Key Points - Chemical Reactions and Equations 

Chemical Reaction: -  Whenever a chemical change occurs we can say that a chemical reaction has taken place

- eg. Food gets digested in our body

- Rusting of iron.

Chemical Equation:-  A chemical reaction can be expressed symbolically by using chemical equation eg magnesium is burnt into air to form magnesium oxide can be represented as
Mg + O2

MgO
We can observe or recognise a chemical reaction by observing change in state, colour, by evolution of gas or by change in temperature.
- Physical state of the reactant and products are mentioned to make chemical reaction more informative. eg we use (g) for gas, (l) for liquid, (s) for solid and (aq) for aqueous.
- Balancing Equation:-  We balance the chemical equation so that no. of atoms of each element involved in the reaction remain same at the reactant and product side. eg Fe + H2O
→ Fe2O3 + H2 can be written as
3 Fe(s) + 4H2O(g) q
→ Fe2O3(s) +4H2(g)
Combination Reaction:-  The reaction in which two or more substances combine to form a new single substance

- eg.
→
CaO(s) + H2O(l)
Ca(OH)2 (aq)
Calcium
Water
Calcium hydroxide oxide (slaked lime)
Quick lime

- Ca(OH)2 slaked lime is used for white washing walls. It reacts will CO2 to form CaCO3 and gives a shiny finish to the walls.
Ca(OH)2

-  (aq)+
CO2
??
→ (g)
CaCO3
 (s)+
Calcium
Calcium
 hydroxide
Carbonate
H2O (l)
Burning of Coal
C(s) + O2(g) ??→ CO2(g) + heat + light

- Formation of water
2H2(g) + O2(g) ??→ 2H2O(l)
- Exothermic Reactions:-  Reaction in which heat is released along with the formation of products. eg. CH4(g) + 2O2(g) ??→ CO2(g) + 2H2O(g)

- Respiration is also exothermic reaction.

- De composition of vegetable matter into compost.
De compositon Reactions:-  The reaction in which a single substance decomposes to give two or more substances. De composition reactions can be of three types
Thermal Decompositon:-  When a decompositon reaction is carried out by heating

Silver bromide behaves similarly
Sunlight 2Ag(s) + Br (g)
2Ag Br ?????
?
→

- The above two reactions are used in black and white photography.

Endothermic Reactions -  The reactions which require energy in the form of heat, light or electricty are called Endothermic Reactions.
2Ba(OH)2 + NH4Cl ??→ 2BaCl2 + NH4OH

- Displacement Reaction: The chemical Reaction in which an element displaces another element from its solution
Fe(s) + CuSO4(aq) ??→ FeSO4 + Cu(s)
Copper (aq)
Sulphate
Iron Sulphate

- The nail becomes brownish in colour and the blue colour of Copper Sulphate solution fade.

- Other examples
Zn(s) + CuSO4 ??→ ZnSO4 + Cu(s) (aq)
 (aq)
Copper
Sulphate
Zinc
Sulphate
Pb(s) + CuCl2 ??→ PbCl2

- + Cu(s)
 (aq)
 (aq)
Copper
Chloride
Lead
Chloride
Zinc and lead are more reactive elements than copper. They displace copper from its compounds.

Double Displacement Reaction: The reaction in which two different atoms or group of atoms are mutually exchanged eg. Na2 SO4

→ BaSO4(s) + 2NaCl
+ BaCl2
 (aq)
 (aq)
 (aq)
Sodium
Barium
Barium
Sodium
Sulphate Chloride
Sulphate Chloride
A white substance is formed due to above reaction. The insoluble substance is called precipitate.
Precipitation Reaction -  Any reaction that produces a precipitate is called a precipitation reaction. eg. Pb(NO3)2
+
 (aq)
2KI ??→ PbI2 ?? +2KNO3 (aq)
 (aq)
Lead Nitrate

- Potassium Lead Potassium
Iodide
Iodide
Nitrate
Oxidation: Oxidation is the gain of oxygen or loss of hydrogen
Heat → 2CuO eg. 2Cu + O2 ????
When Copper is heated a black colour appears. If this CuO is reacted with hydrogen gas then again Cu becomes brown as reverse reaction takes place
Heat → Cu + H2O
CuO + H2 ????

