Combination Reactions and Exothermic Reactions

What is a Combination Reaction?

The simplest kind of chemical reaction — when two or more reactants combine to form a single product, it is called a combination reaction.

General Form

$$A + B \rightarrow AB$$

Things are joining together, not splitting apart — the very word combination means 'coming together'.

Activity 1.4 — Calcium Oxide + Water

Take a small quantity of calcium oxide (quick lime, $CaO$) in a beaker and slowly add water.

Observations:

• The beaker becomes very hot (you can feel it on touching).
• The white $CaO$ powder mixes with water to form a thick slurry.

Reaction:

$$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$$

Here, two substances ($CaO$ and $H_2O$) combine to form a single product $Ca(OH)_2$ (slaked lime / calcium hydroxide) — this is a combination reaction.

Use of Calcium Hydroxide for Whitewashing

The solution of slaked lime is used to whitewash walls. After 2–3 days of whitewashing, the walls develop a shine — why?

$Ca(OH)_2$ slowly reacts with $CO_2$ from the air to form $CaCO_3$ (calcium carbonate), which forms a glossy white coating.

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$

Interesting Fact: Marble has the same chemical formula — $CaCO_3$. That's why a freshly whitewashed wall develops a slight marble-like sheen.

More Examples of Combination Reactions

Combination reactions can occur in three ways —

1. Element + Element → Compound

(i) Burning of coal:

$$C(s) + O_2(g) \rightarrow CO_2(g)$$

(ii) Formation of water from hydrogen:

$$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$$

(iii) Burning of magnesium:

$$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$$

(iv) Formation of ammonia from nitrogen:

$$N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$$

2. Compound + Compound → Compound

(i) The whitewashing reaction:

$$Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$$

3. Element + Compound → Compound

(i) Conversion of sulphur dioxide to sulphur trioxide:

$$2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)$$

Easy Identification

If a reaction has more than one reactant but only one product, it is a combination reaction. Remember the rule:

$$\text{Reactants} > 1 \quad \text{and} \quad \text{Products} = 1 \Rightarrow \text{Combination}$$

[Board Tip] "Two combine to form one" — the simplest rule of combination reactions.

Exothermic Reactions

In Activity 1.4, you saw that the $CaO + H_2O$ reaction made the beaker hot — heat was released. Such reactions are called exothermic reactions.

Definition: Reactions in which heat is also released along with the formation of products are called exothermic reactions.

Symbolic Form

$$\text{Reactants} \rightarrow \text{Products} + \text{Heat}$$

('+ Heat' implies that heat is being released as a kind of 'product')

Major Examples of Exothermic Reactions

(i) Burning of natural gas (methane) — LPG in your kitchen:

$$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{Energy}$$

(ii) Respiration — happening inside our body:

$$C_6H_{12}O_6(aq) + 6O_2(aq) \rightarrow 6CO_2(aq) + 6H_2O(l) + \text{Energy}$$

The glucose that comes from our food reacts with oxygen in our cells to release energy.

(iii) Decomposition of vegetable matter into compost:

Vegetable matter slowly decomposes into manure — this process also releases heat. The interior temperature of a large compost heap can reach 60–70°C.

(iv) Slaking of lime:

$$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$

[Looking Ahead — NEET/JEE] In an exothermic reaction, $\Delta H$ is negative ($\Delta H < 0$) — a concept you will study in Class 11.

The Connection Between Combination and Exothermic

A very important point — not every combination reaction is exothermic, but most combination reactions are exothermic.

Why? — From the Bond-Energy Perspective

When atoms form bonds, energy is released. In combination reactions, new and stable bonds form — and so energy is released.

Classification of Activity 1.1

Burning of magnesium — $2Mg + O_2 \rightarrow 2MgO + \text{Heat} + \text{Light}$

• Combination? Yes — two reactants form one product.
• Exothermic? Yes — a dazzling flame is produced.

Both at once — a single reaction can fall into more than one class.

Physical Processes vs. Chemical Reactions

Some exothermic processes are not chemical at all:

• Freezing of water is an exothermic physical change (latent heat is released) — but no new substance is formed.
• So to call something an exothermic reaction, both a new substance and the release of heat are required.

