This MCQ module is based on: Combination and Decomposition Reactions
TOPIC 2 OF 50
Combination and Decomposition Reactions
🎓 Class 10
Science
CBSE
Theory
Ch 1 — Chemical Reactions and Equations
⏱ ~22 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_10" science_domain="chemistry" difficulty="intermediate"]
1.2 Types of Chemical Reactions
In Part 1, we learnt how to write and balance chemical equations. Now, let us classify chemical reactions into different types based on how the reactants and products are related to each other. The main types we will study are: combination, decomposition, displacement, double displacement, and oxidation-reduction (redox) reactions.
1.2.1 Combination Reactions
A combination reaction is one in which two or more substances (either elements or compounds) combine to form a single product. Think of it as "many things becoming one".
Activity 1.4 — Quicklime and Water
L3 Apply
Predict first: If you add water to quicklime (calcium oxide), do you think the beaker will feel warm, cold, or stay at the same temperature?
- Take a small amount of calcium oxide (quicklime) in a beaker.
- Slowly add water to it.
- Touch the beaker carefully from the outside.
- Observe the change in temperature.
Observations:
A large amount of heat is released when water is added to quicklime. The beaker becomes very hot to touch! The calcium oxide reacts vigorously with water to form slaked lime [calcium hydroxide, Ca(OH)2].
Chemical equation:
Everyday use: This reaction is used by painters and whitewashers. Quicklime mixed with water forms slaked lime, which is applied on walls. It then slowly reacts with CO2 from the air over several days to form CaCO3, giving the wall a shiny finish.
A large amount of heat is released when water is added to quicklime. The beaker becomes very hot to touch! The calcium oxide reacts vigorously with water to form slaked lime [calcium hydroxide, Ca(OH)2].
Chemical equation:
\(\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}\)
This is a combination reaction (two substances combine to form one) and also an exothermic reaction (releases heat).
Everyday use: This reaction is used by painters and whitewashers. Quicklime mixed with water forms slaked lime, which is applied on walls. It then slowly reacts with CO2 from the air over several days to form CaCO3, giving the wall a shiny finish.
More Examples of Combination Reactions
Burning of coal:
\(\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)\)
Formation of water from hydrogen and oxygen:
\(2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)\)
Formation of slaked lime (as seen in Activity 1.4):
\(\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq)\)
Exothermic and Endothermic Reactions
Reactions that release heat energy along with formation of products are called exothermic reactions. The reaction of quicklime with water, burning of fuels, and respiration are all exothermic.
Reactions that absorb heat energy from the surroundings are called endothermic reactions. Decomposition reactions that require heat are endothermic.
Quick Check:
Exothermic: Releases heat → surroundings get warmer (e.g., burning, respiration, CaO + H2O)
Endothermic: Absorbs heat → surroundings get cooler (e.g., photosynthesis, decomposition of CaCO3)
Exothermic: Releases heat → surroundings get warmer (e.g., burning, respiration, CaO + H2O)
Endothermic: Absorbs heat → surroundings get cooler (e.g., photosynthesis, decomposition of CaCO3)
1.2.2 Decomposition Reactions
A decomposition reaction is the exact opposite of a combination reaction. Here, a single substance breaks down (decomposes) into two or more simpler substances. Energy in some form (heat, light, or electricity) is usually needed to break down the compound.
Activity 1.5 — Heating Ferrous Sulphate Crystals
L3 Apply
Predict first: Ferrous sulphate crystals are green in colour. What do you think will happen to their colour when heated strongly?
- Take about 2 g of ferrous sulphate crystals in a dry boiling tube.
- Note the colour of the crystals (green).
- Heat the boiling tube over the flame of a burner.
- Observe the colour change and smell the gas carefully (waft, do not inhale directly).
Observations:
The green colour of ferrous sulphate crystals changes to reddish-brown (ferric oxide, Fe2O3). Gases with a characteristic suffocating smell (SO2 and SO3) are given off.
Chemical equation:
The green colour of ferrous sulphate crystals changes to reddish-brown (ferric oxide, Fe2O3). Gases with a characteristic suffocating smell (SO2 and SO3) are given off.
Chemical equation:
\(2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)\)
The green ferrous sulphate crystals have decomposed upon heating to produce reddish-brown solid (ferric oxide) and two gaseous products.
Activity 1.6 — Heating Lead Nitrate
L3 Apply
Predict first: Lead nitrate is a white crystalline solid. When heated, do you expect it to decompose? What colour might the gas be?
- Take about 2 g of lead nitrate powder in a boiling tube.
- Hold the boiling tube with a holder and heat it over a flame.
- Observe what happens. Note the colour of the fumes.
- Caution: Perform this in a well-ventilated area or fume hood. The fumes of NO2 are toxic.
Observations:
When heated, lead nitrate decomposes to emit brown fumes of nitrogen dioxide (NO2) gas. A yellow residue of lead oxide (PbO) is left behind and oxygen gas is also released.
Chemical equation:
When heated, lead nitrate decomposes to emit brown fumes of nitrogen dioxide (NO2) gas. A yellow residue of lead oxide (PbO) is left behind and oxygen gas is also released.
Chemical equation:
\(2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)\)
Thermal Decomposition
When a decomposition reaction is carried out by heating, it is called thermal decomposition. Another important example is the decomposition of limestone (calcium carbonate):
\(\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)\)
Limestone decomposes upon heating to form quicklime and carbon dioxide. This reaction is the basis of the cement industry.
Activity 1.7 — Electrolysis of Water
L3 Apply
Predict first: Water is made of hydrogen and oxygen. Can we break it back into these gases? What energy source might be needed?
- Take a plastic mug and make two holes in its base. Fix two rubber-covered carbon (graphite) electrodes through these holes.
