattice enthalpy of an ionic solid.

Classify the following properties as extensive and intensive properties: Molar heat capacity, temperature, enthalpy and volume.

Description : Classify the following properties as extensive and intensive properties: Molar heat capacity, temperature, enthalpy and volume.

Last Answer : Ans. Extensive properties : Enthalpy and volume. Intensive properties: Molar heat capacity, temperature.

(i)What are sign conventions used for Heat and Work? (ii).Neither ‘q’ nor ‘w’ is a state function but “ q + w” is a state function. Assign reason.

Description : (i)What are sign conventions used for Heat and Work? (ii).Neither ‘q’ nor ‘w’ is a state function but “ q + w” is a state function. Assign reason.

Last Answer : Ans. (i)Heat absorbed by the system = +ve Heat evolved by the system = -ve Work done by the system = -ve Work done on the system = +ve (ii)ΔU = q + w and ‘∆U’ is a state function, which depends upon initial and final states of the system and not upon its path.

(i)State why heat changes in physical and chemical processes are indicated by enthalpy changes and not by entropy changes? (ii)What are the limitations of First Law of thermodynamics?

Description : (i)State why heat changes in physical and chemical processes are indicated by enthalpy changes and not by entropy changes? (ii)What are the limitations of First Law of thermodynamics?

Last Answer : (i) Most of the processes are carried out in an open system i.e, at constant pressure. (ii)(a) Can’t explain the feasibility of the reaction. (b)It poses no restriction on the direction of flow of heat. (c)Practically it is not possible to convert the heat energy into an equivalent amount of work.

if the equilibrium constant for the reaction is 4.0, what will be the equilibrium constant for the reverse reaction.

Description : if the equilibrium constant for the reaction is 4.0, what will be the equilibrium constant for the reverse reaction.

Last Answer : Ans. Kc=1/4

What will be the value of K ,If reaction is reversed?

Description : What will be the value of K ,If reaction is reversed?

Last Answer : Ans. The value of K will get inversed.

When ΔH>0 and ΔS< 0, reaction is never spontaneous. Explain.

Description : When ΔH>0 and ΔS< 0, reaction is never spontaneous. Explain.

Last Answer : Ans. When ΔH > 0 , energy factor opposes the reaction . When ΔS< 0, i.e. randomness decreases, this factor also opposes the process. As both factors oppose the process, reaction is never spontaneous.

How fishes remain alive under frozen water (ice) ponds?

Description : How fishes remain alive under frozen water (ice) ponds?

Last Answer : When water freezes into ice, the empty spaces are left between water molecules, due to hydrogen bonding. So as a result ice becomes lighter then water and start floating on surface of water. It is also non-conductor of heat, so water beneath it doesn’t freezes and fishes remain alive under it.

Predict the entropy change in- (i) A liquid crystallizes into solid (ii) Temperature of a crystalline solid raised from 0K to 115K.

Description : Predict the entropy change in- (i) A liquid crystallizes into solid (ii) Temperature of a crystalline solid raised from 0K to 115K.

Last Answer : Ans. (i) Entropy decreases because molecules attain an ordered state. (ii) Entropy increases because from 0K to 115K average kinetic of moleculesincreases.

What is spontaneous change? Give one example.

Description : What is spontaneous change? Give one example.

Last Answer : Ans. A process which can take place of its own or upon initiation under some condition. eg. Common salt dissolves in water of its own.

We are consuming a lot of electrical energy, solar energy etc Do you, therefore, conclude that energy of the universe is continuously decreasing? Explain. Which other thermodynamic quantity is continuously increasing or decreasing?

Description : We are consuming a lot of electrical energy, solar energy etc Do you, therefore, conclude that energy of the universe is continuously decreasing? Explain. Which other thermodynamic quantity is continuously increasing or decreasing?

Last Answer : Ans. No, energy of the universe remains constant (law of conservation of energy). Entropy of the universe is continuously increasing.

(i)What happens to the internal energy of the system. (a) if work is done by the system. (b)Work is done on the system. (ii) Why enthalpy of neutralization of strong acid and base is consant?

Description : (i)What happens to the internal energy of the system. (a) if work is done by the system. (b)Work is done on the system. (ii) Why enthalpy of neutralization of strong acid and base is consant?

Last Answer : Ans. (i).(a)ΔU Decreases. (ΔU = q - w) (b)ΔU increases. (ΔU = q + w) (ii) Strong acids and bases dissociate completely into ions in aqueous solution and form 1 mole of water.

Elaborate the statement that physical state of existence at given conditions is a balance between the thermal energy and intermolecular forces.

Description : Elaborate the statement that physical state of existence at given conditions is a balance between the thermal energy and intermolecular forces.

