Chemical Reactions: Types, Factors Affecting Rate of Reaction with Example

A chemical reaction involves the breaking of old chemical bonds and the formation of new chemical bonds. This change may happen spontaneously or it may be facilitated by external forces or energy. Chemistry is all about chemical reactions. In your day-to-day life, you could observe many chemical reactions. A clear understanding of these reactions is essential in order to manipulate them for the sake of human life and the environment. So, chemistry mainly focuses on chemical reactions.

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What Happens During a Chemical Reaction?

In a chemical reaction, the atoms of the reacting molecules or elements are rearranged to form new molecules. Old chemical bonds between atoms are broken and new chemical bonds are formed. Bond breaking absorbs energy whereas bond formation releases energy.

Types of Chemical Reactions

Chemical Reactions are of the following types:

Combination Reactions

A combination reaction is a reaction in which two or more reactants combine to form a compound. It is otherwise called ‘synthesis reaction’ or ‘composition reaction’. When a reactant ‘A’ combines with ‘B’, it forms the product ‘AB’.

• • When two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.
  • Reactions in which heat is released along with the formation of products are called exothermic reactions.
  • During digestion, food is broken down into simpler substances. For example, rice, potatoes, and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cell of our body and provides energy. The special name of this reaction is respiration, which is an exothermic process.

Example:

Hydrogen gas combines with chlorine gas to form hydrogen chloride gas.

Depending on the chemical nature of the reactants, there are three classes of combination reactions:

Element + Element →Compound

Element + Compound → Compound

Compound + Compound → Compound

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Decomposition Reaction 

• • In a decomposition reaction, a single compound splits into two or simpler substances under suitable conditions. It is the opposite of the combination reaction.
  • Breaking of bonds is the major phenomenon in a decomposition reaction and hence it requires energy to break the bonds, depending on the nature of the energy used in the decomposition reaction.

There are three main classes of decomposition reactions. They are

1. Thermal Decomposition Reactions:
   • In this type of reaction, the reactant is decomposed by applying heat. For example, on heating mercury (II) oxide is decomposed into mercury metal and oxygen gas.
   • As the molecule is dissociated by the absorption of heat, it is otherwise called ‘Thermolysis’. It is a class of compound to element/element decomposition. i.e. a compound (HgO) is decomposed into two elements (Hg and Oxygen).
   • In a thermal decomposition reaction, heat is supplied to break the bonds. Such reactions, in which heat is absorbed, are called ‘Endothermic Reactions’.
   • Example:
     
2. Electrolytic Decomposition Reactions:
   • In some of the decomposition reactions, electrical energy is used to bring about the reaction. For example, the decomposition of sodium chloride occurs by passing an electric current through aqueous solution. Sodium chloride decomposes into metallic sodium and chlorine gas. This process is termed ‘Electrolysis’.
   • Example:
3. Photo Decomposition Reactions:
   • Light is a form of energy, which facilitates some of the decomposition reactions. For example, when silver bromide is exposed to light, it breaks down into silver metal and bromine gas. As the decomposition is caused by light, this kind of reaction is also called ‘Photolysis’.
   • Example: 

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Single Displacement Reaction

• • It is a reaction between an element and a compound. When they react, one of the elements of the compound-reactant is replaced by the element-reactant to form a new compound and an element.
  • ‘A’ displaces element ‘B’ from the compound ‘BC’ and hence a single displacement reaction occurs. If zinc metal is placed in hydrochloric acid, hydrogen gas is evolved. Here, hydrogen is displaced by zinc metal and zinc chloride is formed.

• • If an iron nail is placed in an aqueous solution of copper (II) sulphate, the iron displaces copper from its aqueous solution, and the so formed copper deposits over the iron nail.

Double Displacement Reaction

• • When two compounds react, if their ions are interchanged, then the reaction is called a double displacement reaction.
  • The ion of one compound is replaced by the ion of another compound. Ions of identical charges are only interchanged, i.e., a cation can be replaced by other cations. This reaction is also called ‘Metathesis Reaction’.
  • For a double displacement reaction to take place, one of the products must be a precipitate or water. In this way, there are major classes of double displacement reactions.

They are:

1. Precipitation Reactions: When aqueous solutions of two compounds are mixed, if they react to form an insoluble compound and a soluble compound, then it is called a precipitation reaction. Because the insoluble compound, formed as one of the products, is a precipitate and hence the reaction is named so.
2. Neutralization Reactions: It is another type of displacement reaction in which the acid reacts with the base to form salt and water. It is called a ‘Neutralization Reaction’ as both acid and base neutralize each other.

