1.2.2. Decomposition Reaction

n Take about 2 g ferrous sulphate crystals in a dry boiling tube. n Note the colour of the ferrous sulphate crystals. n Heat the boiling tube over the flame of a burner or spirit lamp as shown in Fig. 1.4. n Observe the colour of the crystals after heating.

In this activity, you start by taking ferrous sulphate crystals, which usually have a green color. When these crystals are heated, they undergo a chemical change. This is the first indication of a decomposition reaction, where a single compound breaks down into simpler substances upon heating.

Think of this like baking bread. When you mix dough and heat it in an oven, the ingredients (like flour and yeast) react and change into a new product (the bread). In our case, heating ferrous sulphate causes it to decompose into different substances.

Have you noticed that the green colour of the ferrous sulphate crystals has changed? You can also smell the characteristic odour of burning sulphur.

2FeSO4(s) ⟶ Heat ⟶ Fe2O3(s) + SO2(g) + SO3(g)

When ferrous sulphate is heated, it decomposes into ferric oxide, sulfur dioxide, and sulfur trioxide. This change not only involves a color transformation but also produces gases that can be smelled, showcasing how a single reactant can break down into multiple products during a decomposition reaction.

Imagine a snowman melting in the sun. Initially, there is one shape (the snowman), but as it melts, it transforms into water and possibly some frost, showing a breakdown into simpler forms. This is similar to how ferrous sulphate breaks down into ferric oxide and gases.

Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction used in various industries. CaCO3(s) ⟶ Heat ⟶ CaO(s) + CO2(g)

This example highlights thermal decomposition, specifically how calcium carbonate (commonly found in limestone) breaks down into calcium oxide (or quicklime) and carbon dioxide when heated. This reaction is important in construction, particularly in the manufacture of cement.

Think about how toasting marshmallows changes them. When exposed to heat, they transform from a soft, gooey state to a crispy exterior with bubbling sugar on the outside. Similar to how heat changes calcium carbonate into different products.

Using electricity can also cause decomposition reactions. For example, when water is electrolyzed, it decomposes into hydrogen and oxygen gases. 2H2O(l) ⟶ Electric Current ⟶ 2H2(g) + O2(g)

In this case, applying an electric current to water causes it to split into hydrogen and oxygen. This demonstrates that not only heat but also electricity can trigger decomposition reactions, leading to different products as the compounds break down.

Consider how a battery-powered device uses energy to function. For instance, when you push a toy car with batteries, energy is transformed, activating the car's circuits. Similarly, adding electricity to water prompts its breakdown into gases.

You will see that white silver chloride turns grey in sunlight due to the decomposition of silver chloride into silver and chlorine by light. 2AgCl(s) ⟶ Sunlight ⟶ 2Ag(s) + Cl2(g)

The decomposition of silver chloride in the presence of light illustrates how light energy can facilitate decomposition reactions. When it is exposed to sunlight, silver chloride breaks down into silver and chlorine gas, which is often employed in photography.

It’s similar to how certain colors fade in fabrics when left in the sun for too long. The light breaks down the dyes, just as it breaks down silver chloride into simpler components.

We have seen that the decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.

Decomposition reactions often need energy to break compound bonds. When they absorb energy from heat, light, or electricity, they are categorized as endothermic reactions. This means they take in energy rather than release it.

Think of a sponge soaking up water. While soaking, the sponge absorbs liquids, similar to how a reaction can absorb energy. This allows the reaction to occur, showcasing the energy dynamics in these processes.

Carry out the following Activity: Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?

In this practical activity, mixing barium hydroxide with ammonium chloride results in a temperature drop, indicating an endothermic reaction where the mixture absorbs heat from the surroundings. This allows students to see decomposition reactions in action and understand their thermal energy implications.

This is like making ice cream with salt and ice. The process absorbs heat from the surrounding mixture, creating a cooling effect while the ice melts. Similarly, here, heat absorption can suggest a change in energy states during decomposition.

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