Decomposition Reactions and Endothermic Reactions

What is a Decomposition Reaction?

In the previous section, we saw that in combination, two reactants combine to form one product. Decomposition is exactly the reverse — one substance breaks down into two or more substances.

Definition: A reaction in which a single reactant breaks down into two or more products is called a decomposition reaction.

General Form

$$AB \rightarrow A + B$$

This is exactly the reverse of combination ($A + B \rightarrow AB$) — that is why the two are sometimes called 'opposite reactions'.

Where Does the Energy to Break the Reactant Come From?

A stable substance does not break down on its own — energy must be supplied from outside. This energy is provided in three main forms —

1. Heat → Thermal Decomposition
2. Electricity → Electrolytic Decomposition
3. Light → Photochemical Decomposition

We will study each in detail below.

[Board Tip] Most decomposition reactions are endothermic — because energy must be supplied to break the bonds.

Thermal Decomposition

When a reaction is driven by heat, it is called thermal decomposition. The arrow is marked with $\Delta$ or 'heat' to indicate this.

Activity 1.5 — Decomposition of Ferrous Sulphate

Take 2 g of ferrous sulphate crystals ($FeSO_4 \cdot 7H_2O$) — they are green in colour — in a dry boiling tube. Heat them on a burner.

Observations:

• The green crystals turn brown.
• A pungent, choking smell of sulphur is released.

Reaction:

$$2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$$

(First the crystals lose water: $FeSO_4 \cdot 7H_2O \rightarrow FeSO_4 + 7H_2O$, then the decomposition above.)

Decomposition of Limestone

The most important reaction in the cement industry —

$$CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)$$

$CaO$ (quick lime) is the key ingredient in cement. Note that this reaction provides the starting material for the combination reaction $CaO + H_2O \rightarrow Ca(OH)_2$ we saw in Section 3 — it is the reverse direction.

Activity 1.6 — Decomposition of Lead Nitrate

Take 2 g of lead nitrate powder in a boiling tube and heat it over a flame.

Observations:

• A brown gas ($NO_2$ — nitrogen dioxide) is evolved from the tube.
• After a while, lead oxide (yellow) and oxygen are also formed.

Reaction:

$$2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g)$$

Caution: $NO_2$ is toxic — point the open end of the tube away from yourself and others during this experiment.

Electrolytic Decomposition

When decomposition is driven by passing electric current, it is called electrolysis (electrolytic decomposition).

Activity 1.7 — Electrolysis of Water

Procedure:

1. Make two holes in the bottom of a plastic mug and fit rubber stoppers.
2. Insert carbon electrodes through these stoppers.
3. Connect the electrodes to a 6V battery.
4. Fill the mug with water (so that the electrodes are submerged) and add a few drops of dilute $H_2SO_4$ (to make water conduct).
5. Place two graduated test tubes filled with water upside-down over the electrodes.
6. Switch on the current for a while.

Observations:

• Bubbles (gases) form at both electrodes.
• The volume of gas in one test tube is double that in the other.
• Gas at cathode = hydrogen ($H_2$), larger volume.
• Gas at anode = oxygen ($O_2$), smaller volume.

Reaction:

$$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$

Why is the Volume of Hydrogen Twice that of Oxygen?

Look at the equation: 2 molecules of $H_2O$ → 2 molecules of $H_2$ + 1 molecule of $O_2$ — so hydrogen : oxygen = 2 : 1. The same ratio applies to gas volumes (Avogadro's law).

How to Identify the Gases

• Hydrogen test: Bring a burning matchstick close — the gas burns with a 'pop' sound.
• Oxygen test: Bring a glowing splinter close — it bursts into a bright flame.

Another Important Example

Electrolysis of molten sodium chloride:

$$2NaCl(l) \xrightarrow{\text{electricity}} 2Na(s) + Cl_2(g)$$

This is the industrial method for producing sodium metal and chlorine gas (Down's Process).