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Combustion Reaction

• • A combustion reaction is one in which the reactant rapidly combines with oxygen to form one or more oxides and energy (heat). So in combustion reactions, one of the reactants must be oxygen.
  • Combustion reactions are majorly used as heat energy sources in many of our days to day activities.
  • For instance, we use LPG gas for domestic cooking purposes. We get heat and flame from LPG gas by the combustion reaction of its constituent gases.
  • LPG is a mixture of hydrocarbon gases like propane, butane, propylene, etc.
  • All these hydrocarbons burn with oxygen to form carbon dioxide and water.

Reversible and Irreversible Reactions

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Reactivity Series

Displacement Reactions can only occur when a more reactive metal i.e. the one that forms positive ions more easily) displaces a less reactive metal from a compound.

Example:

Magnesium is more reactive than copper, so magnesium can displace copper from a copper sulfate solution to create magnesium sulfate.

Based on this rule a series of metals are formed. It is called as the Reactivity Series and it consists of a list of metals in order of their reactivity.

Factors Affecting Rate of Reactions

1. Nature of the Reactants: The reaction of sodium with hydrochloric acid is faster than that with acetic acid because Hydrochloric acid is a stronger acid than acetic acid and thus more reactive. So, the nature of the reactants influences the reaction rate.
2. The concentration of the Reactants: Changing the amount of the reactants also increases the reaction rate. The amount of the substance present in a certain volume of the solution is called ‘concentration’. The more the concentration, the more particles per volume exist in it and hence faster the reaction. Granulated zinc reacts faster with 2M hydrochloric acid than 1M hydrochloric acid.
3. Temperature: Most of the reactions go faster at the higher temperatures. Because adding heat to the reactants provides energy to break more bonds and thus speed up the reaction. Calcium carbonate reacts slowly with hydrochloric acid at room temperature. When the reaction mixture is heated the reaction rate increases.
4. Pressure: If the reactants are gases, increasing their pressure increases the reaction rate. This is because on increasing the pressure the reacting particles come closer and collide frequently.
5. Catalyst: A catalyst is a substance that increases the reaction rate without being consumed in the reaction. In certain reactions, adding a substance as a catalyst speeds up the reaction. For example, on heating potassium chlorate, it decomposes into potassium chloride and oxygen gas, but at a slower rate. If manganese dioxide is added, it increases the reaction rate.
6. Surface Area of Reactants: When solid reactants are involved in a reaction, their powdered form reacts more readily. For example, powdered calcium carbonate reacts more readily with hydrochloric acid than marble chips. Because powdering of the reactants increases the surface area and more energy is available on collision of the reactant particles. Thus, the reaction rate is increased.

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Oxidation and Reduction

• • If a substance gains oxygen during a reaction, it is said to be oxidized. If a substance loses oxygen during a reaction, it is said to be reduced.
  • A reaction where one reactant gets oxidized and the other gets reduced is called a redox reaction.

ZnO + C → Zn + CO

Corrosion

• • When a metal is attacked by substance around it such as moisture, acids, etc.. it is said to corrode and this process is corrosion. The black coating on silver and green coating on copper is examples of corrosion along with a coating of reddish-brown powder on the surface of Iron when left for some time. This process is commonly known as rusting.
  • Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metal, especially iron. Corrosion causes serious problems. Every year an enormous amount of money is spent to replace damaged iron.

Read more about the Application of Chemistry in Glass and Ceramics, here.

Rancidity

• • When fats and oils are oxidized, they become rancid, and their smell and taste change. Usually, substances that prevent oxidation (antioxidants) are added to foods containing fat and oils.
  • Keeping food in an airtight container helps slow down the rate of oxidation.
  • Chips manufacturers usually flush bags of chips with a gas such as nitrogen to prevent the chips from getting oxidised.

Two types of rancidity occur which are:

1. Hydrolytic Rancidity: Hydrolytic rancidity occurs when triglycerides are hydrolyzed and free fatty acids are released.
2. Oxidative Rancidity (auto-oxidation): This reaction of lipids with water may require a catalyst, which forms free fatty acids and glycerol. Oxidative rancidity occurs due to degradation by the oxygen which is present in the air. The double bond present in unsaturated fatty acids can be cleaved by a free radical reaction involving molecular oxygen.

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