Reactions — Concepts, Formulas & Examples

Chemical reactions in inorganic chemistry fall into a few main types. Recognising the type helps you predict products and balance equations. CBSE Class 10 covers this as the introduction to inorganic reactions.

Core Concepts

Combination

Two or more substances combine to form one. $A + B \to AB$ . Burning of magnesium, formation of water from hydrogen and oxygen. Usually exothermic.

2\text{Mg} + \text{O}_2 \to 2\text{MgO}

\text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2

\text{NH}_3 + \text{HCl} \to \text{NH}_4\text{Cl}

Quick lime (CaO) reacting with water is a common lab demonstration — the reaction is so exothermic that the beaker becomes too hot to touch. This same reaction is used in self-heating food cans.

Decomposition

One substance breaks into two or more. $AB \to A + B$ . Thermal, electrolytic or photochemical. Heating CaCO3 gives CaO and CO2; electrolysis of water gives H2 and O2.

Types of decomposition by energy source:

Type	Energy Source	Example
Thermal	Heat	$2\text{Pb(NO}_3\text{)}_2 \xrightarrow{\Delta} 2\text{PbO} + 4\text{NO}_2 + \text{O}_2$
Electrolytic	Electricity	$2\text{H}_2\text{O} \xrightarrow{\text{electrolysis}} 2\text{H}_2 + \text{O}_2$
Photochemical	Light	$2\text{AgCl} \xrightarrow{h\nu} 2\text{Ag} + \text{Cl}_2$

The photochemical decomposition of AgCl is why silver halides are used in photography — light breaks down AgCl into silver metal (which appears dark) and chlorine. This same reaction causes silver objects to tarnish.

Decomposition is the reverse of combination. If $A + B \to AB$ is combination, then $AB \to A + B$ is decomposition. Decomposition usually needs energy input (endothermic), while combination usually releases energy (exothermic).

Displacement

A more active element displaces a less active one from its compound. Iron displaces copper from CuSO4. Basis of the reactivity series.

\text{Fe} + \text{CuSO}_4 \to \text{FeSO}_4 + \text{Cu}

\text{Zn} + 2\text{HCl} \to \text{ZnCl}_2 + \text{H}_2 \uparrow

The key to predicting displacement: check the reactivity series. The more reactive element (higher in the series) displaces the less reactive one (lower in the series). This is why zinc displaces copper from CuSO4, but copper cannot displace zinc from ZnSO4.

Double displacement

Two compounds exchange ions to form two new compounds. $AB + CD \to AD + CB$ . Often involves precipitation (AgNO3 + NaCl → AgCl + NaNO3) or neutralisation.

\text{AgNO}_3 + \text{NaCl} \to \text{AgCl}\downarrow + \text{NaNO}_3

\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \to \text{BaSO}_4\downarrow + 2\text{NaCl}

\text{NaOH} + \text{HCl} \to \text{NaCl} + \text{H}_2\text{O}

Neutralisation is a special case of double displacement: acid + base → salt + water. The driving force is the formation of water (a stable, poorly ionised molecule).

Redox

Electrons transfer from one species to another. Oxidation is loss of electrons; reduction is gain. Always occur together. Many combination and displacement reactions are also redox.

OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)

LEO GER: Lose Electrons = Oxidation, Gain Electrons = Reduction

In terms of oxygen/hydrogen:

Oxidation: gain of oxygen OR loss of hydrogen
Reduction: loss of oxygen OR gain of hydrogen

Free elements: oxidation number = 0
Monatomic ions: oxidation number = charge
H is usually +1 (except in metal hydrides: -1)
O is usually -2 (except in peroxides: -1, OF2: +2)
Sum of oxidation numbers in a compound = 0
Sum of oxidation numbers in an ion = charge of the ion

Precipitation

A soluble reactant combines with another to form an insoluble product. Used in analytical chemistry and water treatment.

To predict whether a precipitate forms, you need to know solubility rules:

Most nitrates are soluble
Most chlorides are soluble (except AgCl, PbCl2)
Most sulphates are soluble (except BaSO4, PbSO4)
Most carbonates and hydroxides are insoluble (except Na, K, NH4+ salts)

Worked Examples

$2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$ is combination and redox. $\text{CaCO}_3 \to \text{CaO} + \text{CO}_2$ is decomposition. $\text{Fe} + \text{CuSO}_4 \to \text{FeSO}_4 + \text{Cu}$ is displacement and redox. Look for the pattern.

Combination is usually exothermic (releases heat). Decomposition is usually endothermic (absorbs heat). Not a hard rule but a useful starting point.

Balance: $\text{Fe}_2\text{O}_3 + \text{CO} \to \text{Fe} + \text{CO}_2$

Fe changes from +3 to 0 (reduction, gain of 3 electrons per Fe atom, 6 total for 2 Fe). C changes from +2 to +4 (oxidation, loss of 2 electrons per C atom).

To balance electrons: need 3 CO molecules (giving 6 electrons) for every Fe2O3.

\text{Fe}_2\text{O}_3 + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_2

Check: Fe balanced (2 each side), C balanced (3 each side), O balanced (3 + 3 = 6 on left, 6 on right).

In $\text{CuO} + \text{H}_2 \to \text{Cu} + \text{H}_2\text{O}$ :

Cu goes from +2 to 0 → reduced → CuO is the oxidising agent H goes from 0 to +1 → oxidised → H2 is the reducing agent

The substance that gets reduced is the oxidising agent (it causes oxidation of the other).

Mix NaOH and FeCl3. Exchange partners: NaCl and Fe(OH)3.

