Chemical Reactions and Equations

Any change can be classified as physical change and chemical change. Physical changes can be easily reversed but, it is not easy to reverse a chemical change.In chemical changes, new substances are formed and it is difficult to regenerate the original substances. Chemical changes are more permanent than physical changes.Chemical reaction involves chemical changes.Chemical reactions are the processes in which new substances with new properties are formed.During a chemical reaction, atoms of one element do not change into those of another element. Only a rearrangement of atoms takes place in a chemical reaction.

Magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

Magnesium + Oxygen =Magnesium Oxide

REACTANTS AND PRODUCTS

The substances which take part in a chemical reaction are called reactants.The new substances produced as a result of chemical reaction are called products.

CHARACTERISTICS OF CHEMICAL REACTIONS

In a chemical reaction, reactants are transformed into products.The important characteristics of chemical reaction are:

1.Evolution of a gas

2.Formation of a precipitate

3.Change in colour

4.Change in temperature and

5.Change in state.

 

CHEMICAL EQUATIONS

The method of representing a chemical reaction with the help of symbols and formulas of the substances involved in it is known as chemical equation.

A + B  → C + D

In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.

Chemical equation can be divided into two types –

1. Balanced Chemical Equation

2.  Unbalanced Chemical Equation.

Balanced Chemical Equation: A balanced chemical equation has number of atoms of each element equal on both sides.

Example: Zn + H2SO4 → ZnSO4 + H2

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.

Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.

Fe + H2O  → Fe3O4 + H2

In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation

TYPES OF CHEMICAL REACTION

Chemical reactions can be classified in following type

1. Combination Reaction

2. Decomposition Reaction

3. Displacement Reaction

4. Double Displacement Reaction

5 .Oxidation and Reduction Reaction

COMBINATION REACTION

Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.A general combination reaction can be represented by the chemical equation given here

Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.

Mg + O2  → 2MgOMagnesium + Oxygen ⇨ Magnesium oxide

DECOMPOSITION REACTION

Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.

Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide

CaCO3  →                CaO                + CO2

Calcium carbonate  → Calcium oxide +    Carbon dioxide

 

DISPLACEMENT REACTION

 

Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION.

Zn + 2HCl  →  ZnCl2 + H2

When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal

 

DOUBLE DISPLACEMENT REACTION

Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.

NaOH + HCl  →NaCl + H2O

OXIDATION AND REDUCTION REACTION

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction

CuO  + H2  →          Cu +    H2O

in this reaction, oxygen is removed from copper and oxygen is added to hydrogen

NaOH +        HCl  →           NaCl + H2O

In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide.

CORROSION

Corrosion is defined as the slow and steady destruction of a metal by the environment. It results in the deterioration of the metal to form metal compounds. When a metal is attacked by substances round it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion

 

RANCIDITY

When fats and oils are oxidized, they become rancid and their smell and taste change. Rancidity is the chemical decomposition of fats, oils and other lipids.

 

Question 1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?Answer : When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

Fe        +CuSO₄=        FeSO₄  +          Cu

Iron     + Copper Sulphate  = Iron Sulphate  +   Copper

            Blue colour                 Green Colour

Therefore, the blue colour of copper sulphate solution fades and green colour appears

 

 

Question : What is a balanced chemical equation? Why should chemical equations be balanced?

Answer : A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

 

Question : Why is respiration considered an exothermic reaction? Explain.

Answer : Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

 

 

Question : Why do we apply paint on iron articles?

Answer : Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented. So presence of air and moisture is essential for rusting to take place.

 

Question : Oil and fat containing food items are flushed with nitrogen. Why?

Answer : Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

 

Question :  Explain the following terms with one example each

.(a) Corrosion (b) Rancidity

Answer :(a) Corrosion:Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

 

(b) Rancidity:The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.For example, the taste and smell of butter changes when kept for long.Rancidity can be avoided by:

1. Storing food in air tight containers

2. Storing food in refrigerators

3. Adding antioxidants

4. Storing food in an environment of nitrogen

 

Question : Give an example of a double displacement reaction other than the one givenin Activity 1.10.

Answer : Sodium carbonate reacts with calcium chloride to form calcium carbonate andsodium chloride.

 

 

 

 

In this reaction, sodium carbonate and calcium chloride exchange ions to form two newcompounds. Hence, it is a double displacement reaction.

 

Question Translate the following statements into chemical equations and then balancethem.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and aprecipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

 

Answer