- Fill the mug with water and add a few drops of dilute sulphuric acid to make it conducting.
- Invert the mug in a dish of water. Connect the electrodes to a 6-volt battery.
- Collect the gases formed at each electrode separately by inverting test tubes filled with water over them.
- Test the gas at each electrode. Bring a burning splint near each test tube.
Observations:
Bubbles of gas form at both electrodes. The gas collected at the cathode (negative electrode) is hydrogen -- it burns with a pop sound. The gas at the anode (positive electrode) is oxygen -- it relights a glowing splint. The volume of hydrogen collected is roughly double that of oxygen.
Chemical equation:
Bubbles of gas form at both electrodes. The gas collected at the cathode (negative electrode) is hydrogen -- it burns with a pop sound. The gas at the anode (positive electrode) is oxygen -- it relights a glowing splint. The volume of hydrogen collected is roughly double that of oxygen.
Chemical equation:
\(2\text{H}_2\text{O}(l) \xrightarrow{\text{Electricity}} 2\text{H}_2(g) + \text{O}_2(g)\)
This is an electrolytic decomposition -- water is decomposed using electrical energy.
Activity 1.8 — Decomposition by Sunlight
L2 Understand
Predict first: Silver chloride is a white solid. What do you think will happen if you leave it exposed to sunlight for some time?
- Take a small amount of silver chloride (AgCl) in a watch glass.
- Place it in direct sunlight for some time.
- Observe the change in colour.
Observations:
White silver chloride turns grey when placed in sunlight. This is because it decomposes into silver (grey metal) and chlorine gas.
Chemical equations:
White silver chloride turns grey when placed in sunlight. This is because it decomposes into silver (grey metal) and chlorine gas.
Chemical equations:
\(2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)\)
\(2\text{AgBr}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Br}_2(g)\)
This is a photolytic decomposition -- decomposition caused by light. This reaction is the basis of black-and-white photography. Silver bromide (AgBr) in photographic films decomposes when exposed to light, forming silver metal which creates the dark areas of the image.
Types of Decomposition Reactions
Board Exam Tip: Remember the three types of decomposition with their energy sources: Thermal (heat, symbol △), Electrolytic (electricity), and Photolytic (light/sunlight). Questions frequently ask you to identify the type and give examples.
Competency-Based Questions
In a chemistry lab, three students performed different experiments. Student A heated calcium carbonate in a test tube. Student B passed electric current through water mixed with a few drops of acid. Student C placed silver bromide in sunlight.
Q1. L1 Remember What type of decomposition reaction did Student A perform?
Answer: C. Thermal decomposition. When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide. Since heat is used to bring about the decomposition, it is called thermal decomposition.
Q2. L2 Understand Write the balanced equation for the reaction performed by Student B. What are the two gases produced? (2 marks)
Answer: \(2\text{H}_2\text{O}(l) \xrightarrow{\text{Electricity}} 2\text{H}_2(g) + \text{O}_2(g)\)
The two gases produced are hydrogen (at the cathode) and oxygen (at the anode). Hydrogen is collected in double the volume of oxygen.
The two gases produced are hydrogen (at the cathode) and oxygen (at the anode). Hydrogen is collected in double the volume of oxygen.
Q3. L4 Analyse Student C noticed that the white silver bromide turned grey. Explain why this happens and name the type of reaction. (2 marks)
Answer: Silver bromide (AgBr) undergoes photolytic decomposition in the presence of sunlight. It breaks down into silver metal (grey colour) and bromine gas: \(2\text{AgBr}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Br}_2(g)\). The grey colour is due to the metallic silver deposited.
Q4. L3 Apply Quicklime (CaO) reacts vigorously with water, releasing a large amount of heat. Write the balanced equation and state whether this is exothermic or endothermic. Why? (3 marks)
Answer: \(\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}\)
This is an exothermic reaction because it releases heat to the surroundings. The beaker becomes very hot during this reaction. Any reaction that produces heat energy is exothermic.
This is an exothermic reaction because it releases heat to the surroundings. The beaker becomes very hot during this reaction. Any reaction that produces heat energy is exothermic.
Q5. L5 Evaluate A student claims: "All decomposition reactions are endothermic." Do you agree? Justify with examples. (3 marks)
Answer: The statement is generally true but not universally. Most decomposition reactions require energy input (heat, light, or electricity) and are therefore endothermic. For example, the thermal decomposition of CaCO3 requires continuous heating, and electrolysis of water requires electrical energy. However, the decomposition must be distinguished from the overall energy balance in some cases. For board exams, it is acceptable to state that decomposition reactions require energy, making them endothermic processes.
Assertion-Reason Questions
Assertion (A): The decomposition of silver chloride by sunlight is used in black-and-white photography.
Reason (R): Silver chloride decomposes in sunlight to form silver metal and chlorine gas.
Answer: A. Both are true. Silver chloride (and silver bromide) decomposes in light to form silver metal, and this photolytic decomposition is the chemical basis of black-and-white photography. The Reason correctly explains why this reaction is useful in photography.
Assertion (A): A combination reaction always produces only one product.
Reason (R): In a combination reaction, two or more reactants combine together.
Answer: B. Both are true. A combination reaction does produce a single product (A), and it does involve two or more reactants combining (R). However, the Reason simply describes what happens to the reactants; it does not explain why only one product is formed. The reason for a single product lies in the chemical nature of the bond formation, not merely in the combining of reactants.
Assertion (A): The reaction CaO + H2O → Ca(OH)2 is an endothermic reaction.
Reason (R): This reaction releases a large amount of heat.
Answer: D. The Assertion is false -- this reaction is exothermic, not endothermic, because it releases heat. The Reason is true -- a large amount of heat is indeed released when quicklime reacts with water.
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