Chemistry: CUET Mock Test - 9 - CUET MCQ

Coagulation or precipitation is a process of aggregating together the colloidal particles to change them into large-sized particles which ultimately settle as a precipitate. The different methods to test the type of emulsion are-microscopic, conductance and dye method.

To obtain a stable emulsion, sometimes, small quantities of certain other substances are added during its preparation. The substances thus added to stabilize the emulsions are called emulsifiers or emulsifying agents.

Adsorption is the process which involves the accumulation of the molecules of a substance in higher concentration on the surface of a solid or a liquid. For example, gasses are adsorbed on the surface of charcoal.

Physisorption occurs due to Van der Waals forces. On increasing the temperature, the adsorbate can split into atoms and form chemical bonds with the adsorbent and hence, cause chemisorption.

Chemisorption is stronger than physisorption. Chemisorption involves the forming of a chemical bond between the adsorbent and the adsorbate whereas in physisorption, the molecules are held together by weak Van der Waal’s forces. Therefore, chemisorption is stronger.

Chemisorption is a kind of adsorption that involves a chemical reaction between the adsorbent and adsorbate, resulting in the formation of a chemical bond between the two. Since a bond is to be formed, chemisorption is highly dependent on the reactants(adsorbate and adsorbent) and has a large heat of adsorption. Also, since it involves bond formation, it cannot be multi-layered. Chemisorption is favoured to occur at high temperature.

Sorption refers to the process where adsorption and absorption occur at the same time. Cotton dipped in ink is one of the cases where both adsorption and absorption, that is, sorption occurs.

Physisorption is reversible as the molecules of the adsorbate are held to the adsorbent by weak Van der Waals forces which can be broken easily. Physisorption is exothermic and so, ΔH is negative. Adsorption is a spontaneous process. In adsorption, the movement of the adsorbed molecule is restricted. As a result the entropy change (ΔS) is negative.

Physisorption is an exothermic process. According to Le-Chatelier’s principle, an exothermic reaction is favoured by a decrease in temperature. Therefore, the extent of physisorption decreases on increasing temperature.

In physical adsorption, the molecules of the adsorbate stick to the surface of the adsorbent due to very weak forces which is known as Van der Waals force. Van der Waals force is similar to the forces that cause condensation of gas into liquid.

Given,
k = 3.28 × 10^-4 s^-1
The general formula to find the units for rate constant, k = (mol L^-1)^1-ns^-1 where n is the order of the reaction. The value of n must be 1 for (mol L^-1)^1-ns^-1 to become s^-1.
Therefore,k = 3.28 × 10^-4s^-1 represents a first order reaction.

Given, R=0.6 s^-1 and k= 0.035
For a first order reaction R= k [A]

Acid hydrolysis of ester, CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH
The order of the reaction may be altered sometimes by taking reactant in excess compared to the other.
The rate law R= k [CH₃COOC₂H₅] [H₂O] however water is present in excess.
So, R= k [CH₃COOC₂H₅].

Given,
[A] = 0.75 M, k= 0.02
The reaction belongs to pseudo first order reaction so, the unit is s^-1
R= k [A]= 0.02 × 0.75= 0.015 s^-1.

Given, [A] = 0.5 M, [B] = 0.1 M and k= 0.03
From the rate law it is evident that the order of the reaction is 1+ 0.5 = 1.5 = 32
Therefore the unit of k= (mol L^-1)^1-1.5 s^-1 = (L/mol)^1/2 s^-1
R= k[A]¹[B]^1/2 = 0.03 × 0.5 × 0.1^1/2 = 4.74 × 10^-2(L/mol)^1/2 s^-1.

Since it is an elementary reaction, its rate law r₁= k [A] ³[B]
When the concentrations are changed the new rate will be r₂= k (2[A])³([B]/2) = 4k[A]³[B]
So, r₂ = 4r₁.

Fractional order reactions are reaction whose order is a fraction. This reaction is an example of fractional order reaction, where the order of the reaction is 5/2.
The rate law for the reaction is known to be R= [C₂H₄] [I₂]3/2.

Rusting of iron is one of the most common forms of corrosion. To prevent iron from rusting, generally, more active metals are coated on it. The metal which is often used for coating iron is zinc (which is more active) and this process is called galvanization.

Galvanization is the process of coating iron with zinc to prevent its oxidation. Anti-rust solutions are alkaline phosphate and alkaline chromate solutions. The alkalinity prevents the availability of hydrogen ions. Cathodic protection is the process of connecting the iron object to be protected to a more active metal either directly or through a wire.

Rust is a general term for a series of iron oxides, usually, reddish-brown oxides, formed by the reaction of iron with oxygen in the presence of water or moisture. Water is usually present in the form of water vapour and oxygen is always present in the normal atmosphere. The oxygen needs moisture as a catalyst and reactant to accelerate the reaction, so in the absence of moisture, iron won’t rust.

Magnesium and zinc are highly active. Hence, they corrode at a faster rate than iron. Iron undergoes rusting in the presence of oxygen and moisture. Copper, a noble metal that occurs naturally in its elemental form, is almost totally impervious to corrosion.

Saline medium has extra salts such as sodium chloride dissolved in water. It has a greater concentration of electrolyte than ordinary medium. The ions present will favour the formation of more electrochemical cells and favour the transfer of hydrogen ions and will thus promote rusting or corrosion.

Air and moisture accelerate corrosion. The most important factor in atmospheric corrosion, overriding pollution or lack of it, is moisture. Presence of gases like SO₂ and CO₂ in air catalyse the process of corrosion. Iron, when placed in vacuum, does not rust.

More active metals are readily corroded. Presence of impurities in metals enhances the chances of corrosion. Pure metals do not corrode. Corrosion takes place rapidly at bends, scratches, nicks and cuts in the metal. Electrolytes, if present, also increase the rate of corrosion.

2Fe_(s) → 2Fe²⁺ + 4e^– is the reaction that occurs at the anode.
O₂ (g) + 4H⁺(aq) + 4e^–⟶2H₂O(l) is the reaction that occurs at the cathode.
2Fe_(s) + O_2(g) + 4H⁺_(aq) → 2Fe²⁺_(aq) + 2H₂O_(l) is the overall reaction of rusting.

Aqua regia also known as royal water is a yellow-orange mixture of concentrated nitric acid and hydrochloric acid in the ratio 1:3. It is used by an alchemist to dissolve noble metals like gold and silver.

In the electrolysis of NaCl, if the electrolyte is molten NaCl, then the only ions formed after dissociation are Na⁺ and Cl^– ions. The cathode being a negatively charged electrode attracts the positive Na⁺ ions and neutralizes it to form Sodium metal.

In the electrolysis of aqueous CuSO₄, Cu²⁺, SO₄²⁺, H⁺ and OH^– are the ions formed after dissociation. Copper ions have much higher reduction potential than water. Hence, these ions are easily reduced and deposited as Cu at the cathode.

The electrolyte in electrolysis should contain the metal to be coated, gold in this case. AuCN is used because it is exceptionally stable and doesn’t resist the flow of Au⁺ ions from anode to cathode.

The two electrodes that are used in a Daniell cell are zinc (as anode) and copper (as cathode) electrodes which are dipped in a solution containing its own ions, generally zinc sulphate and copper sulphate.