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Precipitation Reaction

The chemical reaction represents a chemical change that occurs in a substance to form new substance under given reaction conditions. Any chemical reaction can be represented by chemical equations. Chemical equations are symbolic representations of reactants and products involve in the chemical reaction. It provides complete information regarding chemical reaction. There are several types of chemical reactions which you can observe in your daily life such as burning of any substance which is also known as combustion reaction.

Similarly, corrosion or rusting of the metal surface is an example of oxidation reaction. Some other types of chemical reactions are combination reaction, prepetition reaction, redox reaction, displacement reaction, decomposition reaction, neutralisation reaction and precipitation reaction. The combination of hydrogen and oxygen to form water is an example of combination reaction that is also known as synthesis reaction as it results in the formation of new substances.

On the contrary, the decomposition reactions involve the decomposition of a substance into its components. Precipitation reactions are basically double displacement reactions which involve the formation of solid residue that is called as a precipitate. Let’s discuss few more points related to precipitation reaction.

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What is a Precipitate?

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The solid that settles at the bottom of the liquid, after a chemical reaction has occurred is called a precipitate. Some precipitates cannot be filtered directly, as they try to dissolve back into the liquid. Formation of precipitate sometimes indicates that a chemical reaction has occurred. 

For example, When silver nitrate is mixed with hydrochloric acid, a white precipitate of silver chloride settles down the solution. The color of the precipitate depends upon the compound formed. 

The extent of super saturation at the time of nucleation plays a major role in determining the particle size of a precipitate. If super saturation is minimum, relatively large crystals would be obtained.

Chemical reactions occur when two or more chemicals combine to form products. In the process, the combining reactants can be solid, liquid or gas. Let us consider a chemical reaction in which one liquid reacts with a solid or a liquid.

If the product formed settles down as a solid inside the solution in which it is reacting, then such reaction is called as Precipitation reaction. One product formed may be in the form of a solid and the other product a liquid.

The solid formed settles down at the bottom and can be removed by filtration, or some other separation technique.
This process is termed as precipitation.

Precipitation Reaction Definition

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A chemical reaction, in which two compounds react to form a precipitate and another product is present in the aqueous medium is said to be a precipitation reaction. A single or double displacement reaction makes up the precipitate reaction. These are also called as ionic equations. 
  • Since these reactions take place in the ionic state in aqueous medium.
  • When the ions undergo reaction in aqueous medium, products are formed.
  • Some of the products formed are insoluble in water or aqueous medium.
  • These can be identified by solubility rules. And these products are known as precipitates (indicated by showing the down ward arrow.)
  • And they are given with subscript ‘s’.
The main aim of the precipitation reactions is the separation of a pure solid phase in a compound and dense form which could be filtered off. 

Types of Precipitation

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Normally, precipitation is often referred to for rains, snow fall, drizzle, etc. When we talk about precipitation reaction. This precipitation reaction involves three equations.

1. Molecular equation

The complete equation, with the formula of the compounds reacting.
Na2SO4 + BaCl2 BaSO4(s) + 2NaCl
2. Ionic equation

The formula of the compounds are separated into ions, except for the precipitate. This involves the ionic equation.
2Na+ + SO42- + Ba2+ + 2Cl- BaSO4(s) + 2Na+ + 2Cl-
3. Net ionic equations

The ions forming the precipitate only on the reactant side, and precipitate on the product side.
SO42- + Ba2+ BaSO4(s)

Precipitating Agents

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Some Points about Precipitating Agents has given below:
  • A proper precipitate can be obtained, say for example, in case of gravimetric analysis, where a precipitate is necessary, by adding the precipitating agents, in excess from outside.
  • In such cases, excess of the precipitating reagent is either adsorbed on the surface of the precipitate or forms a soluble complex ion with the metal.
  • Hence, where close control of the concentration of a precipitating ion becomes necessary, this method will yield incorrect results.
  • In order to get a precipitate in an analytically desirable form and size, a new technique called precipitation from homogenous medium was developed which in the supersaturation is held to a minimum.
  • In this technique, the precipitating reagent is not added as such but is gradually and uniformly liberated by a homogenous chemical reaction within the solution. This technique is versatile.
  • A large number of precipitating agents can be formed within the solution.
Thus, different types of precipitates can be quantitatively obtained by employing this method. Some examples are

1. Hydroxides and basic salts


Hydroxides and basic salts are precipitated when pH of an acid solution is slowly increased. A 1:1 ammonia solution is generally employed in the laboratory for this purpose. 