Last Answer : Ans. Intermolecular forces tend to keep the molecules together but thermal energy of the molecules tends to keep them apart. The three states of matter are the result of the balance between intermolecular forces and thermal energy of the molecules.

Arrange solid, liquid and gas in order of energy. Give reason.

Description : Arrange solid, liquid and gas in order of energy. Give reason.

Last Answer : Ans. Solid < liquid < gas. This is because solid absorb energy to change into a liquid which further absorb energy to change into a gas.

What do you mean by thermal energy?

Description : What do you mean by thermal energy?

Last Answer : Ans. Thermal energy is the energy of a body arising from motion of its atoms or molecules.

State First law of thermodynamics. Give its mathematical expression.

Description : State First law of thermodynamics. Give its mathematical expression.

Last Answer : Ans. Energy can neither be created nor destroyed but can be transformed from one form to another. ΔU = q + w

Explain the effect of increasing the temperature of a liquid, on intermolecular forces operating between its particles. What will happen to the viscosity of a liquid if its temperature is increased? Arrange the following in increasing order of viscosity-water, hexane, glycerine.

Description : Explain the effect of increasing the temperature of a liquid, on intermolecular forces operating between its particles. What will happen to the viscosity of a liquid if its temperature is increased? Arrange the following in increasing order of viscosity-water, hexane, glycerine.

Last Answer : Ans. On increasing the temperature , the kinetic energy of liquid molecules increases so that it can overcome the attractive forces between the molecules and hence liquid can flow more easily i.e. viscosity decreases. Order of increasing viscosity : hexane < water < glycerine.

What is meant by compressibility factor of gases? How does its value deviate from that of an ideal gas in case of real gases and what does it indicate?

Description : What is meant by compressibility factor of gases? How does its value deviate from that of an ideal gas in case of real gases and what does it indicate?

Last Answer : Ans. It is the ratio of the product PV to nRT. Z = PV/nRT. For ideal gas its value is unity. Z < 1 indicates negative deviation i.e. gas is more compressible due predominance of attractive forces. Z > 1 indicates positive deviation i.e. gas is less compressible due predominance of repulsive forces.

What is kinetic theory of matter? Write down its postulates?

Description : What is kinetic theory of matter? Write down its postulates?

Last Answer : The kinetic theory explains the properties of different states of matter especially that of gases. Its main points are as following: 1- Matter is made up of atoms, molecules or ions. ... is very small. 4- Material particles generally have translational, rotational and vibrational type of movements.

Why is the PH of blood remains almost constant?

Description : Why is the PH of blood remains almost constant?

Last Answer : Ans: Blood is a buffer solution and resist the change in PH

How much is the percentage ionization of an electrolyte to classified is as 'weak electrolyte'?

Description : How much is the percentage ionization of an electrolyte to classified is as 'weak electrolyte'?

Last Answer : Ans:

What is the molarity of pure water?

Description : What is the molarity of pure water?

Last Answer : Ans: 55.5M

What type of substances are generally called 'electrolyte'?

Description : What type of substances are generally called 'electrolyte'?

Last Answer : Ans: Acid, bases and salts

Write the effect of temperature on Equilibrium constant.

Description : Write the effect of temperature on Equilibrium constant.

Last Answer : Ans. The value of Equilibrium constant changes with the temperature. The increase in rate depends upon activation energy .As the activation energy are different for forward & backward direction .So temperature will ... & backward direction. As K= Kf/ Kb ,so the state of equilibrium will be altered.

Why the Equilibrium constant is taken as dimensionless?

Description : Why the Equilibrium constant is taken as dimensionless?

Last Answer : Ans. Activities are used in place of molar Conc. These represent the concentration or pressure with respect to standard state.

What will be the effect on equilibrium if (i) a catalyst is added to it (ii) An inert gas is added at constant volume

Description : What will be the effect on equilibrium if (i) a catalyst is added to it (ii) An inert gas is added at constant volume

Last Answer : Ans. (i) The state of equilibrium is not disturbed on adding catalyst rather it is attained quickly. (ii) No effect on state of equilibrium

Explain why the gas fizzes out when soda water bottle is opened?

Description : Explain why the gas fizzes out when soda water bottle is opened?

Last Answer : Ans. In soda water bottle carbon dioxide gas is dissolved under pressure. As soon as the bottle is opened, the pressure inside the bottle tends to decrease, so the solubility decreases and gas escapes out.

Applying Le- Chatelier's principle, predict the effect of temperature and pressure on evaporation of water.

Description : Applying Le- Chatelier's principle, predict the effect of temperature and pressure on evaporation of water.

