The diagram shows nails A, B and C in agar jelly with indicator: pink shows the metal is corroding fastest by giving up electrons. Nail A is wrapped in magn...
Question 1 Report
The diagram shows nails A, B and C in agar jelly with indicator: pink shows the metal is corroding fastest by giving up electrons. Nail A is wrapped in magnesium, B is bare iron, C is wrapped in tin. Which nail corrodes least?
Answer Details
The indicator turns pink where a metal is giving up electrons, so the nail that stays colourless is the one being protected.
Magnesium is above iron in the reactivity series, so when it is wrapped round a nail it loses electrons in preference to the iron: \( \text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^{-} \). Those electrons flood into the iron and stop it forming \( \text{Fe}^{2+} \), so that nail corrodes least. This is sacrificial protection.
Tin is below iron, so it takes no electrons from the iron; once the jelly reaches the iron through any gap, the iron corrodes faster than if it were bare. The bare nail corrodes at its normal rate, which is worse than the protected one but better than the tin-wrapped case.
Download The App On Google Playstore
Everything you need to excel in your exams
