Chemical Reactions and Equation

Chemical Reaction:

• The process in which new substances with new properties are farmed.

OR

• A process in which breaking of old chemical bonds which exist between the atoms of reacting substances and then making of new chemical bonds between the rearranged atoms of new substances.

Reactant: The substances which take part in a chemical reaction are called reactant.

Product: New substances produced as a result of chemical reaction are called product.

For example:

There are two reactants– Magnesium and Oxygen
And one product- Magnesium oxide

Characteristics of chemical Reactions:

The important characteristics of chemical reactions are:

1. Evolution of a Gas
2. Formation of a Precipitate
3. Change in Colour
4. Change in temperature
5. Change in state

Evolution of a Gas:

• Some chemical reactions are characterised by the evolution of a gas.
• When zinc granules react with dilute sulphuric acid, then bubbles of hydrogen gas are produced.

For example:

Formation of a Precipitate:

• Some chemical reactions are characterised by the formation of a precipitate.
• A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction.
• When dilute sulphuric acid is added to barium chloride solution, then a white precipitate of barium sulphate is formed.

Change in Colour:

• Some chemical reactions are characterised by a change in colour.
• When citric acid reacts with potassium permanganate solution, then the purple colour of potassium. permanganate solution/it becomes colourless.

Change in temperature:

• Some chemical reactions are characterised by a change in temperature.
• When quicklime reacts with water, then slaked lime is formed and lot of heat energy is produced.
• It is an exothermic reaction.
  Note: Calcium hydroxide is known as slaked lime.

Change in state:

• Some chemical reactions are characterised by a change in state.
• The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas.

Chemical Equations:

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

Reactants: The substances which combine or react are known as reactants.
Reactants are always written on the left hand side in an equation.

Products: The new substances produced in a reaction are known as products.

Example:

The Products are always written on the right hand side in an equation.
Note: A chemical equation is a short-hand method of representing a chemical reaction.

Balanced Equation:

A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

For example:
Zinc (Zn) reacts with dilute sulphuric acid (H₂SO₄) to give zinc sulphate (ZnSO₄) and Hydrogen (H₂).

Count the number of atoms of all the elements in the reactants and products separately.

• Thus, there is an equal number of atoms of different elements in the reactants, and products, so the chemical equation is a balanced equation.
• A Balanced chemical equation has equal masses of various elements in reactants and products.

Unbalanced chemical Equation:

An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.

For example:

Hydrogen (H₂) react with Oxygen (O₂) to form water.

Count the number of Hydrogen atoms and oxygen atoms in the reactants as well as product-

• No. of Hydrogen atoms is equal in reactants and products.
• But the no. of oxygen atoms is unequal.
• So, it is an unbalanced chemical equation.
• An unbalanced equation has unequal masses of various elements in reactants and products.
  • Note:
    • Law of Conservation of Mass:
      • According to law of conservation of mass, “matter can neither be created nor destroyed in a chemical reaction”.
      • This means that the total mass of all the reactants must be equal to the total mass of the products.
      • The chemical equation are balanced to Satisfy the law of conservation of mass in chemical equation.

Balancing of Chemical Equations:

• The process of making the number of different types of atoms equal on both sides of an equation is called balancing of equation.
• Balancing of equations by hit and trial method.
• For example:
  • Hydrogen burns in oxygen to form water.

• H₂ and O₂ are reactants and H₂O is the product.
• Count the number of hydrogen atoms and oxygen atoms in reactants and product.

• Number of hydrogen atoms is equal but the number of oxygen atoms is unequal.
• To have two oxygen atoms on the right side, we multiply H₂O by 2

Count the number of various atoms on both sides again-

• The number of oxygen atoms has become equal.
• But the number of hydrogen atoms has now become unequal.
• To have 4 hydrogen atoms on the left side.
• We multiply H₂ by 2 and so that-

• Count the number of various atoms on both sides once again.

• This chemical equation contains an equal number of atoms of hydrogen and oxygen on both sides, so this is a balanced equation.

Chemical equations More Informative:

The chemical equations can be made more informative in three ways-

1. Indicate the physical states of the reactants and products
2. Indicate the heat changes taking place in the reaction
3. Indicate the conditions under which the reaction

Indicate the Physical States of Reactants and Products in an equation:

• There can be four physical states for the reactants and products of a chemical reaction. (solid, liquid, aqueous solution and gas).
• Solid state is indicated by ‘s’.
• Liquid state is indicated by ‘l’.
• Aqueous solution is indicated by ‘aq’.
• Gaseous state is indicated by ‘g’.
• Physical state of the reactants and products are shown by putting the above ‘state symbols’ just after their Symbols / Formula in an equation.

