Video Transcript
At which electrode or electrodes is
a metal deposited when electricity is passed through the following circuit using
inert electrodes?
This is a question about
electrolysis, a process where electric current is passed through a liquid or a
solution, causing it to decompose. To answer the question, we need to
determine which of the electrodes will have a metal deposited on its surface when
electricity is passed through the circuit. Let’s take a closer look at what
happens in each container in order to understand this process. The container on the right contains
molten sodium chloride, in other words, NaCl in its liquid form. The container on the left contains
a sodium chloride solution, or aqueous NaCl. This solution contains sodium ions,
chloride ions, and water molecules.
Notice that no water molecules are
present in the rightmost container. When the power supply is turned on,
the molten sodium chloride will decompose into sodium cations and chloride
anions. These ions are then drawn toward
the electrode of the opposite charge. When electrons are donated or
accepted at each electrode, the products that form are chlorine gas and sodium
metal. These products are easy to predict
because sodium and chlorine are the only elements present in the container.
Now let’s take a look at the
container on the left. When the power supply is turned on,
some of the water molecules break down into a hydrogen ion and a hydroxide ion. As before, the ions are attracted
to the electrode of the opposite charge. We know that at each electrode,
electrons will be transferred, turning one of the ions there into a gaseous or solid
product. But in this container, at each
electrode we have two different ions to choose from. At the anode, either oxygen gas or
chlorine gas will be produced. At the cathode, either hydrogen gas
or sodium metal will be produced. At the anode, one key rule is that
the halogen will react preferentially over the hydroxide ion. So, chloride ions will donate their
electrons to form chlorine gas bubbles.
To determine the product at the
cathode, we can look at the reactivity series and note that hydrogen is more stable
than sodium. As hydrogen is more stable than
sodium, it is more likely to gain an electron to form a stable atom or molecule. Meanwhile, sodium is more reactive
than hydrogen, so it is more likely to remain an ion. As a result, hydrogen gas will be
the product that forms at the cathode.
We now know the product formed at
each electrode and can revisit the question. We want to know at which electrode
or electrodes a metal is deposited. We can see that the only electrode
where a solid metal is deposited is the cathode in the rightmost container. This is electrode D. Therefore, the electrode where a
metal is deposited when electricity is passed through the given circuit is electrode
D.
