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ISC Class 12 Chemistry Solved Question Paper 2007

Short Answer Type

21. Define molecularity of a reaction. Give one difference between the order of a reaction and its molecularity.

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22. Which one of the following O₂⁺ or O ^-₂ , is considered to be more stable ? Justify your answer on the basis of the Molecular Orbital Theory.

straight O subscript 2 superscript plus colon space Number space of space electrons space equals space 8 plus 8 minus 1 space equals space 15 straight sigma 1 straight s squared comma space straight sigma 1 asterisk times straight s squared comma space straight sigma 2 straight s squared comma space straight sigma 2 asterisk times straight s squared comma space straight sigma squared pz squared comma space straight pi squared py squared comma space straight sigma 2 asterisk times pz to the power of 1 Bond space order space space equals space fraction numerator straight N subscript straight b minus straight N subscript straight a over denominator 2 end fraction space equals space fraction numerator 8 minus 3 over denominator 2 end fraction space equals space 2.5 straight O subscript 2 superscript minus space colon space Number space of space electrons space equals space 8 plus 8 plus 1 space equals space 17 straight sigma 1 straight s squared comma space straight sigma 1 asterisk times straight s squared comma space straight sigma 2 straight s squared comma space straight sigma 2 asterisk times straight s squared comma space straight sigma squared pz squared comma space straight pi squared py squared comma space straight sigma 2 asterisk times pz squared comma straight pi 2 asterisk times px to the power of 1 comma straight pi 2 asterisk times py to the power of 1 Bond space space order space space equals space fraction numerator straight N subscript straight b minus straight N subscript straight a over denominator 2 end fraction space equals space fraction numerator 8 minus 5 over denominator 2 end fraction space equals space 1.5 because space Higher space the space bond space order space more space will space be space stability therefore space straight O subscript 2 superscript plus space is space more space stable space than space straight O subscript 2

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23. Draw the electron dot diagrams or any other suitable representation of chloric acid and hypochlorous acid.

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24.

(i) How many sodium ions and chloride ions are present in a unit cell of sodium chloride ?

(ii) What is the co-ordination number of sodium and chloride ions in sodium chloride crystals ?

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25.

Give a reason for each of the following :

(i) Specific conductance decreases with dilution whereas equivalent conductance increases with dilution.

(ii) Lead is precipitated as PbS while zinc is not precipitated when H₂S gas is passed through an acidic solution of lead nitrate and zinc nitrate.

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26. Mention any two factors that influence the rate of a chemical reaction

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27.

(i) Enthalpy change and entropy change of a chemical reaction at 25°C are 177.0 KJ and 160.4 JK^-1 respectively. Calculate free energy change of the reaction and predict whether the reaction is spontaneous or nonspontaneous.

(ii) 2 moles of an ideal gas at 27°C expand reversibly from 2 litres to 20 litres. Find the entropy change for the reaction.

(iii) State the second law of thermodynamics in terms of entropy of the universe.

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28. If enthalpies of formation for C₂H₄, C0₂ and H₂0 at 25°C and 1 atm. pressure are 52 KJ/mole, -394 KJ/mole and -286 KJ/mole respectively, calculate the enthalpy of combustion of C₂H₄.

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