In a school laboratory, a technician bubbles excess chlorine gas through 100 cm^3 of aqueous potassium bromide solution containing 23.8 g of dissolved potassium bromide. The symbol equation for the reaction is: Cl2 + 2KBr → 2KCl + Br2 (a) State the type of reaction occurring, an…

Answer: ((a)) Displacement reaction; the colourless potassium bromide solution turns orange as bromine is formed ((b)) Reactivity decreases going down Group VII, so chlorine (above bromine) is more reactive than bromine and can displace bromide ions to form bromine; iodine (below bromine) is less reactive than bromine, so it cannot displace bromide ions from solution ((c)) 16 g of bromine. Part (a): Type of reaction and colour change Chlorine reacting with potassium bromide solution is a displacement reaction: chlorine takes the place of bromine in the compound, releasing bromine and forming potassium chloride. The potassium…

IGCSE Chemistry 0620: Halogen Displacement Reactions and Reacting-Mass Calculations

In a school laboratory, a technician bubbles excess chlorine gas through $100\ \text{cm}^3$ of aqueous potassium bromide solution containing $23.8\ \text{g}$ of dissolved potassium bromide. The symbol equation for the reaction is:

$\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$

(a) State the type of reaction occurring, and describe the colour change observed in the solution. [2]

(b) Explain why chlorine is able to displace bromide ions from the solution, but iodine could not displace bromide ions from an identical solution. [2]

[ $A_r$ : K = 39, Cl = 35.5, Br = 80] [4]

Part (a): Type of reaction and colour change

Chlorine reacting with potassium bromide solution is a displacement reaction: chlorine takes the place of bromine in the compound, releasing bromine and forming potassium chloride.

The potassium bromide solution starts colourless. As bromine is formed, the solution turns orange (the characteristic colour of bromine dissolved in water).

Part (b): Explaining which halogens can displace which

Reactivity of the Group VII halogens decreases going down the group: chlorine is more reactive than bromine, and bromine is more reactive than iodine.

Chlorine displaces bromide: chlorine is higher in Group VII than bromine, so it is more reactive than bromine. A more reactive halogen can displace a less reactive halide ion from solution, so chlorine successfully displaces bromide ions, forming bromine.
Iodine cannot displace bromide: iodine is lower in Group VII than bromine, so it is less reactive than bromine. A less reactive halogen cannot displace a more reactive halide ion from solution, so iodine cannot displace bromide ions.

Part (c): Calculating the mass of bromine formed

Step 1: Find the relative formula mass of $\text{KBr}$ . $M_r(\text{KBr}) = 39 + 80 = 119$

Step 2: Convert the mass of $\text{KBr}$ to moles. $\text{moles of KBr} = \frac{23.8}{119} = 0.2\ \text{mol}$

Step 3: Use the equation’s mole ratio to find moles of $\text{Br}_2$ .

From the equation, $\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$ , the mole ratio of $\text{KBr}$ to $\text{Br}_2$ is $2 : 1$ . $\text{moles of Br}_2 = \frac{0.2}{2} = 0.1\ \text{mol}$

Step 4: Convert moles of $\text{Br}_2$ to a mass. $M_r(\text{Br}_2) = 80 \times 2 = 160$ $\text{mass of Br}_2 = 0.1 \times 160 = 16\ \text{g}$

Final answers

(a) Displacement reaction; solution turns orange.
(b) Chlorine is more reactive than bromine (displaces bromide); iodine is less reactive than bromine (cannot displace bromide).
(c) Maximum mass of bromine $= \boxed{16}\ \text{g}$ .