A Quick Table — Easy Identification

Example	Combination?	Exothermic?
$CaO + H_2O \rightarrow Ca(OH)_2$	Yes	Yes
$2Mg + O_2 \rightarrow 2MgO$	Yes	Yes
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$	No (multiple products)	Yes
Respiration	No	Yes
Freezing of water	— (Physical)	Yes (latent heat)

Be Careful! Not every exothermic process is a chemical reaction. Only when a new substance forms it is chemical.

Importance in Daily Life

At Home

• Cooking: LPG (mainly $CH_4, C_3H_8$) burns to provide heat.
• Maintaining body temperature: Energy from food is released by respiration.
• Whitewashing walls: $Ca(OH)_2$ + atmospheric $CO_2$ → $CaCO_3$ — a glossy white coating.

In Industry

• Cement industry: $CaO$ is a key ingredient.
• Haber Process: $N_2 + 3H_2 \rightarrow 2NH_3$ — making ammonia for fertilisers. This is an industrially crucial exothermic combination reaction.
• Metallurgy: $2C + O_2 \rightarrow 2CO$ — supplies heat in furnaces.
• Sulphuric acid production: $2SO_2 + O_2 \rightarrow 2SO_3$ (part of the contact process).

In Nature

• Combustion (fire): Burning of fuels.
• Biological processes: Decomposition of vegetables = compost = manure for agriculture.
• Animal metabolism: The energy source for all living beings.

A Curious Question — Why Are We Warm?

Respiration (an exothermic reaction) goes on continuously in our bodies. A healthy adult produces about 80 calories per hour — that's what keeps us at 37°C even in cold weather.

[Board Important] Why is respiration called an exothermic reaction? — A nearly annual question. Standard 3-mark answer:

1. In respiration, glucose + oxygen → CO₂ + H₂O (new substance — chemical)
2. Energy is released (exothermic)
3. This energy keeps us alive.

🧠 Memory Capsule

A one-glance recap to revisit just before the board exam — every key idea about combination and exothermic reactions in one place.

1. Combination Reaction Formula

$$A + B \rightarrow AB \quad (\text{two reactants} \rightarrow \text{one product})$$

2. Exothermic Formula

$$\text{Reactants} \rightarrow \text{Products} + \textbf{Heat}$$

3. Must-Memorise Reactions

Reaction	Type
$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$	Combination + Exothermic
$2Mg + O_2 \rightarrow 2MgO + \text{Heat} + \text{Light}$	Combination + Exothermic
$C + O_2 \rightarrow CO_2 + \text{Heat}$	Combination + Exothermic
$2H_2 + O_2 \rightarrow 2H_2O + \text{Heat}$	Combination + Exothermic
$Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$	Combination (whitewash shine)
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$	Exothermic (but not combination)
Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$	Exothermic

4. Key Facts — One Liners

• $CaO$ = Quick lime (unslaked)
• $Ca(OH)_2$ = Slaked lime — used in whitewashing
• $CaCO_3$ = Calcium carbonate = limestone / marble
• Reason for the wall's shine = formation of $CaCO_3$ from atmospheric $CO_2$

5. Five Sure-Shot Board Questions

1. What is a combination reaction? Give examples.
2. What is an exothermic reaction? Give three examples.
3. Why does a wall painted with slaked lime develop a shine after 2–3 days?
4. Why is respiration considered an exothermic reaction?
5. Identify the type of reaction shown in Activity 1.4.

6. Watch Out!

• Not every combination is exothermic — but most are.
• Not every exothermic reaction is combination — combustion and respiration are exothermic but not combination (multiple products).
• For an exothermic reaction, a new substance must form — heat alone is not enough.

The Bottom Line: Two combine, one forms → combination. Heat released → exothermic.

Solved Examples

Example 1: Identifying Combination Reactions

Which of the following are combination reactions? (i) $2Mg + O_2 \rightarrow 2MgO$ (ii) $2KClO_3 \rightarrow 2KCl + 3O_2$ (iii) $CaCO_3 \rightarrow CaO + CO_2$ (iv) $N_2 + 3H_2 \rightarrow 2NH_3$ (v) $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

Solution:

1. Test: more than one reactant → single product.
2. Analysis:

• (i) Mg + O₂ → MgO — two reactants, one product → Yes, combination ✓
• (ii) one reactant, two products → decomposition ✗
• (iii) one reactant, two products → decomposition ✗
• (iv) N₂ + H₂ → NH₃ — two reactants, one product → Yes, combination ✓
• (v) two reactants, but two products → displacement ✗

1. Answer: (i) and (iv) are combination reactions.

Example 2: NCERT Question — The Whitewashing Mystery

A solution of substance 'X' is used for whitewashing. (i) Name 'X' and write its formula. (ii) Write the reaction of X with water. (iii) Why does the wall develop a shine 2–3 days after whitewashing?

