List of top Chemistry Questions asked in CBSE CLASS XII

The rate constant for the first order decomposition of \(H_2O _2\) is given by the following equation:
\(log\  k = 14.34 - 1.25 \times  10^4\  K/T \)
Calculate \(E_a\) for this reaction and at what temperature will its half-period be 256 minutes?

The rate of a reaction quadruples when the temperature changes from \(293 \ K\) to \(313\  K\). Calculate the energy of activation of the reaction assuming that it does not change with temperature.

What is meant by 'disproportionation'? Give two examples of disproportionation reaction in aqueous solution.

Predict which of the following will be coloured in aqueous solution? Ti³⁺ ,V³⁺ , Cu⁺ , Sc³⁺ , Mn²⁺ , Fe³⁺ and Co²⁺ . Give reasons for each.

For M²⁺/M and M³⁺/M²⁺ systems, the \(E^\ominus\) values for some metals are as follows: 
Cr²⁺/Cr -0.9V 
Cr³ /Cr²⁺ -0.4 V 
Mn²⁺/Mn -1.2V 
Mn³⁺/Mn²⁺ +1.5 V 
Fe²⁺/Fe -0.4V 
Fe³⁺ /Fe²⁺ +0.8 V 
Use this data to comment upon: 
(i) The stability of Fe³⁺ in acid solution as compared to that of Cr³⁺ or Mn³⁺ and 
(ii) The ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

Describe the preparation of potassium permanganate. How does the acidifiedpermanganate solution react with (i) iron(II) ions (ii) SO₂and (iii) oxalic acid? Write the ionic equations for the reactions.

Describe the oxidising action of potassium dichromate and write the ionicequations for its reaction with: (i) iodide (ii) iron(II) solution and (iii) H₂S

Explain giving reasons: 
 (i) Transition metals and many of their compounds show paramagnetic behaviour. 
 (ii) The enthalpies of atomisation of the transition metals are high. 
 (iii) The transition metals generally form coloured compounds. 
 (iv) Transition metals and their many compounds act as good catalyst.

Write down the number of 3d electrons in each of the following ions: Ti²⁺, V²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, CO²⁺, Ni²⁺ and Cu²⁺ . Indicate how would you expect the five 3d orbitals to be occupied for these hydrated ions (octahedral).

The decomposition of hydrocarbon follows the equation
\(k = (4.5 \times  10^{11} s^{-1})e^{-28000 \ K/T }\)
\(Calculate\  E_a\).

Sucrose decomposes in acid solution into glucose and fructose according to the first order rate law, with \(t_{\frac 12} = 3.00\  hours\). What fraction of sample of sucrose remains after \(8 \ hours\)?

Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points: 

1. electronic configurations, 
2. oxidation states, 
3. ionisation enthalpies, and 
4. atomic sizes

Compare the chemistry of the actinoids with that of lanthanoids with reference to: 

1. electronic configuration
2. oxidation states and
3. chemical reactivity.

Use Hund's rule to derive the electronic configuration of Ce³⁺ ion and calculate its magnetic moment on the basis of 'spin-only' formula.

Which is the last element in the series of the actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element.

The chemistry of the actinoid elements is not so smooth as that of the Lanthanoids. Justify this statement by giving some examples from the oxidation state of these elements.

What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104.

What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.

The rate constant for the decomposition of \(N_2O_5\) at various temperatures is given below:

Draw a graph between ln k and \(\frac 1T\) and calculate the values of \(A\) and \(E_a\).
Predict the rate constant at 30 ºC and 50 ºC.