Year 8 Chemistry: Thermal Decomposition

What is Thermal Decomposition?

Thermal decomposition is a chemical reaction that occurs when a compound breaks down into simpler substances when it is heated. Think of it like cooking food, where heat changes the food into something else.

How Does It Work?

When you heat a substance, the heat energy causes the bonds holding the atoms together to break. This leads to the formation of one or more new products.

Key Examples

1. Calcium Carbonate:
   • When calcium carbonate (found in limestone) is heated, it breaks down into calcium oxide (lime) and carbon dioxide gas.
   • Equation: $$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$
2. Hydrated Copper(II) Sulfate:
   • When heated, hydrated copper(II) sulfate (blue crystals) loses water and turns into anhydrous copper(II) sulfate (white powder).
   • Equation: $$\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \rightarrow \text{CuSO}_4 + 5\text{H}_2\text{O}$$

Key Rules

1. Heat is Required: Thermal decomposition only occurs when the substance is heated.
2. One Reactant: Only one compound is usually broken down.
3. Multiple Products: The reaction can produce one or more different substances.

Tips and Tricks

• Visualise It: Think of thermal decomposition like a puzzle being taken apart. The pieces (atoms) come apart when heated.
• Memorise Common Reactions: Familiarise yourself with common examples like calcium carbonate and hydrated copper(II) sulfate.
• Practice Writing Equations: Get comfortable with writing out the chemical equations for these reactions.

Questions on Thermal Decomposition

Easy Level Questions

1. What is thermal decomposition?
2. Is heat needed for thermal decomposition to occur?
3. What happens to a compound during thermal decomposition?
4. Give an example of a substance that undergoes thermal decomposition.
5. What is produced when calcium carbonate is heated?
6. What is the formula for calcium carbonate?
7. What gas is released when calcium carbonate decomposes?
8. What colour are hydrated copper(II) sulfate crystals?
9. What happens to hydrated copper(II) sulfate when heated?
10. Which compound is formed when calcium carbonate decomposes?
11. What does “CaO” stand for?
12. How many water molecules are lost when hydrated copper(II) sulfate decomposes?
13. What is the state of anhydrous copper(II) sulfate?
14. Can thermal decomposition produce gases?
15. What type of reaction is thermal decomposition?
16. Is thermal decomposition a physical or chemical change?
17. What happens to the temperature during thermal decomposition?
18. What is the term for the new substances formed in a reaction?
19. Can all compounds undergo thermal decomposition?
20. What is the chemical equation for the decomposition of calcium carbonate?

Medium Level Questions

1. Write the equation for the thermal decomposition of calcium carbonate.
2. What are the products of heating hydrated copper(II) sulfate?
3. Describe the difference between a reactant and a product.
4. Why is thermal decomposition important in industry?
5. What happens to the bonds in a compound during thermal decomposition?
6. Explain why some substances do not decompose when heated.
7. How does thermal decomposition relate to baking?
8. Can thermal decomposition occur at room temperature?
9. What role does heat play in thermal decomposition?
10. Give two examples of thermal decomposition reactions.
11. How can thermal decomposition be used in the lab?
12. What safety precautions should be taken when performing thermal decomposition?
13. Why might a scientist be interested in the products of thermal decomposition?
14. What is the significance of carbon dioxide in the decomposition of calcium carbonate?
15. How does thermal decomposition differ from combustion?
16. Can thermal decomposition be reversed?
17. How does the size of the particles affect thermal decomposition?
18. What is the formula for hydrated copper(II) sulfate?
19. Why does heating hydrated copper(II) sulfate change its colour?
20. What type of energy is involved in the thermal decomposition process?

Hard Level Questions

1. Write the balanced equation for the thermal decomposition of ammonium dichromate.
2. Explain the difference between endothermic and exothermic reactions in the context of thermal decomposition.
3. How can thermal decomposition be used to identify a substance?
4. Discuss the environmental implications of thermal decomposition reactions.
5. What are some real-world applications of thermal decomposition?
6. Why do some materials require high temperatures for thermal decomposition?
7. Describe an experiment to demonstrate thermal decomposition.
8. What factors can affect the rate of thermal decomposition?
9. Explain how thermal decomposition is involved in the process of making lime from limestone.
10. How does the presence of impurities affect thermal decomposition?
11. What is meant by the term “thermal stability”?
12. Compare the thermal decomposition of carbonates and nitrates.
13. In what way can thermal decomposition be observed in fireworks?
14. Explain why thermal decomposition can be considered irreversible in some cases.
15. How does particle size affect the rate of thermal decomposition?
16. Discuss the role of catalysts in thermal decomposition reactions.
17. Why is it important to control the heat applied during thermal decomposition?
18. How does thermal decomposition relate to the carbon cycle?
19. Can you name a compound that undergoes thermal decomposition and produces nitrogen gas?
20. What is the significance of the products formed from the thermal decomposition of ammonium nitrate?

Answers and Explanations

Easy Level Answers

1. It’s a reaction where a compound breaks down when heated.
2. Yes, heat is needed.
3. It breaks down into simpler substances.
4. Calcium carbonate or hydrated copper(II) sulfate.
5. Calcium oxide and carbon dioxide.
6. CaCO₃.
7. Carbon dioxide gas.
8. Blue.
9. It loses water and turns white.
10. Calcium oxide.
11. Calcium oxide.
12. Five water molecules.
13. White.
14. Yes, it can produce gases.
15. It’s a chemical change.
16. Chemical change.
17. It increases.
18. Products.
19. No, not all compounds can.
20. $$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$

Medium Level Answers

1. $$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$
2. Anhydrous copper(II) sulfate and water.
3. Reactants are what you start with; products are what you end with.
4. It’s used to produce materials like lime.
5. The bonds break.
6. Some are very stable and need lots of energy.
7. Baking produces gases that help the dough rise.
8. No, it needs heat.
9. It breaks the bonds in the compound.
10. Calcium carbonate and hydrated copper(II) sulfate.
11. Use a Bunsen burner for heating.
12. Wear goggles and gloves; heat with care.
13. To understand how materials break down.
14. It contributes to global warming.
15. Combustion reacts with oxygen; decomposition does not.
16. Some can, but not all.
17. Smaller particles decompose faster.
18. $$\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$$
19. It loses water, changing from blue to white.
20. Heat increases the rate of reaction.

Hard Level Answers

1. $$\text{2(NH}_4\text{)Cr}_2\text{O}_7 \rightarrow \text{Cr}_2\text{O}_3 + \text{N}_2 + \text{H}_2\text{O}$$
2. Endothermic absorbs heat; exothermic releases heat.
3. By observing the products formed.
4. Some reactions can release gases harmful to the environment.
5. Used in making lime and other chemicals.
6. They may need more energy to break bonds.
7. Heat a sample of limestone and collect the gas.
8. Temperature and pressure can affect the rate.
9. It’s a method to produce quicklime.
10. Impurities can lower the temperature needed for decomposition.
11. Thermal stability refers to a compound’s resistance to decomposition.
12. Nitrates usually decompose to give off oxygen and nitrogen.
13. They contain compounds that decompose rapidly when heated.
14. Some products can be more stable than the reactants.
15. Smaller sizes often mean faster reactions.
16. Catalysts speed up the reaction without being used up.
17. Too much heat could cause unwanted reactions.
18. Carbon can be released back into the atmosphere.
19. Ammonium nitrate; produces nitrogen gas when decomposed.
20. It helps to understand the chemical behaviour of that compound.

This guide is designed to help you understand thermal decomposition and test your knowledge in a structured way. Good luck with your studies!