A-level Chemistry PapSol Q1-3

(9701_s12_qp_41)

1 (a) (i) What is meant by the term lattice energy?

SOLUTION:

 It is the energy change when one mole of ionic lattice is formed from gaseous ions.

(ii) Write an equation to represent the lattice energy of MgO.  

SOLUTION:

Mg2(g)+O2-(g) ==> MgO(s)

(b) The apparatus shown in the diagram can be used to measure the enthalpy change of formation of magnesium oxide, (MgO). oxygen gas magnesium ribbon water copper spiral stirrer to suction pump small electric heater (to ignite magnesium) List the measurements you would need to make using this apparatus in order to calculate (MgO). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] 

(c) Use the following data, together with appropriate data from the Data Booklet, to calculate a value of ΔH _f^o(MgO). 

 lattice energy of MgO(s) = –3791 kJ mol–1 

 enthalpy change of atomisation of Mg = +148 kJ mol–1 

 electron affinity of the oxygen atom = –141 kJ mol–1 

 electron affinity of the oxygen anion, O– = +798kJ mol–1 

SOLUTION:

  (MgO) = .......................... kJ mol–1 [3] 

(d) Write equations, including state symbols, for the reactions, if any, of the following two oxides with water. Suggest values for the pH of the resulting solutions. 

oxide                             equation                 pH of resulting solution 

Na2O 

MgO

 

[3] 

[Total: 12] 

 

(9701_s12_qp_41)

2. Nitrogen monoxide, NO, is formed in a reversible reaction when air is heated to the temperature of a car engine. 

(a) (i) Suggest a ‘dot-and-cross’ electronic structure for nitrogen monoxide.

SOLUTION:

(ii) The enthalpy change of formation of nitrogen monoxide is +90 kJ mol–1. What is the enthalpy change for the following reaction? 2NO(g) → N2(g) + O2(g) ∆Hr = ..................... kJ mol–1 

SOLUTION:

Hr =-180kJ/mol

1/2 N2 + 1/2O2 ==> NO   ∆H=+90kJ/mol

So, if you multiply the whole equation by 2 you get

N2 + O2 ==> 2NO    ∆H=+180kJ/mol

But the required reaction is 2NO ==> N2 = O2 hwich is the reverse reaction so enthalpy is reversed too. We get -180kJ/mol

(iii) Explain why nitrogen monoxide is formed in the car engine. 

SOLUTION:

Temperature is bery high that drives the equilibrium to the left as forward reaction is exothermic.

(iv) Using bond enthalpy values from the Data Booklet and your answer in ii) above, calculate a value for the bond energy of nitrogen monoxide. bond energy = .......................... kJ mol–1 [5] 

SOLUTION:

break    made  

2N=O     N≡N

                 O=O

Bond breaking energy -Bond making energy = enthalpy change

2(N=O)-(944+496) = -180

E(N=O) = 630kJ/mol

(b) At 800K, nitrogen monoxide reacts with hydrogen according to the following equation. equation I 

2H2(g) + 2NO(g) → 2H2O(g) + N2(g) 

The following table shows how the initial rate of this reaction depends on the partial pressures of the reagents. 

experiment            p(H2)/atm           p(NO)/atm        initial rate /atm s–1 

1                               0.64                     1.60                   1.50 × 10–7 

2                               0.64                     0.80                   3.75 × 10–8 

3                               0.32                     1.60                   7.50 × 10–8 

(i) Find the order of the reaction with respect to each reactant, explaining how you arrive at your answer. 

SOLUTION:

If we double p(No) the rate quadruples so order is 2. If p(H2) is doubled the rate doubles so the order is 1. 

(IF YOU DIDN'T UNDERSTAND THIS TRY RECALLING ORDERS OF REACTIONS.)

 (ii)  Write down the rate equation and the units of the rate constant.

SOLUTION:

 Rate = kp(H2) p(NO)^2

  The following mechanism has been put forward for this reaction.

step 1 NO  +  NO → N2O  +  O 

step 2 H2  +  O → H2O 

step 3 H2  +  N2O → N2  +  H2O 

(iii)  Show how the overall stoichiometric equation I can be derived from the three equations for the individual steps given above.

SOLUTION:

Add all three equtions

2NO+2H2+O+N2O ==> N2O +O +2H2O +N2

Cut the same terms from both sides of the equation to get:

2NO + 2H2==>2H2O + N2

(iv) Suggest which of the three reactions in the mechanism is the rate determining step. Explain your answer.

SOLUTION:

All the reactants of the stoichiometric equation must be in the rate determining equation. We can see this case in Step 2 and Step 3. 

How in step 2:

H2 is there and O is there.Now look in step 1 to see how O was formed. As it was formed from NO, which is present in main eauation I, so you can replace O with NO.

Same is the case for step 3.

(9701_w17_qp_42)

3 (c) The reaction between ClO2 and F2 is shown.

 

                                                          2ClO2 + F2 ————> 2ClO2F 

 The rate of the reaction was measured at various concentrations of the two reactants and the following results were obtained. 

experiment            [ClO2]/moldm–3              [F2]/moldm–3           initial rate /moldm–3 s–1 

       1                          0.010                               0.060                                  2.20 × 10–3 

       2                          0.025                               0.060                              to be calculated 

       3                    to be calculated                     0.040                                  7.04 × 10–3 

 The rate equation is rate = k [ClO2][F2]. 

(i) What is meant by the term order of reaction with respect to a particular reagent? ............................................................................................................................................. ....................................................................................................................................... [1] 

(ii) Use the results of experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k. rate constant, k = ............................. units ............................. [2] 

(iii) Use the data in the table to calculate 

● the initial rate in experiment 2, initial rate = ............................. moldm–3 s–1 

● [ClO2] in experiment 3. [ClO2] = ............................. moldm–3 [2] 

(d) (i) What is meant by the term rate-determining step? ............................................................................................................................................. ....................................................................................................................................... [1] 

(ii) The equation for the reaction between ClO2 and F2 is shown. 

                                                         2ClO2 + F2 2ClO2F 

                                                            rate = k[ClO2][F2] 

The mechanism for this reaction has two steps. 

Suggest equations for the two steps of this mechanism, stating which of the two steps is the rate-determining step. 

step 1 .................................................................................................................................. 

step 2 .................................................................................................................................. 

rate-determining step = ............................. [2] 

(e) By considering the rate equation, explain why the rate increases with increasing temperature. .................................................................................................................................................... .............................................................................................................................................. [1] 

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