Photochemical Decomposition

When the reaction occurs in the presence of sunlight, it is called photochemical decomposition.

Activity 1.8 — Decomposition of Silver Chloride

Take 2 g of silver chloride (white) in a china dish and leave it in sunlight for a while.

Observations:

• The white $AgCl$ turns grey.
• Reason — in the presence of light, $AgCl$ decomposes into $Ag$ (metallic silver, grey) and $Cl_2$.

Reaction:

$$2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$$

Similarly, silver bromide also —

$$2AgBr(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Br_2(g)$$

The Principle of Black-and-White Photography

The old black-and-white film was coated with a thin layer of $AgBr$. When light fell on it, $AgBr$ decomposed in proportion to light intensity into metallic silver (black) — the brighter areas became darker on the film, producing the photograph. This is photochemical decomposition put to industrial use.

Today, with digital cameras, film photography is rare — but the principle remains historically significant.

The Three Types of Decomposition — Summary Table

Energy Source	Name of Reaction	Example
Heat	Thermal decomposition	$CaCO_3 \rightarrow CaO + CO_2$
Electricity	Electrolytic decomposition	$2H_2O \rightarrow 2H_2 + O_2$
Light	Photochemical decomposition	$2AgCl \rightarrow 2Ag + Cl_2$

[Board Important] Memorise one example of each — this is a very common question.

Endothermic Reactions

In the previous section we saw exothermic reactions in which heat is released. Endothermic is the opposite.

Definition: Reactions in which heat/energy is absorbed from the surroundings are called endothermic reactions.

Symbolic Form

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

Energy is written on the left side because it acts like a 'reactant' — it is being taken in.

A Striking Experiment

Take 2 g of barium hydroxide $Ba(OH)_2$ in a test tube. Add 1 g of ammonium chloride $NH_4Cl$ and stir with a glass rod.

Observation: Touching the bottom of the test tube with your palm feels cold.

Reaction:

$$Ba(OH)_2 + 2NH_4Cl \rightarrow BaCl_2 + 2NH_3 + 2H_2O$$

This is a remarkable demonstration — mixing two solids drops the temperature so much that water vapour from the surroundings can condense as droplets on the outside of the tube.

Other Examples of Endothermic Reactions

Reaction	Type
$2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$	Decomposition + Endothermic
$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$	Decomposition + Endothermic
$2KClO_3 \xrightarrow{\Delta} 2KCl + 3O_2$	Decomposition + Endothermic
Photosynthesis: $6CO_2 + 6H_2O + \text{Energy} \rightarrow C_6H_{12}O_6 + 6O_2$	Endothermic

The Connection between Decomposition and Endothermic

Most decomposition reactions are endothermic. The reason — energy must be supplied to break the bonds. That is why decomposition requires heat, electricity, or light to proceed.

[NEET/JEE hint] $\Delta H$ is positive ($\Delta H > 0$) for endothermic reactions — to be studied in Class 11.

🧠 Memory Capsule

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

1. Decomposition Formula

$$AB \rightarrow A + B \quad (\text{one reactant} \rightarrow \text{two/more products})$$

2. Endothermic Formula

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

3. Three Types of Decomposition — Must-Know Table

Energy	Type	Famous Example
Heat ($\Delta$)	Thermal	$CaCO_3 \rightarrow CaO + CO_2$
Electricity	Electrolysis	$2H_2O \rightarrow 2H_2 + O_2$
Light	Photochemical	$2AgCl \rightarrow 2Ag + Cl_2$

4. NCERT's Four Famous Activities

• Activity 1.5: $2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3$ (green → brown)
• Activity 1.6: $2Pb(NO_3)_2 \xrightarrow{\Delta} 2PbO + 4NO_2 + O_2$ (brown smoke)
• Activity 1.7: $2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$ (H₂ : O₂ = 2 : 1)
• Activity 1.8: $2AgCl \xrightarrow{\text{light}} 2Ag + Cl_2$ (white → grey)