For example:
Zinc (Zn) metal reacts with dilute Sulphuric acid (H₂SO₄) to form zinc sulphate solution and hydrogen gas.

this equation is in with physical states

It is more informative because it tells us the physical states.

Indicate the Heat Changes in an equation:

There are two types of reactions on the basis of heat change-

1. Exothermic Reaction
2. Endothermic reaction

Exothermic Reaction:

• Those reactions in which heat is evolved are known as exothermic reaction.

For example:
When carbon burns in oxygen to form carbon dioxide, heat is produced in the reaction-

• An exothermic reaction is denoted by Writing “+Heat” on the products side.
• All the combustion reactions are exothermic reactions.

Another example:

• During respiration, glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy (Heat).

• Respiration is an exothermic process because energy is produced during this process.

Endothermic Reaction:

• Those reactions in which heat is absorbed are known as endothermic reaction.

For example:

• When Nitrogen and oxygen are heated to a very high temperature (about 3000°c) they obtain to form nitrogen monoxide and a lot of heat is absorbed.

• An endothermic reaction is indicated by writing “+Heat” on the reactant side of an equation.
• All the decomposition reactions are endothermic reaction.

Another example:

• When calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide.

• Photosynthesis is an endothermic reaction because sunlight energy is absorbed during the process of photosynthesis.
• The electrolysis of water to form hydrogen and oxygen is also an endothermic reaction.

Indicate the Condition Under Which the Reaction take Place:

• If heat is required for a reaction to take place, then the heat sign ∆ (delta) is put over the arrow of the equation.
• If the reaction take place in the presence of a catalyst, then the Symbol / Formula of the catalyst is also written above / below the arrow sign in the equation.

For example:
When potassium chlorate (KClO₃) is heated in the presence of manganese dioxide catalyst, it decomposes / breaks to form potassium chloride and oxygen gas.

Here delta (∆) stands for heat and MnO₂ is the catalyst.

Types of Chemical Reactions

1. Combination / Addition Reaction
2. Decomposition Reaction
3. Displacement Reaction
4. Double displacement Reaction
5. Oxidation and Reduction Reaction

1. Combination Reaction:

• Those reactions in which two or more substances combine to form a single substance are called combination reactions.

For Example:

a. Magnesium and oxygen combine, when heated to form magnesium oxide.

b. Hydrogen burns in oxygen to form org water.

• The formation of water from hydrogen and oxygen is a combination reaction.

c. Calcium oxide / Lime / Quicklime reacts effective with water to form calcium hydroxide / slaked lime.

• Solid calcium hydroxide which is also called slaked lime.
• Slaked lime is a white powder.
• When calcium hydroxide solution reacts with carbon dioxide gas to form a white solid calcium carbonate.

2. Decomposition Reaction:

• Those reactions in which a compound splits up into two or more simpler substances are known as decomposition reaction.
• In this reaction, heat, light or electricity provide energy which breaks a compound into two or more simpler compound.
• A decomposition reaction is just the opposite of a combination reaction.

For Example:

• When Calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.

• Calcium carbonate (CaCO₃) is also Called ‘limestone’.
• Calcium oxide (CaO) is also called lime or quicklime.
• Calcium oxide is used on a large scale in the manufacture of cement and glass.

a. Thermal decomposition:

• When a decomposition or reaction is carried out by heating, it is called thermal decomposition.
• Thermal means relating to heat.

For Example:

• When potassium chlorate is heated in the presence of manganese dioxide (MnO₂) Catalyst, it decomposes to give potassium chloride and oxygen.

b. Electric Decomposition:

• Those decomposition reactions which are carried out by electricity, it is called electric decomposition.

For Example:

• When electric current is passed through acidified/ acidic water, it decomposes to give hydrogen gas and oxygen gas.

• In this decomposition reaction, water splits up to form two simpler sub- stances, hydrogen and oxygen
• This decomposition reaction by the action of electricity and it is called Electrolysis of water.

Another example:

• When electric current is passed through molten sodium chloride, it decomposes to give sodium metal and Chlorine gas.

