Form 5 Chemistry – PHYSICAL CHEMISTRY – Energetics

CALCULATIONS OF ENTHALPIES BY USING ATOMIZATION DATA

During the reaction reactants are atomization (changed to atoms) while the product energy. Calculate from bond energy.

Example1

Example 2

Calculate the following of formation of CH₄ given that,

Enthalpy of atomization of carbon =

Enthalpy of atomization of hydrogen

Solution

Example 3

Use the information provided in the above example. Then calculate the heat of formation of ethane.

Solution

F.B.E

Example 4

The energy required to atomize 6g of carbon is 357.5 KJ. Calculate the energy required to atomize hydrogen atom. If the heat of formation of methane is given that,

Solution

CALCULATION OF ENTHALPIES BY CALORIMETRY

Calorimetry Is the method determined the heat change reaction by the using calorimeter.

Example of enthalpies that can be determined by calorimetry method;-

i) Enthalpy of hydrogen.

ii) Enthalpy of neutralization.

DETERMINATION OF ENTHALPIES OF NEUTRALIZATION BY CALORIMETRY

What is neutralization reaction?

Neutralization reaction is the reaction between acid and base to give salt and water.

Example

In this method the density of salt solution formed is assumed to be equal to the density of water.

Quantity of that in this method can be calculate by using heat capacity ( C )and by using specific heat capacity ( c ) .

Let the quantity of heat be Q.

By using heat capacity

C = heat capacity.

By using specific heat capacity.

Where by is change in temperature.

c =  specific heat capacity.

If quantity of heat calculated is the same and is also the same, then the relationship between c and C can be,

Enthalpy of neutralization is calculate with respect to the number of moles of water producer.

Reaction.

But number of moles of water depends on the moles of limiting reagent

QUESTION
What is limiting reagent?

Limiting reagent is the reactant compound in the neutralization reaction which have small number of moles.

Example 1

250cof NaOH of 0.4M were added to 250cc of the HCl of 0.4M in a calorimeter. The temperature of the two solutions and the calorimeter was. The mass of calorimeter was 50g and its specific heat capacity was after the reaction the temperature rose to 19.5

Assuming the specific heat capacity of all the solution is. Calculate the standard enthalpy of neutralization.

Solution

Data

Reaction

Limiting reagent

Base

Acid

The same 0.1 mol

Both are limiting reagent.

Moles of water produced

.

Example 2

Define

i) Heat of neutralization.

Is the heat given out when one mole of water is formed from the reaction between base and acid at standard state.

ii) Heat of Ionization.

Is the heat energy required to remove an electron from the outer most shell of a gaseous atom.

iii) Heat of sublimation.

Is the heating absorbed when one mole solid atom is converted to gaseous atom.

iv) Limiting reagent.

Is the reacted compound in the neutralization reaction which have small number of moles.

b) State Hess`s Law of heat summation.

It state’s that,

“If a reaction take place by more than are route the overall heat change is the same for which ever route may be taken”.

Question

For neutralization. The reading was absorbed to rose temperature of both the calorimeter and the solution by 3.4k. Calculate the standard enthalpy of neutralization of . Given that capacity of calorimeter is and specific heat capacity of the solution was 4.2.