iGCSE Chemistry 0620 Extended - 6.1 Physical and chemical changes- Exam Style Questions Paper 2- New Syllabus
▶️ Answer/Explanation
In this energy profile diagram: Label 1 marks the initial chemical energy level, which represents the reactants. Label 4 represents the final resting energy level, which belongs to the products. Label 3 measures the vertical height from the reactant line up to the peak of the curve, representing the activation energy ($E_a$). Label 2 is the difference in energy between reactants and products, which is the overall enthalpy change of the reaction ($\Delta H$). This combination corresponds directly to row A.
✅ Answer: (A)
✅ Answer: (A)
▶️ Answer/Explanation
Let’s calculate the total energy taken in to break reactant bonds and the energy released when making product bonds:
• Bonds broken (Reactants): Ethene ($\text{CH}_2=\text{CH}_2$) has $1 \times (\text{C}=\text{C})$ and $4 \times (\text{C}-\text{H})$. We also break $3 \times (\text{O}=\text{O})$.
$$\text{Energy In} = 612 + 4(412) + 3(496) = 612 + 1648 + 1488 = 3748\text{ kJ/mol}$$
• Bonds formed (Products): $2 \times \text{CO}_2$ has $4 \times (\text{C}=\text{O})$ bonds total. $2 \times \text{H}_2\text{O}$ has $4 \times (\text{O}-\text{H})$ bonds total.
$$\text{Energy Out} = 4(743) + 4(463) = 2972 + 1852 = 4824\text{ kJ/mol}$$
• Overall energy change:
$$\Delta H = \text{Energy In} – \text{Energy Out} = 3748 – 4824 = -1076\text{ kJ/mol}$$
✅ Answer: (B)
• Bonds broken (Reactants): Ethene ($\text{CH}_2=\text{CH}_2$) has $1 \times (\text{C}=\text{C})$ and $4 \times (\text{C}-\text{H})$. We also break $3 \times (\text{O}=\text{O})$.
$$\text{Energy In} = 612 + 4(412) + 3(496) = 612 + 1648 + 1488 = 3748\text{ kJ/mol}$$
• Bonds formed (Products): $2 \times \text{CO}_2$ has $4 \times (\text{C}=\text{O})$ bonds total. $2 \times \text{H}_2\text{O}$ has $4 \times (\text{O}-\text{H})$ bonds total.
$$\text{Energy Out} = 4(743) + 4(463) = 2972 + 1852 = 4824\text{ kJ/mol}$$
• Overall energy change:
$$\Delta H = \text{Energy In} – \text{Energy Out} = 3748 – 4824 = -1076\text{ kJ/mol}$$
✅ Answer: (B)
▶️ Answer/Explanation
A chemical change involves the formation of one or more new substances with different properties from the original materials, and is often difficult or impossible to reverse. Adding sodium to water produces sodium hydroxide and hydrogen gas — entirely new substances — which is clearly a chemical change. Boiling water is a physical change (water just changes state). Dissolving NaCl is a physical change (it can be recovered by evaporation). Option D is misleading; producing water from electrolysis of aqueous NaCl at the cathode is chemical, but option A is the most direct and straightforward example of a chemical change among those given.
✅ Answer: A
✅ Answer: A