IBDP Chemistry - Reactivity 2.1 How much? The amount of chemical change- IB Style Questions For SL Paper 1A - FA 2025
▶️ Answer/Explanation
1. Analyze Reaction Type:
The temperature of the mixture decreases. This means the reaction absorbed heat energy (kinetic energy of particles converted to potential energy of bonds). The reaction is endothermic. The system has increased its internal chemical potential energy.
2. Analyze Equilibration:
Since the mixture is now colder than the surroundings, heat will flow from the surroundings into the system until thermal equilibrium is re-established.
3. Conclusion:
- System: Absorbed heat from the reaction and then absorbed heat from surroundings. Net Energy increases.
- Surroundings: Lost heat to the system. Net Energy decreases.
✅ Answer: (D)
▶️ Answer/Explanation
A systematic error is one that consistently shifts results in the same direction, usually because of a flaw in the experimental setup or technique.
- A. A slightly ajar lid affects how fast gases escape but does not necessarily introduce a consistent bias in the measured mass of magnesium oxide.
- B. A white powdery product is expected for magnesium oxide and does not indicate an error.
- C. Black soot on the crucible indicates incomplete combustion of the fuel, causing contamination and a consistently incorrect heat distribution. This can lead to incomplete reaction of magnesium or extra mass from soot, giving a systematic error in the calculated empirical formula.
- D. A yellow flame colour is an observation but by itself does not directly explain a consistent bias in the mass measurements.
Therefore, the observation most clearly linked to a systematic error in this experiment is the presence of black soot on the crucible.
✅ Answer: (C)
▶️ Answer/Explanation
First find the molar mass of ethane, \(\text{C}_2\text{H}_6\): \[ M(\text{C}_2\text{H}_6) = 2(12.0) + 6(1.0) = 24.0 + 6.0 = 30.0\ \text{g mol}^{-1} \]
One mole contains Avogadro’s number of molecules, \[ N_A \approx 6.02 \times 10^{23}\ \text{molecules mol}^{-1}. \] So the mass of one molecule is \[ m = \frac{30.0\ \text{g mol}^{-1}}{6.02 \times 10^{23}\ \text{molecules mol}^{-1}} \approx 4.98 \times 10^{-23}\ \text{g} \approx 5.0 \times 10^{-23}\ \text{g}. \]
✅ Answer: (B)