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 Form 4 Chemistry: Energy changes in chemical and physical processes lessons

Meaning of heat of displacement and Experiment: To determine the molar enthalpy change in the reaction between #Cu^(2+)# ions and zinc or iron

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Answer Text:
4. Heat of displacement.
- This refers to the enthalpy change that occurs when one mole of a substance is displaced from a solution of its ions.
Experiment: To determine the molar enthalpy change in the reaction between #Cu^(2+)# ions and zinc
or iron.
(i) Procedure:
- A plastic cup or glass beaker is wrapped with a newspaper leaf.
- 25 #cm^3# of 0.2M copper (II) sulphate solution is transferred into the beaker.
- The steady temperature of the solution is noted.
- 0.5g of zinc powder are carefully transferred into the plastic cup and stirred with a thermometer.
- The highest temperature attained by the solution is recorded.
Observations:
- The blue colors of copper
(II) sulphate fades.
- Brown deposits of copper metal are formed in the plastic cup.
Explanation:
- Zinc is higher in the electrochemical series than copper;
- Zinc therefore displaces copper ions from its
solution.
#
equation:
Zn(s)+CuSO(aq) to ZnSO_4(aq) +Cu(s)
Ionically:
Zn(s)+Cu^(2+)(aq) to Zn^(2+)+ Cu(s)
#
- During the reaction the blue #Cu^(2+)# in the solution are replaced by the colorless #Zn^(2+)#.
- Consequently the blue colour of the solution fades, as brown deposits of copper metal are formed in the plastic cup.
- Excess solid (zinc powder) was used to ensure complete displacement of #Cu^(2+)#.
Results:
- Initial temperature of copper sulphate solution, #T_1# = 23˚C
- Highest temperature of the mixture T2 = 33ºC
- Temperature change, ΔT; = T2-T1 =10ºC
- Volume of copper sulphate solution used = 25.0 #cm^3#.
- Mass of zinc powder taken = 0.6g
- Density of the solution = #1gcm^-3#.
- Specific heat capacity of the solution = #4.2Kjkg^-1k^-1#.
Assumptions:
- The volume of the solution remains unchanged after the
reaction. (Cu = 63.5, S = 32, O =16, Zn = 65, Fe =56)
Question:
Using the above data,
calculate, calculate the;
(i). Heat change for the above reaction.
(ii). The molar heat of displacement of copper (II) ions.
figenergy25720201138.JPG
- Since the final temperature of the mixture is higher than initial temperature; it means the Hproducts is lower than Hreactants.
- The reaction is thus exothermic.
- Thus molar heat of displacement of #Cu^(2+) = - 210Kjmol^-1#
#Zn(s)+Cu^(2+)(aq) to Cu(s)+Zn^(2+)(aq), DeltaH=-210Kjmol^(-1)#
Note:
- The experimental value for the heat liberated in this reaction is lower than the theoretical value of #216Kjmol^-1#.
Reasons:
- The heat lost to the surroundings and the heat absorbed by the apparatus is not accounted for in the
calculations.
- This reaction is typically a Redox reaction.
figenergy25720201143.JPG