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Thermodynamics

Short Answer Type

201. At 298K, enthalpy of fusion of sodium metal is +2.6 kJ mol^–1 and enthalpy of vaporisation of liquid sodium is + 98.0 kJ mol^–1. Calculate the enthalpy of sublimation of solid sodium at 298K.

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202.

A swimmer coming out from a pool is covered with a film of water weighing about 80 g. How much heat must be supplied to evaporate this water?

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203.

When 1 gm of liquid naphthalene (C₁₀H₈) solidifies, 49 joules of heat is evolved. Calculate the enthalpy of fusion of naphthalene.

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Long Answer Type

204.

How many grams of methane and volume of oxygen at N.T.P. would be required to produce 445.15 kJ of heat in the following reaction?

$CH subscript 4 left parenthesis straight g right parenthesis space plus space 2 straight O subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow space space space space CO subscript 2 left parenthesis straight g right parenthesis space plus space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon$
$increment straight H space equals space minus 890.3 space k J$

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205. Discuss a method for the measurement of heat of combustion at constant volume (q_v or ∆E).

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206.

Standard vaporisation enthalpy of benzene at its boiling point is 30.8 kJ mol^-1 ; for how long would a 100W electric heater has to operate in order to vaporise a 100g sample of benzene at its boiling temperature?

$open square brackets Power space equals space energy over time. space 1 straight W space equals space 1 space Js to the power of negative 1 end exponent close square brackets$

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207.

State and explain Hess's law of constant heat summation.

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208. How is Hess’s law helpful in calculating the enthalpy of formation of carbon monoxide?

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Short Answer Type

209. What is transition? How is Hess’s law helpful in determining the enthalpy of transition?

Transition means the conversion of one allotropic form of a substance to another. Hess’s law is very useful in determining the enthalpy changes of those reactions which take place extremely slowly. For example, the transformation of rhombic sulphur to monoclinic sulphur is a very slow process. No direct method is available to determine its enthalpy change. It is calculated by Hess’s law. We aim at the equation,

Experimentally it has been found that

Subtracting equation (ii) from (i), we have,

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210.

The thermochemical equations for the formation of water are:

$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis$
$increment subscript straight r straight H to the power of 0 space equals space minus 483. space 8 space kJ$
$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space increment subscript straight r straight H to the power of 0 space equals space minus 571.8 space kJ$
How much energy is needed to convert 2 mol of liquid water to water vapour?

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