Chemical Reactions and Equations-Exercise

Q1.Which of the statements about the reaction below are incorrect?
\(\scriptsize\mathrm{2PbO(s) + C(s) \rightarrow 2Pb(s) + CO_2(g)}\)

1. Lead is getting reduced.
2. Carbon dioxide is getting oxidised.
3. Carbon is getting oxidised.
4. Lead oxide is getting reduced.
   1. 1 & 2
   2. 1 & 3
   3. 1, 2 & 3
   4. all

Answer:
In the given equation: \[\scriptsize\mathrm{2PbO(s) + C(s) \\\quad\rightarrow 2Pb(s) + CO_2(g)}\] \(\mathrm{PbO}\) is losing Oxygen to become \(\mathrm{Pb}\text{ (elemental) }\Rightarrow\) Reduction
or
Lead oxide \(\mathrm{(PbO)}\) turns into \(\mathrm{Pb}\). The lead goes from \(\mathrm{Pb^{2+}}\) in \(\mathrm{(PbO)}\) to \(\mathrm{Pb}\) (elemental), which is a gain of electrons \(\Rightarrow\) Reduction.

\(\mathrm{C}\) is gaining Oxygen to become \(\mathrm{CO_2}\Rightarrow\) Oxidation
or
Carbon \(\mathrm{(C)}\) becomes \(\mathrm{CO_2}\). Here, carbon's oxidation state goes from 0 (elemental) to +4 in \(\mathrm{CO_2}\Rightarrow\) meaning carbon is oxidised.

Option (ii) is correct.

Q2.\(\mathrm{Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe}\)
The above reaction is an example of a

1. combination reaction.
2. double displacement reaction.
3. decomposition reaction.
4. displacement reaction.

Answer:
The reaction: \[ \scriptsize \begin{array}{r|l} \mathrm{Al} & \mathrm{Fe_2O_3\ +\ 2Al\ \\\rightarrow\ Al_2O_3\ +\ 2Fe} \\ \uparrow \\ \mathrm{Fe} & \mathrm{Al}\ \text{ more reactive }\\&\text{than }\ \mathrm{Fe} \end{array} \] is a redox reaction and specifically a displacement (or replacement) reaction \(\Rightarrow\) often known as a thermite reaction.

Option - 4 is correct

Q3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and iron chloride are produced.
2. Chlorine gas and iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

When dilute hydrochloric acid (HCl) is added to iron filings:
A chemical reaction takes place.
Iron reacts with hydrochloric acid to produce iron(II) chloride
You will observe bubbles of gas (effervescence) due to hydrogen being released.
The reaction is: \[\scriptsize\mathrm{Fe(s)+2HCl(aq)\rightarrow FeCl_2 (aq)+H_2(g)}\] Option - 1 is correct

Q4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer:
A balanced chemical equation shows the same number of each type of atom on both sides of the reaction. This means the total atoms of each element are equal before and after the reaction.
Chemical equations must be balanced to follow the Law of Conservation of Mass, which says matter cannot be created or destroyed in a chemical reaction. Balancing ensures that the amount of each element stays the same, showing the reaction correctly.

Q5. Translate the following statements into chemical equations and then balance them.

1. Hydrogen gas combines with nitrogen to form ammonia.
2. Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
3. Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
4. Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer:

1. Hydrogen gas combines with nitrogen to form ammonia.
   
   When hydrogen gas combines with nitrogen to form ammonia, the reaction is called the Haber process.
   \[\scriptsize\mathrm{N_2(g) + 3H_2(g)\rightarrow 2NH_3(g)}\]
2. Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
   
   When hydrogen sulphide \(\scriptsize\mathrm{(H_2S)}\) gas burns in air, it reacts with oxygen to form water and sulphur dioxide. \[\scriptsize\mathrm{2H_2S(g) + 3 O_2(g)\rightarrow H_2O(l) + 2SO_2(g)}\]
3. Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
   
   When barium chloride reacts with aluminium sulphate, aluminium chloride and barium sulphate (as a precipitate) are formed.
   
