Exercises: Simple electrochemical processes

A cell is a device that transforms chemical energy into electrical energy thanks to a chemical reaction taking place inside it. Explain in your own words what this phrase means and name two devices in your home that run on batteries.

At the science fair, a group inserts a zinc plate and a copper plate into a lemon, connects them to an LED, and it lights up: they have built a "lemon battery", as shown in the diagram.

1. a) What acts as the electrolyte (the medium that conducts the current inside the cell) in this setup?
2. b) Why do you think two different metals are needed and two identical plates would not work?

Electrolysis is the reverse process of a cell: an electric current forces a chemical reaction to occur that would not happen on its own. Complete the summary with the missing words: "In a cell, a chemical reaction $\dots$ electric current; in electrolysis, electric current $\dots$ a chemical reaction".

When direct current is passed through water (with a few drops of acid so that it conducts electricity), bubbles of two different gases appear at the electrodes, as shown in the diagram.

1. a) Write the balanced equation for the decomposition of water.
2. b) Name the two gases given off.

Classify these cells and batteries as "single-use" or "rechargeable": the alkaline battery of a remote control, the lithium-ion battery of a cell phone, the lead battery of a car and the button cell of a watch. Explain in your own words what it means for a battery to be rechargeable.

When you plug your cell phone into the charger, the chemical reaction in its battery occurs "in reverse". Explain, using the concept of a reverse process, what the charger does with the electric current and how recharging a battery is similar to electrolysis.

In the electrolysis of water, $2\ H_2O \rightarrow 2\ H_2 + O_2$, the volume of hydrogen collected is always twice that of oxygen, as shown in the diagram.

Explain, using the coefficients of the balanced equation, why this happens.

Chrome-plated taps or silver-plated cutlery are produced by electroplating: the steel object is connected to the negative terminal of a power supply and immersed in a solution containing ions of the metal (chromium or silver), which is deposited onto it, forming a very thin layer. Explain two reasons why it is useful to coat steel with these metals.

The Daniell cell has a zinc electrode immersed in zinc sulphate solution and a copper electrode in copper sulphate solution, connected to each other, as shown in the diagram.

While the cell is operating, the zinc electrode loses mass and the copper electrode gains mass.

1. a) Which metal is passing into the solution in the form of ions?
2. b) Which metal is being deposited and where do its atoms come from?
3. c) Explain where the electrons "travel" that cause the light bulb connected to the cell to shine.

You want to light up an LED using lemons at the science fair.

1. a) State the three essential "ingredients" of your homemade cell (two electrodes and an electrolyte) and what material you will use for each one.
2. b) Predict what will happen if you insert two identical copper plates into the lemon instead of one copper plate and one zinc plate, and justify your answer.
3. c) A single lemon provides a very low voltage: explain how you would connect several lemons to achieve a higher voltage.

In a water electrolysis cell, $4\ \text{mol}$ of hydrogen have been collected.

1. a) Using the equation $2\ H_2O \rightarrow 2\ H_2 + O_2$, calculate how many moles of oxygen have been given off.
2. b) How many moles of water have decomposed in total?

In the laboratory, water is electrolysed, collecting the gases in two inverted graduated tubes placed over the electrodes, as shown in the diagram.

When the hydrogen tube contains $20\ \text{mL}$, what volume of oxygen do you expect to find in the other tube, measured under the same conditions? Explain the ratio using the coefficients of the balanced equation.

A cell phone battery works "in both directions".

1. a) Describe the energy conversion taking place when the cell phone is in use (discharging) and when it is plugged into the charger (charging).
2. b) Reason why we say that recharging the battery is, at heart, an electrolysis process.

To silver-plate a spoon, the spoon is connected to the negative terminal of a power supply and a silver sheet to the positive terminal, both immersed in a silver nitrate solution ($AgNO_3$), as shown in the diagram.

1. a) On which of the two electrodes is the silver deposited?
2. b) The solution would gradually lose silver ions were it not for the fact that the sheet dissolves: explain what role the sheet connected to the positive terminal plays.
3. c) Reason what would happen to the spoon if it were accidentally connected to the positive terminal.

Compare a cell and electrolysis by answering these three questions for each process:

1. a) Which process produces electric current and which consumes it?
2. b) In which is the chemical reaction spontaneous and in which is it forced by the current?
3. c) What energy conversion occurs in each?