- Reduction: Reduction is the loss of oxygen or gain of hydrogen.

- Redox Reaction: The reaction in which one reactant gets oxidised while other gets reduced
eg.
ZnO + C ??→ Zn + CO
MnO2 + 4HCl ??→ MnCl2 + 2H2O + Cl2
7

Corrosion: When a metal is attacked by substances around it such as moisture, acids etc. eg.  Reddish brown coating on iron.
 (ii) Black coating on Silver.

- Rancidity: When fats and oils are oxidised they become rancid and their smell and taste change.

- Antioxidants are added to foods containing fats and oil.
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Question Bank
1 Mark

Answer the following questions very breifly

1. What happens when the milk is left at room temperature during summer?

2. Write a chemical equation when magnesium is burnt in air to give magnesium oxide.

3. A substance under goes chemical reactions to produce simpler products, what type of reaction is this?

4. Why do copper vessels lose their shine when exposed to air?

5. Which gas is produced by the action of dilute hydrochloric acid on zinc granules?

6. Fe2O3 + 2Al ??→ Al2O3 + 2Fe
The above reaction is an example of which type of reaction.

7. Name the type of reaction in which energy is absorbed.

8. Why does the colour of copper sulphate solution change when an iron nail is dipped into it?

9. Give an example of decomposition reaction which proceeds by absorbing electric energy.

10. Why do we balance the chemical equation?

- Answer the following questions briefly
(2 mark)

1. Write down the observations which indicate the occurence of a chemical reaction.

2. Why is respiration considered as an exothermic reaction? Explain.

3. Transfer the following statements into Chemical equations and then balance them.

4.  a) Hydrogen gas combines with nitrogen to form ammonia.
b) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Identify the substances that are oxidised and the substances that are reduced to the folllowing reactions.

1. Na
+
O2
 (s)
 (g)
CuO
+
 (s)
H2
??
→ Na2O
 (s)
 (g)
??
→ Cu + H O
2 (l) (s)

5. What happens when silver chloride is exposed to sunlight? Give one practical application of this reaction. Write the equation also.

6. Why is the bag used for potato chips flushed with nitrogen gas?
Answer the following questions in detail

1.  (3 marks)
Write down the balanced chemical equations for the following reactions. a) Zinc Carbonate(s) ??→ Zinc Oxide + Carbon Dioxide(g) b) Aluminium(s) + Chlorine(g) ??→ Aluminium Chloride(s) c) Magnesium(s)+ Water(l)

2. Heat →
Magnesium Hydroxide(l)+ Hydrogen(g)
Choose combination, displacement and double displacement reactions out of the given reactions.
+ 4HCl(l) ??→ MnCl2
 i)
MnO2
 ii)
CaO
 iii)
2AgCl(s) ??→ 2Ag(s) + Cl2
 (s)
 (s)
+ CO2
 (g)
??
→ CaCO3
9
 (s)
+ Cl2
 (g)
+ 2H2O
 (l)
 (s)
 (g)

What happens when CO2 is passed through slaked lime? Write the balanced (g) chemical equation. Write the type of reaction that has occured.
Explain the following questions detail

(5 marks)
Balance the following chemical equation and identify the type of reaction they represent
KClO3 ??→ KCl + O2
NH3 + O2 ??→ NO + H2O
Na2O + H2O ??→ NaOH
Na + H2O ??→ NaOH + H2
FeCl3 + NaOH ??→ Fe (OH)3 + NaCl.

2. Define various types of chemical reactions. Write one chemical equation for each type.