Solution:

(i) Substance 'X' is calcium oxide (quick lime), formula = $\textbf{CaO}$.

(ii) Reaction of X with water:

$$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$$

This is an exothermic combination reaction. The $Ca(OH)_2$ (slaked lime) so formed is what gets applied to the wall.

(iii) Reason for the shine:

$Ca(OH)_2$ slowly reacts with the $CO_2$ in the atmosphere —

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$

A thin coating of $CaCO_3$ (calcium carbonate) builds up on the wall over 2–3 days. Marble also has the formula $CaCO_3$ — which is why the wall ends up with a slight marble-like shine.

[Repeatedly asked in Board — 3 marks]

Example 3: Definition of Exothermic Reaction

Define an exothermic reaction. Give three examples.

Solution:

Definition: Chemical reactions in which heat is released along with the formation of products are called exothermic reactions.

Three examples:

(i) Burning of natural gas (methane):

$$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{Energy}$$

(ii) Respiration:

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$$

(iii) Slaking of lime:

$$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$

[Board Important — 3-mark question]

Example 4: Why is Respiration an Exothermic Reaction?

Why is respiration considered an exothermic reaction? Explain.

Solution:

The respiration reaction:

$$C_6H_{12}O_6(aq) + 6O_2(aq) \rightarrow 6CO_2(aq) + 6H_2O(l) + \text{Energy}$$

It is an exothermic reaction because —

1. It is a chemical reaction: Glucose and oxygen combine to form new substances ($CO_2$ and $H_2O$).
2. Energy is released: The reaction releases energy, stored as ATP and used for various body functions.
3. Bodily evidence: Running or exercise makes the body feel warm — a direct sign of the heat released by faster respiration.
4. This same energy keeps us alive — it maintains body temperature at 37°C, drives muscle contraction, and powers the brain.

Conclusion: Respiration produces new substances and releases energy — both conditions met, hence it is an exothermic chemical reaction.

[Repeatedly asked in Board — almost every year — 3 marks]

Example 5: Combination + Exothermic Combined Question

Consider the reaction: $\quad CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$ (a) What type of reaction is this? (b) Is it exothermic? Give a reason. (c) State one daily-life use of the product.

Solution:

(a) Type: This is a combination reaction because two reactants ($CaO$ and $H_2O$) combine to form a single product, $Ca(OH)_2$.

(b) Exothermic? Yes, this is an exothermic reaction because heat is released along with the reaction — the beaker becomes warm to the touch.

(c) Use: The product $Ca(OH)_2$ (slaked lime) is used for whitewashing walls. After 2–3 days, it reacts with $CO_2$ in the air to form a shiny white coating of $CaCO_3$.

Lesson: A single reaction can belong to more than one type — both combination and exothermic at the same time.

Example 6: Composting — A Chemical Analysis

What type of reaction is the composting of vegetable matter? Explain.

Solution:

1. Process: Vegetables (containing carbohydrates, proteins and other organic compounds) slowly break down by the action of microorganisms into humus (manure).
2. Chemical Nature:

• It is a chemical reaction — new substances are produced.
• It is exothermic — the interior temperature of a compost heap can reach 60–70°C, clear evidence of heat being released.

1. Overall: It is an exothermic reaction (a kind of decomposition) where complex organic matter breaks into smaller molecules and minerals.
2. Daily-Life Importance:

• Compost is a natural fertiliser for agriculture.
• An eco-friendly way to manage waste.
• Very large compost heaps can sometimes catch fire from the heat generated inside.

Answer: Composting is an exothermic chemical reaction.

Example 7: Synthesis of Ammonia — The Haber Process

What type of reaction is the formation of ammonia from nitrogen and hydrogen? Write the balanced equation and explain its classification.

