Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Chemistry Class 12

Created by: Mohit Rajpoot

Class 12 : Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

The document Theory & Procedure, Thermochemistry Class 12 Notes | EduRev is a part of the Class 12 Course Chemistry Class 12.
All you need of Class 12 at this link: Class 12

Objective

To determine:

  • Water equivalent of calorimeter.
  • Enthalpy of dissolution of given salt.

The Theory

Thermochemistry is the study of heat and energy associated with a chemical reaction or a physical transformation.  A reaction may absorb or release energy. The reaction which absorbs energy in the form of heat is called endothermic reaction and that which releases energy in the form of heat is called exothermic reaction.

Measurement of heat changes are carried out in vessels called calorimeters. Metallic calorimeters are not used for measuring thermochemical changes because metals may react with substances. Stainless steel or gold plated copper calorimeters may be used. Reactions may also be carried out in beakers placed in a thermo flask, in a thermally insulated box or in a styrofoam cup. During measurement of heat changes, the calorimeter also absorbs some heat; this amount of heat should also be known. It is called calorimeter constant or water equivalent of calorimeter. To determine the water equivalent of calorimeter, known volume of hot water at a specified temperature is added to known volume of water contained in the calorimeter at room temperature. Since energy is conserved, the heat taken by calorimeter and cold water should be equal to heat given by hot water. Thus, we can write the following equation,

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

It is known that when a solute is dissolved in a solvent, heat is absorbed or evolved. Thus, dissolution of a solute in a solvent is accompanied by enthalpy change (∆H) of the system. If heat is absorbed, ∆H is positive. If heat is evolved, ∆H is negative.

The enthalpy change per mole of a solute dissolved varies with the concentration of the solution. The enthalpy of dissolution is defined as the enthalpy change per mole of a solute when it is dissolved in a pure solvent to give a solution of specified concentration.

For example, when one mole of anhydrous calcium chloride is dissolved in 400 moles of water, 78.60 kJ heat is evolved. The thermochemical equation is written as:

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Similarly, when one mole of hydrated calcium chloride is dissolved in 400 moles of water, 18.83 kJ heat is absorbed. The thermochemical equation is written as:

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Learning Outcomes

  • Students understand the terms, water equivalent of calorimeter, enthaply of dissolution of salt etc.
  • Students acquire the skill to perform the experiment in the real lab, once they understand the different steps.

(A) Determination of Water Equivalent of Calorimeter

Materials Required

  • Calorimeter
  • Wooden box with lid
  • Stirrer  
  • Thermometer  
  • Distilled water
  • Cotton  
  • 250 ml beakers
  • Hot plate
  • Measuring jar                                

Real Lab Procedure

  • Using a measuring jar, take 50 ml of distilled water in the calorimeter.
  • Using a thermometer, measure the temperature of water in the calorimeter. Let it be t1 °C.
  • Place the calorimeter in the wooden box and fill cotton in the space between the calorimeter and the wooden box to avoid heat loss.
  • Now insert a stirrer in the calorimeter.
  • Using the measuring jar, pour 50 ml of distilled water in a beaker.
  • Heat water in the beaker to a temperature 10-20 °C (~47 °C) higher than that of room temperature.
  • Stop heating and note the temperature of this warm water. Let it be t2 °C.
  • Add the warm water into the calorimeter without any loss of time.
  • Immediately close the wooden box.
  • Quickly insert the thermometer into the calorimeter.
  • Now, stir the water using the stirrer and note the thermometer reading.
  • Continue stirring until the temperature becomes constant. Let it be t3 °C.

Observations

  • Volume of water taken in the calorimeter = 50 ml

Therefore, mass of water = 50 g (assuming specific density of water is unity)

  • Temperature of water = t1 °C
  • Volume of warm water added = 50 ml
  • Therefore mass of warm water = 50 g
  • Temperature of warm water = t2 °C
  • Temperature after mixing = t3 °C

Calculations:

 
Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

(B) Determination of Enthalpy of Dissolution of Salt in Water at Room Temperature

Materials required

  • Calorimeter
  • Wooden box with lid
  • Stirrer  
  • Thermometer  
  • Distilled water
  • Cotton  
  • Measuring jar
  • Sample
  • Ammonium chloride

Real Lab Procedure

  • Using the measuring jar, take 50 ml of distilled water in the calorimeter.
  • Note the temperature of water using the thermometer. Let it be t1 °C.
  • Place the calorimeter in the wooden box and fill cotton in the space between the calorimeter and the wooden box to avoid heat loss.
  • Insert a stirrer in the calorimeter.
  • Add 5 g of ammonium chloride into the calorimeter.
  • Immediately close the wooden box.
  • Quickly insert the thermometer into the calorimeter.
  • Now stir the water using the stirrer and note the thermometer reading.
  • Continue stirring until the temperature becomes constant. Let it be t2 °C.
  • At this temperature, the substance just dissolves.

Observations

  • Weight of ammonium chloride dissolved, w = 5 g
  • Volume of water taken in the calorimeter = 50 ml
  • Therefore, mass of water taken in the calorimeter = 50 g (assuming specific density = 1)
  • Temperature of water = t1 °C
  • Temperature after dissolving ammonium chloride = t2 °C
  • Water equivalent of calorimeter, W = 11.11 g
  • Molecular mass of ammonium chloride, M = 53.49 g/mol

Calculations

Assuming density and specific heat of the solution to be same as that of water, heat evolved or absorbed for dissolution of w g of the solute,

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Theory & Procedure, Thermochemistry Class 12 Notes | EduRev

Precautions

  • Do not stir the liquid in the calorimeter vigorously because vigorous stirring does cause some increase in temperature.  
  • The space between the calorimeter and the wooden box should be filled with cotton to avoid heat loss.
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