Predicting Products at Electrodes Flashcards

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When you predict products at the electrodes in electrolysis of an aqueous solution, what is the first thing you should do?

First list all the ions in the solution, including H+ and OH− from the water.

At the negative electrode (cathode), which type of ions do you look at when predicting the product?

You look at the positive ions (cations), because they move to the cathode and gain electrons.

In an aqueous solution, what is the rule for deciding if a metal or hydrogen is made at the cathode?

If the metal is more reactive than hydrogen, hydrogen gas is made. If the metal is less reactive than hydrogen, the metal is made.

Name two metals that are less reactive than hydrogen and so are usually produced at the cathode from their aqueous solutions.

Copper and silver are less reactive than hydrogen and are usually produced at the cathode.

In the electrolysis of aqueous copper(II) sulphate with inert electrodes, what forms at the cathode and why?

Copper metal forms at the cathode because copper ions are less reactive than hydrogen ions.

In the electrolysis of aqueous sodium chloride with inert electrodes, what forms at the cathode and why?

Hydrogen gas forms at the cathode because sodium is more reactive than hydrogen, so hydrogen ions are discharged instead of sodium ions.

At the positive electrode (anode), which type of ions do you look at when predicting the product?

You look at the negative ions (anions), because they move to the anode and lose electrons.

In an aqueous solution, what is usually formed at the anode if the solution contains halide ions such as chloride, bromide or iodide?

The halogen gas is formed at the anode, such as chlorine from chloride ions or bromine from bromide ions.

In an aqueous solution with no halide ions, such as sodium sulphate solution, what gas is usually made at the anode?

Oxygen gas is usually made at the anode, from hydroxide ions in the water.

What are the three main steps to predict the products at the electrodes for electrolysis of an aqueous solution?

First list all ions present. Then apply the cathode rule (metal or hydrogen). Finally apply the anode rule (halogen or oxygen).

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