In addition, give a real-life example of each process.

The battery of a gas car contains $6$ lead cells connected in series, and each cell provides about $2\ \text{V}$, as shown in the diagram.

1. a) What total voltage does the battery provide?
2. b) Explain why the cells are connected in series and not in parallel.
3. c) The starter motor requires a current of $120\ \text{A}$ for $2\ \text{s}$. Calculate the electric charge passing through it ($Q = I \times t$).

An LED requires at least $1.8\ \text{V}$ to light up and each lemon cell provides about $0.9\ \text{V}$.

1. a) What is the minimum number of lemon cells connected in series needed to light up the LED? Justify your answer.
2. b) Draw in your notebook the setup with $3$ lemons, indicating which electrode (copper or zinc) of each lemon connects to which electrode of the next one.
3. c) If one of the three lemons is connected with reverse polarity relative to the others, what total voltage does the combination produce? Reason whether the LED will light up.

You can do it on paper and send a photo with your answer.

In the Daniell cell, these two half-reactions occur: at the zinc electrode, $Zn \rightarrow Zn^{2+} + 2\ e^-$; at the copper electrode, $Cu^{2+} + 2\ e^- \rightarrow Cu$.

1. a) Add the two half-reactions together and write the overall reaction of the cell.
2. b) Which electrode acts as the negative terminal, that is, the one that donates electrons to the circuit?
3. c) Explain, using the half-reactions, why the zinc electrode loses mass and the copper electrode gains mass while the cell is operating.

An off-grid house has solar panels that charge a battery during the day, and at night the battery powers the lights.

1. a) Describe the complete chain of energy transformations from the moment light hits the panel until a bulb lights up at night.
2. b) Locate in that chain a process that operates like a cell and another that operates like electrolysis, and explain each one.
3. c) In each step, some energy is lost as heat: reason why no part of the chain is $100\ \%$ efficient and what general principle about energy is always fulfilled.

In an electrolytic cell, water is decomposed and, between the two electrodes, a total of $72\ \text{mL}$ of gas is collected, measured under the same conditions.

1. a) What volume of hydrogen and what volume of oxygen have been collected?
2. b) Under these conditions, $1\ \text{mol}$ of any gas occupies $24\ \text{L}$: calculate the number of moles of each gas.
3. c) Using the molar mass of water, $18\ \text{g/mol}$, calculate what mass of water has decomposed.

To electrolyse water in the laboratory, a few drops of sulfuric acid or a pinch of sodium sulfate must be added.

1. a) Explain what role the dissolved ions play in conducting current through water.
2. b) Predict what you would observe if you tried to electrolyse completely pure distilled water using a $9\ \text{V}$ battery, and justify your answer.
3. c) Propose a test for identifying the gas in each tube, knowing that oxygen relights a glowing splint and that hydrogen burns with a squeaky "pop" when a flame is brought near it.

Aluminum is extracted by the electrolysis of its molten oxide (alumina), a process that consumes huge amounts of electricity.

1. a) Explain, using the concept of a "reaction forced by current", why producing aluminum is so energy-intensive.
2. b) Recycling a can consumes only $5\ \%$ of the energy required to produce new aluminum. For every $100\ \text{kWh}$ it would cost to produce primary aluminum, calculate how much energy is saved if that aluminum is obtained by recycling.
3. c) Using this data, argue why recycling cans is an important measure from both a chemistry and an environmental perspective.

The steel hull of ships has zinc blocks welded to it called "sacrificial anodes": it is the zinc that corrodes instead of the iron, as shown in the diagram.

1. a) Taking into account what happens in the Daniell cell, where zinc wears away by losing electrons more easily than copper, explain why a zinc block protects the iron of the hull.
2. b) Reason what would happen if the blocks were made of copper instead of zinc.
3. c) Why must these blocks be inspected and replaced periodically?

An alkaline battery costs $\$10$ and lasts for a single use. A rechargeable battery costs $\$80$ and allows $500$ recharges; the charger and electricity for all those recharges cost a total of about $\$120$.

1. a) Calculate how much $500$ uses cost with alkaline batteries and how much with the rechargeable battery.
2. b) Besides the financial savings, reason what environmental advantage the rechargeable battery has in terms of the waste generated.
3. c) Used batteries must not be thrown into the normal rubbish bin: explain why from the perspective of the chemistry they contain.