Fig. 1 shows a pilot plant at a coastal water-treatment site. The plant removes bromine from concentrated salt water left after desalination. The salt water...
Question 1 Report
Fig. 1 shows a pilot plant at a coastal water-treatment site. The plant removes bromine from concentrated salt water left after desalination. The salt water contains sodium bromide and sodium chloride. Chlorine gas enters a closed contact vessel, where it reacts with bromide ions. Air then transfers the bromine formed to a collection unit. The recovered liquid is kept away from the storage tank because bromine is harmful. A technician records the bromide concentration before the reaction in Table 1.
Fig. 1
| volume of salt water processed | concentration of sodium bromide | percentage of bromine recovered |
|---|---|---|
| 2.50 m3 | 0.0180 mol dm-3 | 82.0% |
The relative formula mass of Br2 is 160.
(a) State the colour expected in the contact vessel when bromine is produced. [1]
(b) Complete the chemical equation for the reaction in the vessel.
Cl2 + 2NaBr → ........ + ........ [2]
(c) Explain why chlorine can displace bromine from sodium bromide solution. [2]
(d) Calculate the mass, in g, of bromine recovered from the volume of salt water in Table 1. [4]
(e) Give one reason why any chlorine remaining in the treated water should be removed before the water is returned to the storage tank. [1]
Answer Details
(a) Bromine is orange or orange-brown in the contact vessel. [1]
(b) The completed equation is:
\[\mathrm{Cl_2+2NaBr\rightarrow2NaCl+Br_2}\]
[2]
(c) Chlorine is more reactive than bromine. It gains electrons from bromide ions, displacing them to form bromine. [2]
(d) Convert the salt-water volume:
\[2.50\text{ m}^3=2500\text{ dm}^3\]
Calculate the amount of sodium bromide:
\[n(\mathrm{NaBr})=2500\times0.0180=45.0\text{ mol}\]
Two moles of sodium bromide form one mole of bromine, and \(82.0\%\) is recovered:
\[n(\mathrm{Br_2})=\left(\frac{45.0}{2}\right)\times0.820=18.45\text{ mol}\]
\[m(\mathrm{Br_2})=18.45\times160=2952\text{ g}\]
The mass recovered is \(2952\text{ g}\), accepted as \(2950\text{ g}\) or \(2.95\times10^3\text{ g}\). [4]
(e) Remaining chlorine should be removed because it is toxic and could harm people or kill aquatic organisms when the water is returned to storage. [1]
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