Last Answer : Ans Evaporation of water can be represented as water + heat water vapours: i) With increase in pressure- the equilibrium tend to shift in direction where there is decrease in the vol. i.e. ... left. ii) with increase in temperature- the equilibrium shifts to right as it is an endothermic reaction.

Write the significance of equilibrium constant?

Description : Write the significance of equilibrium constant?

Last Answer : Ans.The numerical value of equilibrium constant is a measure of the extent to which the reactants have been converted into products. A large value of K indicates large amount of ... its low value indicates less amount of reactants converted into products before achieving the equilibrium state.

What are the conditions for getting maximum yield of NH3 by Haber’s process?

Description : What are the conditions for getting maximum yield of NH3 by Haber’s process?

Last Answer : Ans. High conc. Of N2 &H2, Low Temp. & High Pressure.

What happens to the dissociation of PCl5 in a closed vessel if He gas is introduced into it at the same temperature?

Description : What happens to the dissociation of PCl5 in a closed vessel if He gas is introduced into it at the same temperature?

Last Answer : Ans. No effect.

Which measurable property become constant in Water Water vapors equilibrium at constant temperature

Description : Which measurable property become constant in Water Water vapors equilibrium at constant temperature

Last Answer : Ans. Vapour pressure.

what do you mean by homogenous and heterogeneous equilibrium?

Description : what do you mean by homogenous and heterogeneous equilibrium?

Last Answer : Ans. Homogenous;-It is that equilibrium reaction in which all the reactants and the products are in the same phase. Heterogeneous equilibrium: - It is that equilibrium reaction in which all the reactants and the products are in two or more phases

Write Application of Equilibrium constant?

Description : Write Application of Equilibrium constant?

Last Answer : Ans. (i) To predict the extent of the reaction. (ii) To predict the direction of the reaction. (iii) To calculate the equilibrium concentration.

Write characteristics of Chemical equilibrium.

Description : Write characteristics of Chemical equilibrium.

Last Answer : Ans. (i) At equilibrium the rate of forward reaction becomes equal to rate of backward reaction. (ii) A chemical equilibrium established only if none of the products are allowed to escape out ... can be attained from either direction. (iv) Catalyst does not alter the state of equilibrium.

what is Equilibrium?

Description : what is Equilibrium?

Last Answer : Ans : Equilibrium represents the state of process in which properties such as temperature, pressure ,concentration of the system do not show any change with the passage of time. OR The state in which rate of forward reaction becomes equal to the rate of backward reaction.

Explain Hess’s law.

Description : Explain Hess’s law.

Last Answer : Ans. It states that the enthalpy change of the reaction remains the same whether the reaction is carried out in one step or several steps, i.e., ∆H = ∆H1 + ∆H2 + ∆H3 +…………

Water can be lifted into the water tanks at the top of the house with the help of a pump. Then why is not considered to be spontaneous?

Description : Water can be lifted into the water tanks at the top of the house with the help of a pump. Then why is not considered to be spontaneous?

Last Answer : Ans. A spontaneous process should continue taking place by itself after initiation . But this is not so in the given case because water will go up so long as the pump is working.

What is the value of ∆G where ice and water are in equilibrium?

Description : What is the value of ∆G where ice and water are in equilibrium?

Last Answer : Ans. ∆G=0

What is the sign of enthalpy of formation of highly stable compound?

Description : What is the sign of enthalpy of formation of highly stable compound?

Last Answer : Ans. Negative.

Which have more entropy real crystal or ideal crystal and why?

Description : Which have more entropy real crystal or ideal crystal and why?

Last Answer : Ans. Real crystal has more entropy because it has more disorderness.

Out of carbon(diamond)and carbon (graphite) whose enthalpy of formation is taken as zero?

Description : Out of carbon(diamond)and carbon (graphite) whose enthalpy of formation is taken as zero?

Last Answer : Ans. Enthalpy of formation of carbon in the form of graphite is taken as zero because it is a more commonly found in stable form of carbon.

Predict the sign of ∆S for (a) the process of evaporation (b) the process of freezing.

Description : Predict the sign of ∆S for (a) the process of evaporation (b) the process of freezing.

Last Answer : Ans. (a) ∆S =+ve (b) ∆S=-ve

Write an expression in the form of chemical equation for the standard enthalpy of formation (∆Hf) of CO (g).

Description : Write an expression in the form of chemical equation for the standard enthalpy of formation (∆Hf) of CO (g).

Last Answer : Ans. C(s) +1/2 O2(g) → CO (g)

A real crystal has more entropy than an Ideal crystal. Why?

Description : A real crystal has more entropy than an Ideal crystal. Why?

Last Answer : Ans. A real crystal has some disorder due to presence of defects in their structural arrangement, and Ideal crystal does not have any disorder.