   Balanced chemical equation: \[\scriptsize\mathrm{3BaCl_2(aq)+Al_2(aq)\rightarrow 2AlCl_3(aq)+3BaSO_4(s)}\] Barium sulphate \(\scriptsize\mathrm{(BaSO_ 4)}\) forms as an insoluble white precipitate.
   This is an example of a double displacement (precipitation) reaction.
4. Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
   
   When potassium metal reacts with water, potassium hydroxide and hydrogen gas are formed.
   
   Balanced chemical equation: \[\scriptsize\mathrm{2K (s)+2H_2O (l)\rightarrow 2KOH (aq)+H_2(g)}\] Potassium \(\scriptsize\mathrm{(K)}\) reacts vigorously with water \(\scriptsize\mathrm{(H_2O)}\)
   It forms potassium hydroxide \(\scriptsize\mathrm{(KOH)}\) solution and hydrogen gas \(\scriptsize\mathrm{(H_2)}\) (with effervescence).

Q6. Balance the following chemical equations.

1. \(\scriptsize\mathrm{HNO_3+ Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O}\)
2. \(\scriptsize\mathrm{ NaOH + H_2SO_4 \rightarrow Na_2SO_4 + H_2O}\)
3. \(\scriptsize\mathrm{NaCl + AgNO_3 \rightarrow AgCl + NaNO_3}\)
4. \(\scriptsize\mathrm{ BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl}\)

Answer:
Balancing Chemical Equation:

1. \(\scriptsize\mathrm{HNO_3+ Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O}\)
   
   

No of Atoms	In Reactant (LHS)	In Product (RHS)
Ca	1	1
N	1 in \(\scriptsize\mathrm{HNO_3}\) 2 \(\scriptsize\mathrm{\times (HNO_3)}\)	2 in \(\scriptsize\mathrm{Ca(NO_3)_2}\)
Partially Balanced Eqn: \(\scriptsize\mathrm{2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O}\)
O	6 in \(\scriptsize\mathrm{2HNO_3}\) + 2 in \(\scriptsize\mathrm{Ca(OH)_2}\) =8	6 in \(\scriptsize\mathrm{Ca(NO_3)_2}\) + 1 in \(\scriptsize\mathrm{H_2}\) =7
	for 8 Oxygen: 2\(\scriptsize\mathrm{\times (H_2O)}\)
Partially Balanced Eqn: \(\scriptsize\mathrm{2HNO_3 +Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O}\)
H	2 in \(\scriptsize\mathrm{2HNO_3}\) + 2 in \(\scriptsize\mathrm{Ca(OH)_2}\) =4	4 in \(\scriptsize\mathrm{H_2O}\) =4
Balanced Eqn: \(\scriptsize\mathrm{2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O}\)

2. \(\scriptsize\mathrm{ NaOH + H_2SO_4 \rightarrow Na_2SO_4 + H_2O}\)
   
   

   No of Atoms	In Reactant (LHS)	In Product (RHS)
   N	1 in \(\scriptsize\mathrm{NaOH}\) 2\(\scriptsize\mathrm{\times NaOH}\) =2	2 in \(\scriptsize\mathrm{Na_2SO_4}\)
   Partially Balanced Eqn: \(\scriptsize\mathrm{2NaOH + H_2SO_4\rightarrow Na_2SO_4 + H_2O}\)
   S	1 in \(\scriptsize\mathrm{H_2SO_4}\)	1 in \(\scriptsize\mathrm{Na_2SO_4}\)
   O	2 in \(\scriptsize\mathrm{2NaOH}\)+ 4 in \(\scriptsize\mathrm{H_2SO_4}\) =6	4 in \(\scriptsize\mathrm{Na_2SO_4}\) + 1 in \(\scriptsize\mathrm{H_2O}\) =5
   	for 6 Oxygen: 2\(\scriptsize\mathrm{\times H_2O}\)
   Partially Balanced Eqn: \(\scriptsize\mathrm{2NaOH+H_2SO_4\rightarrow Na_2SO_4 + 2H_2O}\)
   H	2 in \(\scriptsize\mathrm{2NaOH}\)+ 2 in \(\scriptsize\mathrm{H_2SO_4}\)= 4	4 in \(\scriptsize\mathrm{2H_2O}\)= 4
   Balanced Eqn: \(\scriptsize\mathrm{2NaOH + H_2SO_4\rightarrow Na_2SO_4 + 2H_2O}\)