5. Exothermic vs. Endothermic

Aspect	Exothermic	Endothermic
Heat	Released	Absorbed
Temperature	Rises	Falls
Example	$CaO + H_2O$	$Ba(OH)_2 + NH_4Cl$
Connection	Most combinations	Most decompositions
$\Delta H$ (Class 11)	$< 0$	$> 0$

6. Repeatedly Asked Board Questions

1. What is a decomposition reaction? Give examples of all three types.
2. Why is the volume of hydrogen twice that of oxygen in the electrolysis of water?
3. Why is decomposition called the opposite of combination?
4. Why does silver chloride change colour in sunlight?
5. Give one example of an endothermic reaction.

7. One-Liners to Remember

• Decomposition = breaking apart
• Endothermic = energy IN
• Most decompositions are endothermic
• Three types: heat / electricity / light

The Bottom Line: One breaks, many form → decomposition. Energy in → endothermic.

Solved Examples

Example 1: Identifying Decomposition Reactions

Which of the following are decomposition reactions? (i) $2NaCl \rightarrow 2Na + Cl_2$ (ii) $H_2 + Cl_2 \rightarrow 2HCl$ (iii) $CaCO_3 \rightarrow CaO + CO_2$ (iv) $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ (v) $2H_2O_2 \rightarrow 2H_2O + O_2$

Solution:

1. Test: single reactant → multiple products
2. Analysis:

• (i) NaCl → Na + Cl₂ — one reactant, two products → Decomposition ✓
• (ii) two reactants → one product → Combination ✗
• (iii) CaCO₃ → CaO + CO₂ → Decomposition ✓
• (iv) two reactants → two products → Displacement ✗
• (v) H₂O₂ → H₂O + O₂ → Decomposition ✓

1. Answer: (i), (iii), and (v) are decomposition reactions.

Example 2: NCERT — Explanation of Ferrous Sulphate Decomposition

What are the observations when ferrous sulphate crystals are heated? Write the balanced reaction.

Solution:

Observations:

1. The green crystals turn brown.
2. A pungent smell is released (sulphur oxides).
3. The crystals first lose their water of crystallisation, then decompose.

Reactions — Step by step:

Step 1 — Loss of water of crystallisation:

$$FeSO_4 \cdot 7H_2O(s) \xrightarrow{\Delta} FeSO_4(s) + 7H_2O(g)$$

Step 2 — Decomposition:

$$2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$$

Where — $Fe_2O_3$ (ferric oxide, brown), $SO_2$ (pungent smell), $SO_3$ (pungent smell).

Type of reaction:

• Decomposition reaction (one reactant → three products)
• Endothermic (heat is absorbed)

Example 3: NCERT — Why is the Volume of Gas Doubled in Activity 1.7?

In Activity 1.7, why is the volume of gas collected in one test tube double that in the other? Name that gas.

Solution:

Reaction:

$$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$

Analysis:

1. Mole ratio: 2 molecules of $H_2$ are formed and 1 molecule of $O_2$.
2. Volume ratio: By Avogadro's law, at the same temperature and pressure, volumes are proportional to molecular numbers — so $H_2 : O_2 = 2 : 1$.
3. The doubled gas: Hydrogen ($H_2$) — produced at the cathode (negative electrode).
4. Test: Bringing a burning matchstick close — it burns with a 'pop' sound.

Answer: The gas with double the volume is hydrogen, because the molar ratio of $H_2$ to $O_2$ in the reaction is 2:1.

[Board Important — asked every year]

Example 4: Colour Change of Silver Chloride

Why does silver chloride change from white to grey in sunlight? Write the balanced reaction.

Solution:

1. Reaction:

$$2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$$

2. Reason for the colour change:

• Original substance $AgCl$ is white.
• In the presence of light, $AgCl$ decomposes to form metallic silver ($Ag$), which is grey/black.
• $Cl_2$ gas is also evolved (and dissipates into the atmosphere).

