Tag: surface chemistry

Questions Related to surface chemistry

Which of the following are correct about the catalyst?

adsorption theory of heterogeneous catalysis chemistry surface chemistry adsorption and colloids catalysis enzymes and catalysts

  1. They participate in the reaction but recovered at last

  2. It does not affect $\Delta G $

  3. It increases $\Delta G $

  4. It alters the mechanism of reaction

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Correct Option: A,B,D
Explanation:

The catalyst is the substance which increases the rate of reaction by lowering the activation energy which alters the mechanism of the reaction.

Which statement of the following is false for a catalyst?

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. A catalyst can initiate a reaction.

  2. It does not alter the position of equilibrium in a reversible reaction.

  3. A catalyst remains unchanged in quality and composition at the end of the reaction.

  4. Catalysts are sometimes very specific in reaction.

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Correct Option: A
Explanation:

Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst which is not consumed in the catalysed reaction and can continue to act repeatedly.It increase rate of forward as well as backward reaction.

In presence of a catalyst, the activation energy is lowered by 3 kcal at $27^o C$. Hence, the rate of reaction will increase by:

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. 32 times

  2. 243 times

  3. 3 times

  4. 48 times

Show answer
Correct Option: C
Explanation:

$K={Ae}^{\dfrac{-Ea}{RT}}$ 

So Ratio, ${\dfrac{K _2}{K _1}}={e^\dfrac{(Ea _2-Ea _1)}{RT}}$
$\dfrac{K _2}{K _1}$=$e^{\dfrac{(3\times1000)}{(8.314\times300)}}$=$e^{{3000}/{2494}}$ = $e^{1.2}$ = $3.3$
Nearly 3 times.

Which of the following statements is correct about solid catalysts?

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. Catalyst is required in equal amount as the reactants present in the reaction.

  2. Catalytic activity of a solid catalyst does not depend upon the extent of chemisorption.

  3. Desorption is not important for a solid to act as good catalyst.

  4. Same reactants may give different products by using different catalysts.

Show answer
Correct Option: D
Explanation:
Substances, which accelerate the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction, are known as catalysts, and the phenomenon is known as catalysis.
  • For a given chemical reaction the amount of catalyst used is not necessarily to be equal to that of reactants.
  • Catalytic activity of a solid catalyst depends upon the extent of chemisorption because of the various bonds formed.
  • A catalyst is called a good catalyst if not only catalyses the reaction but also gives space for further reaction over its surface .i.e Desorption is a important characteristic.
  • It is not necessary that a reaction to give same products if we use different catalysts, for different cataysts the products may be different.

An inhibitor is described as:

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. a substance that slows down or stops a reaction

  2. a substance which inhibits the properties of a catalyst

  3. a substance formed during the reaction and does not participate in the reaction

  4. a substance which prevents formation of products in a reaction being most reactive

Show answer
Correct Option: A
Explanation:

A reaction inhibitor is a substance that decreases the rate of, or prevents, a chemical reaction. A catalyst, in contrast, is a substance that increases the rate of a chemical reaction.

Presence of traces of arsenious oxide ($As _{2}O _{3}$) in the reacting gases $SO _{2}$ and $O _{2}$ in presence of platinised asbestos in contact process acts as:

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. catalytic promoter

  2. catalytic poison

  3. dehydrating agent

  4. drying agent.

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Correct Option: B
Explanation:

  • Promoters are substances that enhance the activity of a catalyst while poisons decrease the activity of a catalyst.
  • In the manufacturing of sulfuric acid, a step involves the reaction of $SO _2$ and $O _2$ in presence of platinized asbestos to give $SO _3$
                    .i.e. $SO _2+O _2 \rightarrow SO _3$ 
In the above reaction if traces of arsenious oxide ($As _2O _3$)  is used then it acts as the catalytic poison by reducing the catalytic activity of Platinised asbestos.

Which are not examples of autocatalysis?

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. Breakdown of atomic fuel in reactor.

  2. Shock decomposition of glycerol trinitrate.

  3. ZSM-5 used to convert alcohol in petrol.

  4. Storage of chloroform along with little ethanol.

Show answer
Correct Option: A,C,D
Explanation:

A chemical reaction is said to have undergone autocatalysis if the reaction product itself is the catalyst for that reaction.
Shock decomposition of glycerol trinitrate is autocatalysis due to the mixture of gases which acts as autocatalyst. The reaction is as follows:
$4C _3 H _5 (ONO _2) _3 \rightarrow 12CO _2 + 10H _2O + 6N _2 +O _2$

Select the correct statement(s).

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. Enzyme catalysed reactions are highly specific

  2. Enzyme catalysed reactions are highly susceptible to $pH$ and temperature

  3. Decomposition of glyceryl trinitrate is an example of autocatalysis

  4. The acid hydrolysis and alkaline hydrolysis of ester are reversible

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Correct Option: A,B,C

Which of the following statements is/are correct ?

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. The efficiency of a heterogeneous catalyst depends upon its surface area.

  2. Catalyst operates by providing alternate path for the reaction that involves a lower activation energy.

  3. Catalyst lowers the energy of activation of the forward direction without affecting the energy of activation of the backward direction

  4. Catalyst does not affect the overall enthalpy change of the reaction.

Show answer
Correct Option: A,B,D
Explanation:
  • A catalyst that is in a separate phase from the reactants is said to be a heterogeneous, or contact, catalyst. An example of heterogeneous catalysis is the use of finely divided platinum to catalyze the reaction of carbon monoxide with oxygen to form carbon dioxide. The efficiency of a heterogeneous catalyst depends on the surface area, more the surface area more easier the catalysis occurs.
  • Collisions only result in a reaction if the particles collide with certain minimum energy called the activation energy for the reaction. A catalyst provides an alternative route for the reaction with lower activation energy. It does not "lower the activation energy of the reaction".
  • Lowering the Activation Energy of a Reaction by a Catalyst. The only effect of the catalyst is to lower the activation energy of the reaction. The catalyst does not affect the energy of the reactants or products. Thus it doesn't affect $\Delta H$.

A catalytic poison renders the catalyst ineffective because:

enzymes and catalysts catalysis adsorption and colloids surface chemistry chemistry adsorption theory of heterogeneous catalysis

  1. It is preferentially occupy surface of the catalyst.

  2. It adsorbs the molecules of the reactants.

  3. It combines chemically with the catalyst.

  4. It combines with one of the reactants.

Show answer
Correct Option: A
Explanation:

The poison is adsorbed in the surface of the catalyst and hence the catalyst cannot contribute to the reaction and even poisoning can react chemically with the catalyst.
Hence option A is correct.