3. \(\scriptsize\mathrm{NaCl + AgNO_3 \rightarrow AgCl + NaNO_3}\)
   
   

   No of Atoms	In Reactant (LHS)	In Product (RHS)
   Na	1	1
   Ag	1	1
   Cl	1	1
   N	1	1
   O	3	3
   Already Balanced Eqn: \(\scriptsize\mathrm{NaCl + AgNO_3\rightarrow AgCl+NaNO_3}\)

4. \(\scriptsize\mathrm{ BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl}\)
   
   

   No of Atoms	In Reactant (LHS)	In Product (RHS)
   Ba	1	1
   Cl	2 in \(\scriptsize\mathrm{BaCl_2}\)	1 in \(\scriptsize\mathrm{HCl}\)
   	for 2 Chlorine: 2\(\scriptsize\mathrm{\times HCl}\)
   Partially Balanced Eqn: \(\scriptsize\mathrm{BaCl_2+H_2SO_4\rightarrow BaSO_4 + 2HCl}\)
   O	4 in \(\scriptsize\mathrm{H_2SO_4}\)	4 in \(\scriptsize\mathrm{BaSO_4}\)
   H	2 in \(\scriptsize\mathrm{H_2SO_4}\)	2 in \(\scriptsize\mathrm{2HCl}\) =4
   Balanced Eqn: \(\scriptsize\mathrm{BaCl_2+H_2SO_4\rightarrow BaSO_4+2HCl}\)

Q7. Write the balanced chemical equations for the following reactions.

1. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
2. Zinc + Silver nitrate → Zinc nitrate + Silver
3. Aluminium + Copper chloride → Aluminium chloride + Copper
4. Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer:

1. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
   
   Balanced Chemical Formula
   
   \[\scriptsize\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O (l)}\]
2. Zinc + Silver nitrate → Zinc nitrate + Silver
   
   Balanced Chemical Formula
   
   \[ \scriptsize\mathrm{Zn\ (s)\ +\ 2AgNO_3\ (aq)\ \\\quad\quad\rightarrow\ Zn(NO_3)_2\ (aq)\ +\ 2Ag\ (s)} \]
3. Aluminium + Copper chloride → Aluminium chloride + Copper
   
   Balanced Chemical Formula
   
   \[ \scriptsize\mathrm{2Al\ (s)\ +\ 3CuCl_2\ (aq)\\\quad\quad \rightarrow\ 2AlCl_3\ (aq)\ +\ 3Cu\ (s)} \]
4. Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
   
   Balanced Chemical Formula
   
   \[ \scriptsize\mathrm{BaCl_2\ (aq)\ +\ K_2SO_4\ (aq)\\\quad\quad\rightarrow\ BaSO_4\ (s)\ +\ 2KCl\ (aq)} \]

Q8. Write the balanced chemical equation for the following and identify the type of reaction in each case

1. Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
2. Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
3. Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
4. Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Answer:

1. Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s) \[\scriptsize\mathrm{2KBr (aq)+BaI_2(aq)\rightarrow 2KI (aq)+BaBr_2(s)}\]
   Type of reaction: Double displacement (precipitation) reaction
2. Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g) \[\scriptsize\mathrm{ZnCO_3(s)\rightarrow ZnO(s)+CO_2(g)}\]
   Type of reaction: Thermal decomposition reaction
3. Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g) \[\scriptsize\mathrm{H_2(g)+Cl_2(g)\rightarrow 2HCl(g)}\]
   Type of reaction:Combination (synthesis) reaction
4. Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g) \[\scriptsize\mathrm{Mg(s)+2HCl(aq)→MgCl_2(aq)+H_2 (g)}\]
   Type of reaction: ,Single displacement (redox) reaction

Q9. What does one mean by exothermic and endothermic reactions? Give examples.