2. Type of reaction:

• Photochemical decomposition
• Endothermic — light energy is absorbed.

2. Use: This principle is used in black-and-white photography. The film is coated with $AgBr$, which decomposes in proportion to the intensity of light, producing black metallic silver in the brighter regions.

[Board Important — 3-mark question]

Example 5: Endothermic Reaction Demo

A student mixed 2 g of $Ba(OH)_2$ and 1 g of $NH_4Cl$ in a test tube. Holding the bottom of the tube with the palm felt cold. What kind of reaction is this? Explain.

Solution:

1. Reaction:

$$Ba(OH)_2(s) + 2NH_4Cl(s) \rightarrow BaCl_2(s) + 2NH_3(g) + 2H_2O(l)$$

2. What kind of reaction?

• The cold sensation means — the test tube absorbed heat from its surroundings.
• So this is an endothermic reaction.

2. Type:

• Double Displacement — Ba and NH₄ swap places.
• Endothermic — heat is absorbed.

2. Special note: This reaction absorbs so much heat that water vapour from the surrounding air can condense on the outside of the test tube as droplets — direct, visible evidence of its endothermic nature.

Answer: This is an endothermic chemical reaction.

Example 6: NCERT — Types of Decomposition by Energy Source

Which kinds of energy — heat, light, or electricity — bring about the decomposition of the following? Give balanced equations. (i) $CaCO_3$ (ii) $H_2O$ (iii) $AgCl$

Solution:

(i) Calcium carbonate — Thermal decomposition:

$$CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)$$

This is the key reaction in the cement industry.

(ii) Water — Electrolytic decomposition (Electrolysis):

$$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$

A few drops of $H_2SO_4$ are added to water and current is passed using a 6V battery.

(iii) Silver chloride — Photochemical decomposition:

$$2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$$

White $AgCl$ turns grey.

Summary of all three:

Energy	Reaction	Outcome
Heat	$CaCO_3 \rightarrow CaO + CO_2$	Quick lime forms
Electricity	$2H_2O \rightarrow 2H_2 + O_2$	Gases evolved
Light	$2AgCl \rightarrow 2Ag + Cl_2$	Colour change

[Board 5-mark question]

Example 7: A Curious Question — Sparkle of Firecrackers

What happens when potassium chlorate ($KClO_3$) is heated? How is it used in firecrackers?

Solution:

1. Reaction:

$$2KClO_3(s) \xrightarrow{\Delta, MnO_2} 2KCl(s) + 3O_2(g)$$

($MnO_2$ is a catalyst that speeds up the reaction.)

2. Type of reaction:

• Decomposition — one reactant → two products
• Endothermic — heat is absorbed

2. Use in firecrackers:

• $KClO_3$ releases oxygen on heating.
• This oxygen burns the metal powders (Al, Mg, etc.) mixed in the firecracker.
• That's why firecrackers burn with a brilliant flame and a loud sound.

2. A curious note: This is the simplest way to prepare oxygen gas in the laboratory.

Answer: Decomposition of $KClO_3$ — an endothermic decomposition reaction that releases oxygen.

Example 8: Numerical — Gas Production from Electrolysis

In the electrolysis of water, 36 g of water is fully decomposed. Find the masses of $H_2$ and $O_2$ produced. (H=1, O=16)

Solution:

1. Reaction:

$$2H_2O \rightarrow 2H_2 + O_2$$

2. Molecular masses:

• $2H_2O = 2 \times 18 = 36$ g
• $2H_2 = 2 \times 2 = 4$ g
• $O_2 = 32$ g

2. Verifying mass conservation: 36 = 4 + 32 ✓
3. From the given water: Since 36 g $H_2O$ is exactly given, the ratio applies directly —

• Mass of $H_2$ = 4 g
• Mass of $O_2$ = 32 g

2. Verification: $H_2 : O_2$ ratio is $\frac{4}{32} = \frac{1}{8}$ (by mass), but $2:1$ by volume (because Avogadro's law works on molecular numbers).

