Chemical reactions are a part of our daily lives. We cannot get our basic necessities fulfilled without relying on chemical reactions. Chemical reactions are involved in our cooking, cleaning, transportation and hygiene. All these reactions have different natures and thus, the products formed are different.
Definition: The reaction in which two ionic liquids are mixed together to obtain a solid mass that is not soluble in the parent solution is called a Precipitation reaction. The solid mass formed during the precipitation reaction is called Precipitate.
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Precipitation Reactions Questions with Solutions
Q1: Which of the following reactions is a precipitation reaction?
- CaCl2 (aq) + K2CO3 (aq) → 2KCl (aq) + CaCO3 (s)
- 2NaBr (aq) + Cl2 (g) → 2NaCl (aq) + Br2 (l)
- Cl2 (aq) + 2I- (aq) → I2 (aq) + 2Cl- (aq)
- None of the above
Answer: (a.)
Explanation: For a reaction to be a precipitation reaction, one of the products must be in a solid state.
Q2. The clear liquid remaining after the solid precipitate settles down is called ______.
- Mother Liquor
- Supernatant Liquid
- Ionic solution
- Acidic solution
Answer: (b.)
Explanation: In a precipitation reaction, the newly formed solid mass settles down and the rest of the liquid appears clear and can be separated by transferring into another beaker. There is no need for filtration of this liquid. While a mother liquor is the liquid left after filtration of a solution.
Q3. During the drilling of an oil well, which product is extracted first?
- Marsh Gas
- Coal Gas
- Bio Gas
- Natural Gas
Answer: (d.)
Explanation: As natural gas exists just above the Precipitation Reactions in the Precipitation Reactions sources such as oil wells, natural gas comes out first.
Q4. Which process is used to separate out different hydrocarbon compounds from Precipitation Reactions?
- Distillation
- Centrifugation
- Boiling
- Fractional Distillation
Answer: (d.)
Explanation: Fractional Distillation is a method of separating a mixture of liquids that have different boiling points.
Q5. Till what temperature is the crude Precipitation Reactions heated in the Fractional Distillation method to separate out the other hydrocarbon products found in it?
- 1000 oC
- 800 oC
- 600 oC
- 400-500 oC
Answer: (d.)
Explanation: The boiling point of the liquid constituents of the crude Precipitation Reactions goes as high as ~370 oC. Hence, the crude Precipitation Reactions is heated in the temperature range of 400-500 oC.
Q6. Write the complete and net ionic equations of the given molecular equation.
K2C2O4 (aq) + Ba(OH)2 (aq) → 2KOH (aq) + BaC2O4 (s)
Answer: The given reaction is a precipitation reaction. The balanced molecular equation can be written into the complete ionic equation as:
2K+ (aq) + C2O42- (aq) + Ba2+ (aq) + 2OH- (aq) → 2K+ (aq) + 2OH- (aq) + BaC2O4 (s)
The 2K+ (aq) and 2OH- (aq) cancel out each other and hence the net ionic equation is:
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Ba2+ (aq) + C2O42- (aq) → BaC2O4 (s)
Q7. Out of the given equations, answer the following questions.
(i) H2O (l) → H2O (s)
(ii) CH3OH (g) + O2(g) → CO2 (g) + H2O (g)
(iii) 2H2O (l) → 2H2 (g) + O2 (g)
- Which equation shows a physical change?
- Which equation shows a chemical change?
- Which equation represents a combustion reaction?
Answer: a. A physical change is when the identity or the intramolecular bonds in the reactants do not change. This change comprises the change in the physical state of the matter. In (i), water undergoes a change in its physical state from being a liquid to solid. During this change, the chemical compound water (H2O) does not change its chemical formula. Hence, (i) is the physical change.
b. A chemical change comprises a change in the identity of the chemical compounds. In a chemical change, the old chemical bonds break to form new bonds between rearranged molecules. Hence, in a chemical change, the chemical formula of the reactants change and products with a new chemical formula are formed. So, (ii) and (iii) are chemical changes.
c. Combustion reaction involves burning of a fuel in the presence of oxygen to form salt and water. Hence, (ii) is the combustion reaction.
Q8. What is the condition for a precipitation reaction to occur? How can this condition be achieved?
Answer: Precipitation takes place when two aqueous saturated solutions are mixed up. The precipitate forms when its concentration in the solution exceeds its solubility. This may happen due to change in temperature, loss of solvent by solvent evaporation, or just by mixing solvents. Precipitation reactions occur at a faster rate in a supersaturated solution.
Q9. Bromine is prepared by treating the bromide salts with chlorine. Write a chemical reaction for the same.
Answer: Bromide salt (brine) such as NaBr reacts with chlorine to release Br2 (l) as follows:
2NaBr (aq) + Cl2 (g) → 2NaCl (aq) + Br2 (l)
Q10. Which stage comes before precipitation?
Answer: Nucleation is the onset stage of precipitation characterised by the formation of small solid particles in the solution. The formation of particles initiates the separation of the two interfaces. During the nucleation process, the energy changes must be in the favour of the precipitation to enhance.
Q11. What is a Walden reductor?
Answer: The walden reductor makes the process of reductive precipitation possible by the action of silver metal accumulated over a copper spring. The walden reductor is made by immersing a copper spring into a silver nitrate solution and the metallic silver starts depositing on the copper spring.
During the process, the reduction and precipitation reactions undergo simultaneously. The Walden reactor is used to obtain elements in their low-valence state or to obtain the compound in small amounts.
For example: the Walden reactor can be used to reduce UO22+ to U4-.
Q12. Why do some precipitates look coloured?
Answer: The colour is the property of metals incorporated in the salt. The colour appears as per the valency of the metal atom. Some common colours associated (but not obvious) with the fixed valence state of a metal are given hereunder.
Metal
Colour(s)
Iron (II)
Dirty Green
Iron (III)
Reddish Brown
Copper
Blue
Cobalt (hyd.)
Pink
Nickel
Green
Q13. Explain how the colour of the precipitate helps in the cation analysis?
Answer: The Precipitation reaction can be used to analyse the type and nature of the cation present within a given solution. This can be done by adding an alkali solution to the unknown salt solution; hence converting the cation (present in the salt) into its hydroxide. Therefore, the colour and solubility of the precipitate in excess of the solution is determined to carry out the cation analysis. This way the colour of the precipitate helps in the cation analysis.
Q14. Define the term “pellet” in reference to the precipitation reactions.
Answer: For precipitation to happen, there must be strong enough interparticle attractive forces so as to hold the solid particles (formed during the reaction) together within the solution. However, if the attractive forces between the particles are weak, the particles do not get sufficient mass inorder to settle down in the solution by the means of gravity. This leads to the formation of a colloid as the solid particles stay suspended in the solution. Hence, the solution is centrifuged in order to separate the small solid particles from the solution. The compact solid mass obtained upon centrifugation is termed a pellet.
Q15. What is precipitate ageing?
Answer: Precipitate ageing or digestion occurs when the precipitate particles are left unseparated at a high temperature within the same solution. This results in the formation of bigger recrystallised particles. The process is termed as Ostwald ripening.
Practise Questions on Precipitation Reactions
Q1. Identify the spectator ions in the given reaction.
AgNO3 (aq) + NaCl (aq) →
- Ag+ and NO3- ions
- Na+ and NO3- ions
- Na+ and Cl- ions
- Ag+ and Cl- ions
Q2. What is a supersaturated liquid?
Q3. Mention a precipitation reaction involving organic compounds.
Q4. Write two applications of Precipitation reactions in biochemistry.
Q5. What is the principle of Precipitation reactions?
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