Give enthalpy (H) of all elements in their standard state.

Give enthalpy (H) of all elements in their standard state.
by

1 Answer

Answer :

Ans. In standard state enthalpies of all elements is zero.
by

Related questions

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)

(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.

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.

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.

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.

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 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.

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.

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.

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

Define i) surface tension ii) viscosity. Give their units.

Description : Define i) surface tension ii) viscosity. Give their units.

Last Answer : Ans.i) The force acting parallel to the surface and perpendicular to a line of unit length anywhere in the surface is called surface tension. Its unit is Nm-1 . ii) The force of friction which one part of the liquid offers to another part of the liquid is called viscosity. Its unit is poise.

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.

Give relation between Cp and Cv.

Description : Give relation between Cp and Cv.

Last Answer : Ans. Cp – Cv = R

Give three differences between evaporation and boiling.

Description : Give three differences between evaporation and boiling.

Last Answer : Ans.i) Evaporation occurs at all temperatures while boiling occurs at boiling point. ii) Evaporation is surface phenomenon while boiling occurs throughout the bulk of liquid. iii) Evaporation is slow while boiling is rapid.

Give one example for each of the following types of inter-particle forces (a) London dispersion forces (b)Dipole-Dipole forces (c) Hydrogen bond (d) Ion-dipole forces

Description : Give one example for each of the following types of inter-particle forces (a) London dispersion forces (b)Dipole-Dipole forces (c) Hydrogen bond (d) Ion-dipole forces

Last Answer : Ans. (a) noble gases. (b) between HCl molecules. (c) Between water molecules or HF molecules. (d)Between NO3- and water 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 is a thermodynamic state function?

Description : What is a thermodynamic state function?

Last Answer : Ans. A function whose value is independent of path. eg. P, V, E, H

State First Law of thermodynamics.

Description : State First Law of thermodynamics.

Last Answer : Ans. Energy can neither be created nor destroyed. The energy of an isolated system is constant. ∆U=q+ w.

(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.

Name the state variables that remain constant in isobaric and isochoric process.

Description : Name the state variables that remain constant in isobaric and isochoric process.

Last Answer : Ans. Pressure and Volume .

Differentiate between path fuction and state function.

Description : Differentiate between path fuction and state function.

Last Answer : Ans. Path Function : Those properties which depend upon path followed.e.g,work. Sate Function : Those properties which depend upon the initial and final staes of the system and not upon the path followed. E.g, enthalpy.

State with expression Dalton’s law of partial pressure?

Description : State with expression Dalton’s law of partial pressure?

Last Answer : Ans. The total pressure exerted by the mixture of non reactive gases is equal to the sum of the partial pressures of individual gases. Ptotal = p1 + p2 + p3 +…………………

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.

According to you, what are the factors that determine the state of matter

Description : According to you, what are the factors that determine the state of matter

Last Answer : Ans. The factors that determine the different states of the matter are pressure, temperature .

Name two intermolecular forces that exist between HF molecules in liquid state.

Description : Name two intermolecular forces that exist between HF molecules in liquid state.

Last Answer : Ans. Dipole-dipole interaction and hydrogen bonding.

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.

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

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 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.

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 +…………

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.

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

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.

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