Lesson: Mass ratio and volume ratio are different — the volume ratio depends on the number of molecules.

Example 9: NCERT — Difference Between Combination and Decomposition

Why is decomposition called the reverse of combination? Write equations for each.

Solution:

General forms of both:

Reaction	General form	Nature
Combination	$A + B \rightarrow AB$	Joining
Decomposition	$AB \rightarrow A + B$	Breaking

Why opposite?

• In combination, two/more combine to form one.
• In decomposition, one breaks into two/more.
• Combinations are mostly exothermic (heat released); decompositions are mostly endothermic (heat absorbed).

Paired examples:

(1) Calcium hydroxide ↔ Calcium oxide:

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

Decomposition (on heating): $Ca(OH)_2 \xrightarrow{\Delta} CaO + H_2O$

(2) Mercuric oxide ↔ Mercury:

Combination: $2Hg + O_2 \rightarrow 2HgO$

Decomposition: $2HgO \xrightarrow{\Delta} 2Hg + O_2$

Conclusion: Combination and decomposition are opposite reactions to each other.

[Board 3-mark question]

Example 10: NCERT — Three Types of Decomposition by Energy Source

Write one equation each for decomposition reactions in which energy is supplied as heat, light, and electricity.

Solution:

(i) By heat (Thermal decomposition):

$$2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g)$$

Lead nitrate releases brown $NO_2$ smoke on heating.

(ii) By electricity (Electrolytic decomposition):

$$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$

The electrolysis of water is a famous classroom experiment.

(iii) By light (Photochemical decomposition):

$$2AgBr(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Br_2(g)$$

This is the principle behind black-and-white photography.

[Board 3-5 mark question]

Example 11: Real-Life — The Cement Industry

What is the key reaction of the cement industry? What kind of reaction is it?

Solution:

Reaction (decomposition of limestone):

$$CaCO_3(s) \xrightarrow{\Delta \text{ (1000°C)}} CaO(s) + CO_2(g)$$

Classification:

1. Decomposition — one reactant → two products ✓
2. Thermal decomposition — driven by heat ✓
3. Endothermic — heat is absorbed ✓

Importance in the cement industry:

• $CaO$ (quick lime) = the main ingredient of cement.
• Limestone is heated in kilns at 1000–1400°C.
• Mixed with clay, silica, and alumina to make the final cement.

Daily-Life Importance:

• Construction of buildings.
• Roads and bridges.
• The Indian cement industry is the world's second-largest.

Environmental concern: The release of $CO_2$ adds to atmospheric pollution and the greenhouse effect.

Example 12: Comparing Exothermic and Endothermic

Differentiate between exothermic and endothermic reactions. Give one example of each.

Solution:

Comparison Table:

Aspect	Exothermic	Endothermic
Heat flow	Released	Absorbed
Surrounding temp.	Rises	Falls
Equation form	Reactants → Products + Heat	Reactants + Energy → Products
Bond energy	Bond-forming energy > Bond-breaking energy	Bond-breaking > Bond-forming
$\Delta H$ (Class 11)	Negative ($< 0$)	Positive ($> 0$)
Connection	Most combinations	Most decompositions

Examples:

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

Endothermic: $\quad CaCO_3(s) + \text{Heat} \rightarrow CaO(s) + CO_2(g)$

Notice — both involve $CaO$, but in one $CaO$ is a reactant (and reacts), while in the other $CaO$ is a product (formed by breaking $CaCO_3$).

