Question

Select combination reaction from the reactions listed above. Is this reac tion endothermic or exothermic?

Answer 1

The combination reaction from the given list of chemical equations is:

1. \( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \)

In this reaction, calcium oxide (commonly known as quick lime) reacts with water to form calcium hydroxide (slaked lime). It is called a combination reaction because two reactants — calcium oxide and water — combine to form a single product, calcium hydroxide. Such reactions typically involve the union of two or more substances to form one compound.

This reaction is also a classic example of an exothermic reaction. When calcium oxide comes in contact with water, a considerable amount of heat is released. This heat is sufficient to make the reaction mixture hot and even produce steam if water is added in large amounts. The release of heat indicates that energy is being given out to the surroundings.

Practical application: This exothermic combination reaction is commonly used in whitewashing walls. The heat produced helps the slaked lime to set and stick better on the surface.

Therefore, the reaction \( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \) is both a combination reaction and an exothermic reaction.

Answer 2

The reaction that represents a displacement reaction from the given list is:

3. \( \text{CuSO}_4 + \text{Fe} \rightarrow \text{FeSO}_4 + \text{Cu} \)

This is a displacement reaction because a more reactive metal, iron (Fe), displaces a less reactive metal, copper (Cu), from its compound copper sulfate (CuSO₄). In displacement reactions, one element replaces another element from a compound, and this typically occurs when the free element is more reactive than the element it replaces.

Colour change observed:
In this reaction, the blue colour of copper sulfate solution gradually fades as iron starts reacting with it. This happens because copper sulfate (CuSO₄) is blue in colour, but iron sulfate (FeSO₄), which is formed in the reaction, is pale green in colour. Also, reddish-brown deposits of copper metal are seen forming on the surface of the iron.

So, the observable changes are:
- The solution colour changes from blue to green
- Reddish-brown copper metal gets deposited

Therefore, the reaction is a displacement reaction with a blue to green colour change and the formation of copper deposits.

Answer 3

The reaction from the list that involves thermal decomposition is:

2. \( \text{Pb(NO}_3)_2 \rightarrow \text{PbO} + \text{NO}_2 + \text{O}_2 \)

This is a thermal decomposition reaction because lead nitrate (\( \text{Pb(NO}_3)_2 \)) breaks down into simpler substances — lead oxide (PbO), nitrogen dioxide (NO₂), and oxygen (O₂) — when heated. In thermal decomposition reactions, a single compound breaks down into two or more simpler substances when heat is applied.

Another common example of thermal decomposition is that of ferrous sulphate (FeSO₄). When ferrous sulphate is heated, it decomposes to form ferric oxide (Fe₂O₃), sulfur dioxide (SO₂), and sulfur trioxide (SO₃). This reaction is accompanied by a change in colour and the release of gases.

Balanced chemical equation:
\[ 2\text{FeSO}_4 \xrightarrow{\text{heat}} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3 \]

Observation: Upon heating, green crystals of ferrous sulphate turn brown and release gases with a strong smell, indicating the formation of SO₂ and SO₃ gases.

Thus, thermal decomposition is a process in which a compound breaks down due to heat, and both lead nitrate and ferrous sulphate undergo such reactions when heated.

Answer 4

The double displacement reaction from the list is:

4. \( \text{Na}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 + \text{NaCl} \)

Word equation:
Sodium sulfate + Barium chloride → Barium sulfate + Sodium chloride

This reaction is a double displacement reaction because the cations (positive ions) and anions (negative ions) of the two reactants exchange partners to form new compounds. In this case, sodium (Na⁺) swaps places with barium (Ba²⁺), leading to the formation of barium sulfate (a white insoluble precipitate) and sodium chloride (a soluble salt).

Is this a redox reaction?
No, this is not a redox reaction. A redox (reduction-oxidation) reaction involves a change in the oxidation states of the elements, where one substance gets oxidized (loses electrons) and another gets reduced (gains electrons).

In this reaction, the oxidation states of all the elements remain the same before and after the reaction. There is no transfer of electrons, only an exchange of ions. Therefore, it is purely an example of a double displacement reaction and a precipitation reaction, but not a redox reaction.