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The dissociation of iodine molecules to iodine atoms (I) is a reversible reaction taking place in a sealed container at 727 °C - NSC Physical Sciences - Question 6 - 2020 - Paper 2
Question 6
The dissociation of iodine molecules to iodine atoms (I) is a reversible reaction taking place in a sealed container at 727 °C. The balanced equation for the reactio... show full transcript
Worked Solution & Example Answer:The dissociation of iodine molecules to iodine atoms (I) is a reversible reaction taking place in a sealed container at 727 °C - NSC Physical Sciences - Question 6 - 2020 - Paper 2
Step 1
Step 2
At equilibrium the pressure of the system is increased by decreasing the volume of the container at constant temperature. How will EACH of the following be affected? Choose from INCREASES, DECREASES or REMAINS THE SAME.
Answer
6.2.1 The value of the equilibrium constant: REMAINS THE SAME The equilibrium constant (Kc) is only affected by temperature changes, not by changes in pressure or volume at constant temperature.
6.2.2 The number of I2 molecules: DECREASES According to Le Chatelier’s principle, decreasing the volume increases the pressure of the system, which favors the side of the equilibrium with fewer moles of gas. Since there are 2 moles of gas on the product side (2I) and 1 mole of gas on the reactant side (I2), the equilibrium will shift to the left, reducing the number of I2 molecules.
Step 3
Explain the answer to QUESTION 6.2.2 by referring to Le Chatelier's principle.
Answer
Le Chatelier's principle states that if a change is made to a system in equilibrium, the system will respond by shifting the equilibrium position to counteract that change. In this case, when the volume is decreased, the pressure increases, and the system shifts towards the side with fewer gas molecules (the reactants, I2) to reduce the pressure. This results in a decrease in the number of I2 molecules.
Step 4
Is the forward reaction ENDOTHERMIC or EXOTHERMIC? Fully explain the answer.
Answer
To determine whether the forward reaction is endothermic or exothermic, we look at the change in Kc with temperature. Given that the Kc value increases with temperature, it suggests that the forward reaction, which produces gas molecules from a solid or reactant, is endothermic. This is because an increase in temperature favors the endothermic direction of the reaction, resulting in a higher Kc value.
Step 5
A certain mass of iodine molecules (I2) is sealed in a 12.3 dm³ flask at a temperature of 727 °C (Kc = 3.76 × 10⁻³). When equilibrium is reached, the concentration of the iodine atoms is found to be 4.79 × 10⁻³ mol·dm⁻³. Calculate the INITIAL MASS of the iodine molecules in the flask.
Answer
To find the initial mass of I2, we start by calculating the changes at equilibrium. Let x be the change in moles of I2. At equilibrium:
- Concentration of I: 2x = 4.79 × 10⁻³ mol·dm⁻³ → x = 2.395 × 10⁻³ mol·dm⁻³
- Concentration of I2 at equilibrium = Initial concentration - change = 0.0259 - 2.395 × 10⁻³ mol·dm⁻³ Thus:
Equilibrium concentration of I2 = 2.395 × 10⁻³ mol·dm⁻³. Using the Ideal Gas Law & Kc values, we find the initial concentration and use the molar mass of I2:
Mass of I2 = moles × molar mass. Molar mass of I2 = 254 g/mol. Initial mass of I2 = 2.395 × 10⁻³ mol × 254 g/mol = 0.607 g or approx 26.543 g.