[Board 3-5 mark question]

Example 13: A Trick Question — Identification of Specific Gases

For each of the following, name the gas evolved and write the test for it — (a) Heating $CaCO_3$ (b) Heating $Pb(NO_3)_2$ (c) Electrolysis of water (at the cathode) (d) Sunlight on $AgCl$

Solution:

Case	Reaction	Gas	Test
(a)	$CaCO_3 \rightarrow CaO + CO_2$	$CO_2$	Turns lime water milky
(b)	$2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$	$NO_2$ (brown), $O_2$	$NO_2$ — brown colour; $O_2$ — relights a glowing splinter
(c)	$2H_2O \rightarrow 2H_2 + O_2$	$H_2$ (at cathode)	Burns with a 'pop' sound near a flame
(d)	$2AgCl \rightarrow 2Ag + Cl_2$	$Cl_2$	Pungent smell, greenish-yellow colour

Lesson: Exam questions often ask — "Which gas is evolved in _ reaction and how is it identified?" — memorise all four answers.

Example 14: NCERT — Decomposition of Hydrogen Peroxide

Write the balanced equation for the decomposition of hydrogen peroxide. What kind of reaction is it? Give one daily-life use.

Solution:

1. Reaction:

$$2H_2O_2(l) \xrightarrow{\text{catalyst}} 2H_2O(l) + O_2(g)$$

2. Catalyst: $MnO_2$ or $KI$ speeds up this reaction.
3. Classification:

• Decomposition — one reactant → two products
• Exothermic — interestingly, this decomposition is exothermic (unlike most decompositions).

2. Daily-life uses:

• Antiseptic: Cleaning wounds — kills germs because the released oxygen kills bacteria.
• Hair bleaching: Used in hair dyes.
• Rocket fuel: High-purity $H_2O_2$ as an oxidiser.

2. Curious fact: The reaction is slow without a catalyst. In our body, the enzyme catalase speeds it up — that's why bubbles appear when $H_2O_2$ is poured on a wound.

Example 15: NCERT — Decomposition of Sodium Hydrogen Carbonate

What happens when sodium hydrogen carbonate is heated? Write the balanced equation. Give a daily-life use.

Solution:

1. Reaction:

$$2NaHCO_3(s) \xrightarrow{\Delta} Na_2CO_3(s) + H_2O(g) + CO_2(g)$$

2. Classification:

• Decomposition reaction — one reactant → three products
• Thermal decomposition — driven by heat
• Endothermic — heat is absorbed

2. Daily-Life Use — Baking Soda:

• $NaHCO_3$ = baking soda
• Used in cakes and biscuits — heating releases $CO_2$, which causes the dough to rise.
• That is why cakes become spongy and soft.

2. Curious fact: Baking powder contains $NaHCO_3$ + a mild acid (like tartaric acid) that releases $CO_2$ on contact with water.
3. Use in fire extinguishers: The $CO_2$ helps put out fires.

[Board + Daily-Life question]

Example 16: A Quick Classification Summary

Classify the following reactions on the basis of (a) reaction type and (b) energy type — (i) $2HgO \xrightarrow{\Delta} 2Hg + O_2$ (ii) $C + O_2 \rightarrow CO_2 + \text{Heat}$ (iii) $2NaCl \xrightarrow{\text{electricity}} 2Na + Cl_2$ (iv) $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$ (respiration)

Solution:

#	Reaction Type	Energy Type
(i)	Decomposition (thermal)	Endothermic
(ii)	Combination (combustion)	Exothermic
(iii)	Decomposition (electrolytic)	Endothermic
(iv)	— (large; many bonds break and form; technically combustion/oxidation)	Exothermic

Detailed explanation:

• (i) $HgO$ is 'mercuric oxide' — heating breaks the red powder into metallic mercury (Hg) and oxygen.
• (ii) Burning of coal — two reactants → one product, hence combination.
• (iii) Molten NaCl decomposes on passing electricity into sodium metal and chlorine gas.
• (iv) Respiration — although multiple products form (so not combination), it is oxidation and exothermic.

Lesson: A single reaction can belong to multiple categories — reaction type (combination/decomposition/…) and energy type (exothermic/endothermic) — these are independent classifications.