Answer:
Exothermic reactions: are chemical reactions that release heat to the surroundings. You can feel the container getting warm.
Example: When quicklime (calcium oxide) is added to water, it forms slaked lime and gives off heat:
\[\scriptsize\mathrm{CaO(s)+H_2O (l)\rightarrow Ca(OH)_2(aq)+heat}\] Endothermic reactions: are chemical reactions that absorb heat from the surroundings. The container feels cold.
Example: When barium hydroxide reacts with ammonium chloride, the mixture becomes cold: \[\scriptsize\mathrm{Ba(OH)_2(s)+2NH_4Cl(s)\\\rightarrow BaCl_2(aq)+2NH_3(g)+2H_2O (l)}\]

Q10. Why is respiration considered an exothermic reaction? Explain.

Answer:
Respiration is called an exothermic reaction because it releases energy when glucose (a sugar) breaks down in our cells.
During respiration, glucose reacts with oxygen to produce carbon dioxide, water, and energy in the form of heat and ATP (a usable energy molecule). This released energy keeps our body warm and powers activities like moving, thinking, and growing.
In simple words: just like a fire gives out heat when wood burns, respiration gives out energy when sugar burns inside our body — so it gives off energy, making it exothermic.

Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer:
Combination reactions happen when two or more substances join together to form a single product. It's like pieces coming together to make a whole. \[\scriptsize\mathrm{2H_2(g)+O_2(g)\rightarrow 2H_2O(l)}\] Decomposition reactions are the opposite because one substance breaks down into two or more simpler substances. It's like breaking a big thing into smaller parts.
So, decomposition reactions reverse what combination reactions do. \[\scriptsize\mathrm{2H_2O(l)\xrightarrow{\text{Electricity}} 2H_2(g)+O_2(g)}\]

Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer:
Decomposition by Heat (Thermal decomposition):
When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide: \[\scriptsize\mathrm{CaCO_3(s) \xrightarrow{\text{Heat}}CaO(s) + CO_2(g)\uparrow}\] Decomposition by Light (Photolytic decomposition):
When silver chloride is exposed to sunlight, it breaks down into silver and chlorine gas: \[\scriptsize\mathrm{2AgCl(s)\xrightarrow{\text{Sunlight}}2Ag(s) + Cl_2(g)\uparrow}\] Decomposition by Electricity (Electrolytic decomposition):
When electricity is passed through water, it splits into hydrogen and oxygen gases: \[\scriptsize\mathrm{2H_2O(l)\xrightarrow{\text{Electricity}}2H_2(g)\uparrow + O_2(g)\uparrow}\]

Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:
Displacement reaction: In a displacement reaction, a more reactive element replaces a less reactive element from its compound.
Only one element is displaced. \[\scriptsize\mathrm{Zn(s)+CuSO_4(aq)\rightarrow ZnSO_4(aq) + Cu(s)}\] Here, zinc displaces copper from copper sulphate.

Double displacement reaction: In a double displacement reaction, two compounds exchange their ions to form two new compounds.
Both partners swap parts with each other. \[\scriptsize\mathrm{Na_2SO_4(aq)BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\] Here, sodium sulphate and barium chloride exchange ions to form barium sulphate (a precipitate) and sodium chloride.

Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer:
In the refining of silver, copper metal is used to recover silver from silver nitrate solution by displacement.
The reaction involved is: \[\scriptsize\mathrm{Cu(s)+AgNO_3(aq)\rightarrow Cu(NO_3)_2(aq) + 2Ag(s)}\]

Q15. What do you mean by a precipitation reaction? Explain by giving examples.

