(i) According to Hess's law it is possible to determine the enthalpy of formation of a substance which cannot be measured directly-explain.
(ii) What do you mean by heat of transition?

(i) What is the difference betweenn the enthalpy of formation and the enthalpy of reaction? Explain with proper explain. (ii) what do you understand by the standard state of liquid benzene at 25^(@)C ?

Which of the following statement is incorrect about internal energy? a) The absolute value of internal energy cannot be determined b) The internal energy of one mole of a substance is same at any temperature or pressure c) The measurement of heat change during a reaction by bomb calorimeter is equal to the internal energy change d) Internal energy is an extensive property

X, Y and Z are three substance, On passage of electricity through X, two substances A and B are formed, Y is a liquid which on evaporation generates C as a residue and D was vaporized. Z could not be split up into any simpler substances further. A is found to be a good conductor of heat and electricity while B was found to be a gas. it is possible to split C and D into simpler substances chemically. what do you infer regarding the nature of these substances with respect to composition?

What do you mean by heat of reaction or enthalpy of reaction? What are the factors on which heat of reaction depends?

Explain from newton's third law of motion, what do you mean by an unbalanced force?

What do you mean by evaporation ? Describe the factors which determine the rate of evaporation of a liquid .

(i) Account for the following : (a) White silver chloride turns grey in sunlight. (b) Brown coloured copper powder on heating in air turns into black coloured substance. (ii) What do you mean by (a) Displacement reaction (b) Reduction reaction (c) Combination reaction ? Write balanced chemical equation.

Hess's law of heat of summatiion is based on law of conservation of energy. It has found significance in deriving heats of many reactions which either do not take place or if take place, than other side reactions also accompany it. For example heat of reaction for C+(1)/(2)O_(2) rarr CO can not be studied directly as it occurs with simultaneous reaction of oxidation of CO to CO_(2) . Bond energy is referred as heat enthalpy when a bond is broken to produce isolated gaseous atoms. In case of breaking up of bonds between unlike atoms e.g., C-H bond in CH_(4) , bond energy is referred as avarage bond energy as four C-H bonds are broken up. Which of the following statements are correct ? (1) The dissociation of a ond is always endothermic (2) The formation of a bond is always exothermic. (3) Heat of formation of an atom =1//2xx bond energy like atoms covalent bond. (4) The heat enthalpy change in a chemical reaction is equal but opposite to the heat enthalpy if reaction is reversed. (5) Hess's law can be verified experimentally.