Short Answer TypeA 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?
When 1 gm of liquid naphthalene (C10H8) solidifies, 49 joules of heat is evolved. Calculate the enthalpy of fusion of naphthalene.
Long Answer TypeHow 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?
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?
State and explain Hess's law of constant heat summation.
This law states that if a reaction is the sum of two or more constituent reactions, then ∆ H for the overall process must be the sum of all ∆H of the constituent reactions. In brief, the enthalpy change for a reaction is the same whether it occurs in one step or in a series of steps.
Consider a reaction in which reactant S1 is converted to product S4. There are two reactions for converting S1 to S4:
(i) Direct reaction: Reactant S1 is directly converted to product S4 in one step.
Let ∆rH° be the enthalpy change during this conversion.
(ii) Indirect reactions: Let reactant S1 be directly converted to S4 in a number of steps.
be the enthalpy changes in the first (a), second (b) and third reaction (c) respectively.
According to Hess's law
Here a, b, c .....(as stated above) refer to the balanced thermochemical equation that can be summed to give the equation for the desired reaction.
Illustration of Hess’ law: Carbon when burnt in oxygen forms CO2 in two ways: First way:
Second way: Carbon may be first converted into CO and then changed into CO2.
Total amount of enthalpy changes in a second way
= -110.5 - 283.0
= -393.5 kJ
which is the same as in the first case.
Short Answer TypeThe thermochemical equations for the formation of water are:
How much energy is needed to convert 2 mol of liquid water to water vapour?
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