Answer:
A precipitation reaction is a type of chemical reaction where two solutions are mixed and an insoluble solid (called a precipitate) forms and settles out of the solution.
Example 1:
When solutions of barium chloride and sodium sulphate are mixed, a white solid called barium sulphate forms: \[\scriptsize\mathrm{BaCl_2(aq)+ Na_2SO_4(aq)\\\qquad\rightarrow \underset{\text{Precipitate}}{BaSO_4(s)}+2NaCl(aq)}\] Example 2:
When silver nitrate reacts with sodium chloride solution, a white precipitate of silver chloride is formed: \[\scriptsize\mathrm{AgNO_3(aq) + NaCl\\\qquad\rightarrow\underset{\text{Precipitate}}{AgCl(s)}+NanO_3(aq)}\]

Q16. Explain the following in terms of gain or loss of oxygen with two examples each.
Oxidation
Reduction

Answer:
Oxidation: Oxidation means a substance gains oxygen during a reaction. You can remember this as “adding oxygen”. Examples:
\[\scriptsize\mathrm{Cu+O_2 \rightarrow CuO}\] Here, copper \(\scriptsize\mathrm{(Cu)}\) gains oxygen to form copper oxide \(\scriptsize\mathrm{(CuO)}\). So, copper is oxidised.

\[\scriptsize\mathrm{H_2S+O_2 \rightarrow H_2O+SO_2}\] Here, hydrogen sulphide \(\scriptsize\mathrm{(H_2S)}\) gains oxygen and becomes sulphur dioxide \(\scriptsize\mathrm{(SO_2)}\); thus, \(\scriptsize\mathrm{H_2S}\) is oxidised.

Reduction: Reduction means a substance loses oxygen during a reaction. Think of it as “removing oxygen”.
Examples:
\[\scriptsize\mathrm{CuO+H_2 \rightarrow Cu+H_2O}\] In this reaction, copper oxide \(\scriptsize\mathrm{(CuO)}\) loses oxygen to become copper \(\scriptsize\mathrm{(Cu)}\). So, copper oxide is reduced.

\[\scriptsize\mathrm{Fe_2O_3 + 3CO\rightarrow 2Fe + 3CO_2}\] Here, iron(III) oxide \(\scriptsize\mathrm{(Fe_2O_3)}\) loses oxygen to become iron \(\scriptsize\mathrm{(Fe)}\), so it is reduced during the process.

Q17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer:
The shiny brown coloured element ‘X’ is copper (Cu).
When copper is heated in air, it reacts with oxygen and forms a black coloured compound called copper(II) oxide \(\scriptsize\mathrm{(CuO)}\).
Element ‘X’: Copper \(\scriptsize\mathrm{(Cu)}\)
Black coloured compound: Copper(II) oxide \(\scriptsize\mathrm{(CuO)}\) \[ \scriptsize\mathrm{2Cu (s) + O_2 (g) \xrightarrow{\text{heat}} 2CuO(s)} \]

Q18. Why do we apply paint on iron articles?

Answer:
We apply paint on iron articles to prevent rusting. Paint forms a protective coat on the iron surface and stops air and moisture (water) from coming into contact with the iron. This prevents the chemical reaction (oxidation) that causes iron to rust.

Q19.Oil and fat containing food items are flushed with nitrogen. Why?

Answer:
Oil and fat containing food items are flushed with nitrogen to prevent them from getting spoiled. Nitrogen gas pushes out oxygen from the container, which stops oils and fats from reacting with oxygen. This reaction is called oxidation, and it makes food rancid, giving it a bad smell and taste. Flushing with nitrogen helps the food stay fresh for a longer time.

Q20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity

Answer:
Corrosion: is the slow destruction of metals when they react with substances like air, water, or chemicals around them. The most common example is rusting of iron. When iron reacts with oxygen and moisture, it forms a reddish-brown layer called rust.
Example:
Iron gates or railings develop a reddish-brown coating (rust) after being exposed to rain and air for some time.

Rancidity: is the process in which oils and fats in food get spoiled when they react with oxygen, leading to a bad smell and taste. This happens due to oxidation of fats and oils.
Example:
Chips or fried snacks left open for a long time taste and smell unpleasant because the oils in them have become rancid.