Solution:

Balanced equation:

$$N_2(g) + 3H_2(g) \xrightarrow[\text{High pressure}]{\text{Fe catalyst}} 2NH_3(g) + \text{Heat}$$

Classification:

1. Combination reaction? Yes — two reactants ($N_2$ and $H_2$) form a single product ($NH_3$).
2. Exothermic? Yes — heat is released (interestingly, the industrial process still uses high temperature, because the reaction proceeds very slowly otherwise).

Importance of the Haber Process:

• It is the foundation of the ammonia industry.
• Ammonia is used to make fertilisers (urea, ammonium sulphate).
• Critically important for agriculture — about half the world's population is fed using fertilisers from the Haber process.

[NEET/JEE hint] You will revisit the Haber process in Class 12 in the context of chemical equilibrium.

Example 8: NCERT — Burning of Magnesium

Identify the type of reaction in Activity 1.1, in which heat is released along with the formation of a single product.

Solution:

Reaction:

$$2Mg(s) + O_2(g) \rightarrow 2MgO(s) + \text{Heat} + \text{Light}$$

Analysis:

1. Reactants: Mg (atoms combined) and $O_2$ (one molecule) — two reactants.
2. Product: Only $MgO$ — single product.
3. Released: Heat and light.

Classification:

• Combination reaction — two reactants → one product ✓
• Exothermic reaction — heat is released ✓

Answer: This is a combination + exothermic chemical reaction.

A note: Burning magnesium produces such intense light that it was once used in flash photography. Even today it is used in fireworks and certain emergency flares.

Example 9: Burning of Coal

Write the balanced equation for the burning of coal and state which classes of reaction it belongs to.

Solution:

1. Reaction:

$$C(s) + O_2(g) \rightarrow CO_2(g) + \text{Energy}$$

2. Atom check: C=1=1 ✓, O=2=2 ✓ — already balanced.
3. Classification:

• Combination? Yes — two reactants (C and $O_2$) → one product ($CO_2$).
• Exothermic? Yes — combustion releases a lot of energy.

2. Daily-life: Most of India's electricity is generated by burning coal in thermal power plants.
3. Environmental note: $CO_2$ is a greenhouse gas, so burning coal contributes to global warming.

Answer: Burning of coal is a combination + exothermic reaction.

Example 10: Formation of Water from Hydrogen

Write the balanced equation for the formation of water from hydrogen and oxygen. What kind of reaction is it?

Solution:

1. Unbalanced:

$$H_2 + O_2 \rightarrow H_2O$$

2. Balancing:

• Balance O: coefficient 2 on $H_2O$ → RHS O = 2
• Balance H: coefficient 2 on $H_2$ → LHS H = 4

$$2H_2(g) + O_2(g) \rightarrow 2H_2O(l) + \text{Energy}$$

3. Verify: H=4=4 ✓, O=2=2 ✓
4. Classification:

• Two reactants → one product = Combination
• Heat released = Exothermic

3. An interesting fact: A mixture of hydrogen and oxygen is explosive — that's the principle behind the 'hydrogen bomb' name. In controlled combustion, however, this reaction is used as a rocket fuel.

Answer: This is a combination + exothermic reaction.

Example 11: True/False

State whether each of the following is true or false, with reasons — (a) All combination reactions are exothermic. (b) All exothermic reactions are combination reactions. (c) Respiration is an exothermic reaction. (d) The reaction of lime with water is endothermic.

Solution:

(a) False — Most combination reactions are exothermic, but not all. Some can be endothermic, e.g., $N_2 + O_2 \rightarrow 2NO$ (which absorbs heat).

(b) False — Respiration and combustion of methane are exothermic but not combination reactions (they yield multiple products).

(c) True — In respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$, energy is released.

(d) False — The reaction of lime with water is exothermic ($CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$), since the beaker becomes hot.

Example 12: Identification by Temperature Change

A student does two experiments — (A) adds water to $CaO$ in a beaker. (B) dissolves sodium chloride in water in another beaker. Based on the temperature change in each, identify which is a chemical reaction.

Solution:

(A) $CaO + H_2O$:

• Temperature rises sharply (beaker becomes very hot).
• New substance $Ca(OH)_2$ is formed.
• This is a chemical reaction — specifically an exothermic combination reaction.

$$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$$

(B) NaCl + water (solution):

• Practically negligible change in temperature.
• No new substance — NaCl is still NaCl, just dissolved.
• This is only a physical change, not a chemical reaction.