3\text{NaOH} + \text{FeCl}_3 \to 3\text{NaCl} + \text{Fe(OH)}_3\downarrow

Fe(OH)3 is insoluble (reddish-brown precipitate). NaCl is soluble. The precipitate is the driving force for this reaction.

Common Mistakes

Confusing displacement and double displacement. Displacement has an element on both sides; double displacement has two compounds.

Saying all reactions are reversible. Many go to completion.

Writing that decomposition is always thermal. It can also be electrolytic or photochemical.

Confusing the oxidising agent with the substance being oxidised. The oxidising agent itself gets reduced. It causes oxidation of others.

Forgetting to balance equations. CBSE awards zero marks for an unbalanced equation, even if the products are correct. Always check atom counts on both sides.

Exam Weightage and Revision

Chemical reactions is a high-weightage chapter in CBSE Class 10, carrying 5-8 marks. Questions include: classify reactions, balance equations, identify oxidising/reducing agents. NEET uses these concepts in thermodynamics and electrochemistry but does not test reaction classification directly.

Question Type	CBSE Frequency	Marks
Classify given reactions	Every year	2-3
Balance equations	Every year	2
Identify oxidising/reducing agent	Most years	2
Predict products	Most years	2-3
Give examples of each type	Every year	3

For CBSE Class 10, memorise two examples of each reaction type with balanced equations. The exam will ask you to classify reactions and write balanced equations — having examples ready saves time.

Practice Questions

Q1. Classify: $2\text{FeSO}_4 \xrightarrow{\Delta} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$

This is a decomposition reaction. One compound (FeSO4) breaks down into multiple products (Fe2O3, SO2, SO3) on heating. It is also a redox reaction: Fe goes from +2 to +3 (oxidised), S goes from +6 to +4 in SO2 (reduced).

Q2. Will a reaction occur if a copper strip is placed in ZnSO4 solution? Explain.

No. Copper is below zinc in the reactivity series. A less reactive metal cannot displace a more reactive one from its salt solution. For displacement to occur, the free metal must be more reactive than the metal in the salt. Zn can displace Cu, but Cu cannot displace Zn.

Q3. Identify the type of reaction: $\text{Na}_2\text{SO}_4 + \text{BaCl}_2 \to \text{BaSO}_4 + 2\text{NaCl}$

Double displacement (exchange of ions) and precipitation (BaSO4 is insoluble). The Na+ and Ba2+ ions exchange partners. The formation of the insoluble BaSO4 precipitate drives the reaction forward.

Q4. In the reaction $\text{MnO}_2 + 4\text{HCl} \to \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$ , identify the substance oxidised and the substance reduced.

Cl in HCl goes from -1 to 0 in Cl2 → oxidised. HCl is the reducing agent. Mn in MnO2 goes from +4 to +2 in MnCl2 → reduced. MnO2 is the oxidising agent. Note that only some HCl molecules are oxidised (to Cl2); others simply provide Cl- to form MnCl2.

FAQs

Can a reaction be both combination and redox?

Yes. $2\text{Mg} + \text{O}_2 \to 2\text{MgO}$ is combination (two substances forming one) and redox (Mg is oxidised, O is reduced). The categories are not mutually exclusive.

What drives a chemical reaction forward?

Several factors: formation of a precipitate (removes product from solution), formation of a gas (escapes), formation of water (very stable), release of energy (exothermic). If none of these driving forces exist, the reaction may not proceed or may be reversible.

How do I know which reactions are irreversible?

If the products are very stable (like water or CO2 gas), the reaction effectively goes to completion. If a precipitate forms and settles out, the reverse reaction is unlikely. Combustion reactions are essentially irreversible at normal conditions.

Memorise five reaction types with one example each. Covers most one-line questions.

Balancing Equations — A Systematic Approach

Many students lose marks not because they do not know the reaction but because they cannot balance the equation. Here is a reliable method:

Count metal atoms on both sides. Add coefficients to balance.

Count each non-metal. Add coefficients.

Count O atoms on both sides. Add coefficient to balance.

Count H atoms. Add coefficient. Check that everything else is still balanced.

Example: Balance $\text{Al} + \text{HCl} \to \text{AlCl}_3 + \text{H}_2$

Al: 1 on each side (balanced). Cl: 1 on left, 3 on right → put 3 before HCl. H: 3 on left (from 3HCl), 2 on right → need 3H on right, so 3/2 H2. Multiply everything by 2 to get integers:

2\text{Al} + 6\text{HCl} \to 2\text{AlCl}_3 + 3\text{H}_2

Corrosion and Rancidity as Chemical Reactions

Two everyday chemical reactions that students should recognise:

Corrosion: Metals react with oxygen and moisture. Iron → rust (Fe2O3·xH2O). Silver → tarnish (Ag2S). Copper → green patina (Cu2(OH)2CO3). These are all oxidation reactions.

Rancidity: Fats and oils in food react with oxygen (oxidation), developing an unpleasant smell and taste. Prevention: antioxidants (BHA, BHT), vacuum packing, nitrogen flushing, refrigeration, or airtight containers.

Endothermic and Exothermic — Quick Classification

Type	Example	Evidence
Exothermic	Burning fuel, neutralisation, respiration	Temperature rises, heat released
Endothermic	Photosynthesis, dissolving NH4Cl, decomposition of CaCO3	Temperature falls, heat absorbed

A quick test in the lab: if the container feels warm, the reaction is exothermic. If it feels cold, endothermic. For combustion reactions, the answer is always exothermic.

Reactions are the verbs of chemistry. Once you know the types, predicting what happens next becomes routine.