But, when ammonia is used as a precipitating agent, the pH cannot be controlled. The possibilities of high concentration of ammonia can be avoided, if it is not added from the outside, but is slowly liberated within the solution by a chemical reaction. 

2. Sulphates


Sulfamic acid has been used to liberate ions within a solution:
NH2SO3H + H2O $\to$ NH4+ H+ + SO42-
Di methyl sulfate is also a good source of sulphate ions. The hydrolytic reaction has been used for the precipitation of barium, strontium, calcium, etc.

3. Trimethyl phosphate hydrolysis forms phosphate ions which may be used for aiding the precipitation of insoluble phosphates.

Selective precipitation

If a chemical solution is a mixture of two salts, containing two different cations, but same anions, say for example, Iron (ii) and silver. When another outside compound like sodium chloride is added to this, one of the ions, silver ions, comes out of the solution in the form of a white precipitate, leaving behind iron and sodium ions. This is called as selective precipitation, where using another compound, a precipitating agent, one ion can be removed from the mixture.

Net Precipitation Reaction

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Net precipitation reaction, also called net ionic equation shows only the ions (cations and anions) which form the precipitate. It does not indicate the ions which are present in the aqueous solution. This is a very important part of study of displacement reactions because it showed the exact chemical reaction that has occurred in the solution.

When ionic compounds such as KI and Pb(NO3)2 dissolve in water, the solutions are mixtures of ions and water molecules.

KI ⇔ K+ (aq) + I - (aq)
Pb(NO3)2 ⇔ Pb2+ (aq) + 2NO3- (aq)

The reaction is better represented by listing all of the ions in the reactants and products as a total ionic equation.

2K+(aq) + 2 I -(aq) + Pb2+(aq) + 2NO3-(aq) PbI2(s) + 2K+(aq) + 2NO3-(aq)

When these four ions are mixed, two of them, Pb2+ and I- bond to form a solid, lead iodide. The other two, K+ and NO3 - appear on both sides of the total ionic equation. 

Because they remained unchanged, these are called as spectator ions. On writing only the ions involved in a reaction, and omitting all spectator ions, the above ionic equation would become a net ionic equation or net precipitation equation.
2K+ (aq) +2 I - (aq) + Pb2+ (aq) + 2NO3- (aq) → PbI2 (s) + 2K+ (aq) + 2NO3- (aq)

2 I - (aq) + Pb2+ (aq) PbI2 (s)

This is the Net precipitation reaction showing the ions which form the precipitate.

Colors of the precipitate formed depend upon the compound formed as a precipitate. Some of the colors of precipitates are
Lead(II) iodide, lead chromate - Yellow color.
Sulphates of Barium, calcium, strontium - white precipitate.
Chromate of potassium, Aluminium - Yellow.
Potassium di chromate - Orange.
Silver chloride - White.

Black Precipitate


The compounds formed by Black precipitate is formed by copper(II) oxide, cobalt sulfide, Iron sulfide, lead sulfide, etc.

Precipitation Reaction Problems

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The following are precipitation reaction problems .

Solved Examples

Question 1: Write the net ionic equation for:

AgNO3(aq) + NaCl(aq) AgCl+ NaNO3 (aq)

Solution:
Ionic Equation:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) → AgCl ↓ + NO3-(aq) + Na+(aq)
The ions in red are spectator ions.

Net Ionic Equation:
Ag+ (aq) + Cl-(aq) → AgCl↓

Question 2: What will be the net ionic equation for the below mentioned molecular reaction:

Mg(OH)2(s) + 2HCl (aq) → MgCl2(aq) + 2H2O(l)
Solution:
Ionic Equation:
Mg2+(s)+ 2OH-(s) + 2H+ (aq) + 2Cl-(aq) → Mg2+(aq) + 2Cl-(aq) + 2H2O (l)
The ions in red are spectator ions.

Net Ionic Equation:
2OH-(s) + 2H+ (aq) → 2H2O (l)


Precipitation Reaction Examples

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If a chemical reaction takes place with one of the products forming a precipitate, those reactions are said to be precipitate reactions. 

Some of the examples of precipitation reactions or precipitate reactions are

Example:

AgNO3 (aq)+ NaCl (aq) AgCl(s) + NaNO3(aq)


In the above reaction, silver chloride is the insoluble salt, and thus makes up the precipitate. Thus, on reaction between silver nitrate and sodium chloride, the silver chloride formed is the precipitate and thus this is a precipitation reaction.

Example: 2

2KOH(aq) + CaCl2(aq) Ca(OH)2(aq) + 2KCl(aq)

In the above reaction, calcium chloride formed is the precipitate, since its an insoluble salt. Calcium hydroxide forms a white precipitate.

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