Conclusion:

• (A) Chemical reaction (exothermic combination).
• (B) Only a physical change (dissolution).

Lesson: A large temperature change is a sign of a chemical reaction, but temperature alone is not decisive — always check whether a new substance has formed.

Example 13: Applications of Exothermic Reactions

Give three major daily-life applications of exothermic reactions.

Solution:

1. Cooking (combustion of LPG / natural gas):

$$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{Energy}$$

The heat released cooks our food.

2. Transportation (combustion of fuel in vehicles):

$$2C_8H_{18}(l) + 25O_2(g) \rightarrow 16CO_2(g) + 18H_2O(g) + \text{Energy}$$

(Petrol is mainly a mixture of octane.) The energy released drives engines.

3. Power generation (from coal or natural gas):

$$C(s) + O_2(g) \rightarrow CO_2(g) + \text{Energy}$$

Most thermal power plants in India work on this principle.

Other applications:

• Energy from respiration in our bodies.
• Furnaces in cement manufacture.
• Composting for agriculture.
• Combustion of rocket fuels.

[Board 5-mark question]

Example 14: Sub-classes of Combination Reactions

Classify the following combination reactions into the three sub-classes — element+element, compound+compound, element+compound: (i) $C + O_2 \rightarrow CO_2$ (ii) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$ (iii) $2SO_2 + O_2 \rightarrow 2SO_3$ (iv) $H_2 + Cl_2 \rightarrow 2HCl$ (v) $CaO + H_2O \rightarrow Ca(OH)_2$

Solution:

Reaction	Type of Reactants	Sub-class
(i) $C + O_2$	element + element	Element + Element
(ii) $Ca(OH)_2 + CO_2$	compound + compound	Compound + Compound
(iii) $2SO_2 + O_2$	compound + element	Element + Compound
(iv) $H_2 + Cl_2$	element + element	Element + Element
(v) $CaO + H_2O$	compound + compound	Compound + Compound

Note: The number of products (single) decides whether it is a combination reaction; the nature of the reactants decides the sub-class.

Example 15: Numerical — Quantity of Heat

If 1 mol of $CaO$ and 1 mol of $H_2O$ react to release 65 kJ of heat, how much heat will be released by the complete reaction of 5.6 g of $CaO$? (Ca = 40, O = 16)

Solution:

1. Reaction:

$$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + 65 \text{ kJ}$$

2. Ratio: 1 mol $CaO$ = $40 + 16 = 56$ g — 65 kJ heat.
3. Given: 5.6 g $CaO$.
4. Calculation:

$$\text{Heat} = \frac{65}{56} \times 5.6 = 6.5 \text{ kJ}$$

5. Answer: 6.5 kJ of heat will be released by the complete reaction of 5.6 g $CaO$.

Lesson: This numerical gives an early taste of mass, mole concept, and thermodynamics — concepts you will study in detail in Class 11.

Example 16: Application — A Kitchen Question

What reaction takes place when LPG burns in your kitchen? How is it classified? What is required for complete combustion?

Solution:

LPG is mainly a mixture of butane ($C_4H_{10}$) and propane ($C_3H_8$). Let's take butane as the example —

1. Unbalanced:

$$C_4H_{10} + O_2 \rightarrow CO_2 + H_2O + \text{Energy}$$

2. Balancing:

• C: coefficient 4 on $CO_2$ (4 C atoms)
• H: coefficient 5 on $H_2O$ (10 H atoms)
• O: RHS now has $4 \times 2 + 5 = 13$ → coefficient 13/2 on $O_2$
• Multiply the entire equation by 2 (the odd-vs-even trick):

$$2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(g) + \text{Energy}$$

3. Classification:

• Combustion — Yes
• Exothermic — Yes (the flame heats your kitchen)
• Combination — No (multiple products)
• Oxidation — Yes (O is being added to $C_4H_{10}$)

3. Required for complete combustion:

• Sufficient oxygen — with insufficient oxygen, incomplete combustion forms toxic CO and soot (carbon particles).
• This is why kitchens must always be well-ventilated — burning LPG in a closed room can produce poisonous CO that may be fatal.

[Board + Real-Life] This question is also important from the environmental-awareness perspective.