• It is called electrolysis of molten sodium chloride.

c. Light Decomposition:

• Those decomposition reactions which are carried out by light energy, it is called light decomposition.

For Example:

• When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

Another Example:

• When silver bromide is exposed to light, it decomposes to form silver metal and bromine vapour.

• Digestion of food in the body is an example of decomposition reaction.
• Decomposition reactions carried out by electricity are used to extract several metals from their natural compounds like chlorides / oxides.

3. Displacement Reaction:

• Those reactions in which one element takes the place of another element in a compound are known as displacement reaction.
• In this reaction, a more reactive element displaces a less reactive element from its Compound.

For Examle:

• When a zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained.

• Zinc is more reactive than copper. So, zinc is able to displace the copper.

Another example:

• When a magnesium metal is placed in copper sulphate solution, then magnesium sulphate solution and copper metal are formed.

• Magnesium is able to displace copper from copper sulphate, solution because magnesium is more reactive than copper.

4. Double Displacement Reaction:

• Those reaction in which two compounds react by an exchange of ions to form two new compounds are called double displacement reaction.

For Example:

• If barium / chloride (BaCl₂) solution is added to copper sulphate (CuSO₄)  solution, then a white precipitate of barium sulphate is produced and along with copper chloride solution.

For Another Example:

• When hydrogen sulphate gas is passed through copper sulphate solution, then a black precipitate of copper sulphide is formed along with sulphuric acid.

• Any reaction in which an insoluble solid I his formed that separates from the solution is called a precipitation reaction.

5. Oxidation and Reduction Reaction

a. Oxidation:

• Addition of Oxygen to a substance is called oxidation. OR Removal of hydrogen from a substance is also called oxidation.

b. Reduction:

• Addition of hydrogen to a substance is called reduction. OR Removal of oxygen from a substance is also called reduction.
• Thus, the process of reduction is just the opposite of process of oxidation.

i. Oxidising agent:

• The substance which gives oxygen or removes hydrogen for oxidation is called an oxidising agent..

ii. Reducing agent:

• The substance which gives hydrogen and removes oxygen for reduction is called a reducing agent.

Redox Reaction:

• The oxidation and reduction reactions are also called redox reaction.
• Redox, the term ‘red’ stand for reduction and ‘ox’ stands for “oxidation”.

For Example: When copper oxide is heated with hydrogen, the copper and water are formed.

Above Oxidation- reduction reaction:

• H₂ – Substance oxideised
• CuO – Substance reduced
• CuO – Oxidising agent
• H₂ – Reducing agent

Another Example: When hydrogen sulphide reacts with chlorine, then sulphur and hydrogen chloride, are formed.

Above oxidation – reduction reaction

• H₂S – Substance oxidised
• Cl₂ – Substance reduced
• Cl₂ – Oxidising agent
• H₂S – Reducing agent

Effects of oxidation reactions in everyday life:

There are two common effects of oxidation reactions which we observe in daily life.

1. Corrosion of Metals
2. Rancidity of food

1. Corrosion:

• The process in which metals are eaten up gradually by the action of air, moisture or a chemical on their surface; it is called corrosion.
• The oxidation of metals by the oxygen of air.
• Rusting of iron metal is the most common form of Corrosion.
• When an iron metal is left in air, it gets covered with a red – brown flaky substance called ‘rust’. And this is called rusting of iron.

• Rusting of iron is a redox reaction.
• Rust is a soft and porous substance which gradually falls off from the Surface of an iron object and then the iron below starts rusting.
• Corrosion weakens the iron and steel objects and structures.

2. Rancidity:

• The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
• Rancidity spoils the food materials prepared in fats and oils.
• Rancidity is called ‘vikritgandhita’ in Hindi.

Preventation of Rancidity:

1. Adding anti-oxidants to foods containing fats and oils.

• Anti-oxidant is a substance which prevents oxidation.
• The two common anti-oxidants are BHA (Butylated Hydroxy-Anisole) and BHT (Butylated Hydroxy – Toluene).

2. Packaging fat and oil containing foods in nitrogen gas.

• Nitrogen is an unreactive gas.

3. Keeping food in a refrigerator. When the food is kept in a refrigerator, the oxidation of fats and oils in it is slowed down due to low temperature.

4. Staring food in air-tight containers.

5. storing food away from the light. In the absence of light, the oxidation of fats and oils present in food is slowed down.