6. Electrochemistry

ELECTROLYSIS What is a redox reaction? Oxidation is the addition of oxygen to a substance and reduction is the removal of oxygen from a substance. Which substances are oxidized and reduced in this reaction? oxygen removed reduction lead oxide + carbon

lead carbon + monoxide oxygen added oxidation Reduction and oxidation always take place together. Why is this type of reaction called a redox reaction? redox = reduction and oxidation

Redox and electrons Magnesium burns in oxygen to form magnesium oxide. It is obvious that the magnesium has been oxidized, but what has happened to the oxygen? A redox reaction can also be explained in terms of the gain or loss of electrons. What happens to the atoms and electrons in this reaction? magnesium + oxygen magnesium oxide

2Mg(s) + O2(g) 2MgO(s) Oxidation and electron loss When magnesium burns in oxygen to form magnesium oxide, what happens to magnesium and its electrons?

oxidized (electrons lost) Mg + O Mg2+

O2- The magnesium has been oxidized. The Mg atom has lost 2 electrons to form a Mg2+ ion. Oxidation is the loss of electrons. Oxidation and electron gain When magnesium burns in oxygen to form magnesium oxide, what happens to oxygen and its electrons? reduced (electrons gained)

Mg + O Mg2+ O2- The oxygen has been reduced.

The O atom has gained 2 electrons to form a O 2- ion. Reduction is the loss of electrons. Redox and OILRIG An easy way to remember what happens to the electrons during oxidation and reduction is to think OILRIG! Using OILRIG What does OILRIG stand for in terms of redox reactions? Oxidation Is

Loss of electrons Reduction Is Gain of electrons What is a half-equation? Redox reactions involve the transfer of electrons. Equations written to show what happens to the electrons during oxidation and reduction are called half-equations. What are the half-equations for the oxidation and reduction processes in this reaction? magnesium + oxygen magnesium oxide

2Mg (s) + O2 (g) 2MgO (s) oxidation: Mg Mg2+ + 2ereduction: O2 + 4e- 2O2- What are ionic compounds?

Ionic compounds are made up of positive metal ions and negative non-metal ions. What ions are in sodium chloride? positive sodium ions negative chloride ions The positive and negative ions in an ionic compound attract each other strongly. It takes a lot of energy to separate them. How does structure affect the properties of ionic compounds? Do molten ionic compounds conduct?

What is electrolysis? An ionic compound conducts electricity when it is molten or in solution. The current causes the ionic compound to split up and form new substances. This process is called electrolysis, a word which comes from Greek and means splitting by electricity. Electrolysis has many uses, including: purifying copper plating metals with silver and gold extracting reactive metals, such as aluminium making chlorine, hydrogen and sodium

hydroxide. What happens during electrolysis? In electrolysis, the substance that the current passes through and splits up is called the electrolyte. The electrolyte contains positive and negative ions. What happens to these ions during electrolysis? Negative ions move to the positive electrode and lose electrons. This is oxidation.

heat Positive ions move to the negative electrode and gain electrons. This is reduction. Electrolysis of molten lead bromide Electrolysis of molten PbBr2 redox equations What redox processes occur at the electrodes during the electrolysis of molten lead bromide (PbBr2)?

At the negative electrode: Pb2+ + 2e- Pb (reduction) At the positive electrode: 2Br- Br2 + 2e- (oxidation) What is the overall equation for the electrolysis of molten lead bromide ? lead bromide lead + bromine PbBr2 (l) Pb (l) + Br2 (g) Why and how is aluminium extracted?

Aluminium is one of the most useful metals in the world. Electrolysis is used to extract aluminium from its ore. Why is it not possible to extract aluminium by heating its ore with carbon? Aluminium ore (bauxite) has a very high melting point (2050 C). For electrolysis, the ore is dissolved in a compound called cryolite (Na3AlF6), which lowers the melting point to 700 C. Why is this important economically?

Extracting aluminium Extracting aluminium redox equations What redox processes occur at the electrodes during the electrolysis of aluminium oxide (Al2O3)? At the negative electrode: Al3+ + 3e- Al (reduction) At the positive electrode: 2O2- O2 + 4e- (oxidation)

What is the overall equation for the extraction of aluminium by electrolysis? aluminium oxide aluminium + oxygen 2 Al2O3 (l) 4 Al (l) + 3 O2 (g) What are the products of electrolysis? Purifying copper using electrolysis Purifying copper redox equations

What happens at the electrodes during the purification of copper by electrolysis ? At the negative electrode: Cu2+ + 2e- Cu (reduction) At the positive electrode: Cu Cu2+ + 2e- (oxidation) This process is carried out on a huge scale in industry and the copper formed on the negative electrodes is 99.99% pure. The precious metals recovered from the impurities are also sold off and help to make this industrial process profitable.

Purifying copper true or false? Electrolysis of NaCl solution Products of electrolysis of NaCl solution The electrolysis of sodium chloride solution produces three very useful products: Chlorine used for killing bacteria in water, for bleach and making plastics like PVC. Hydrogen used for making margarine and fertilizers, and for rocket fuel. Sodium hydroxide used in many

chemical reactions, such as making soap, neutralizing acids and making paper. Chlorine is expected as a product of this process but hydrogen and sodium hydroxide are surprising products. What happens at the electrodes to form these products? How does the chlorine form? In the electrolysis of NaCl solution, the negative chloride ions (Cl-) are attracted to the positive electrode. Here, the Cl- ions lose electrons to make

chlorine atoms, which then form chlorine molecules (Cl2). Are the Cl- ions oxidized or reduced? oxidized How many electrons are lost by each Cl- ion? one How many Cl- ions join to make a Cl2 molecule? two What is the half-equation for this redox process? 2Cl- Cl2 + 2e- (oxidation) Why is sodium not formed? In the electrolysis of sodium chloride solution, the Na+ ions might be expected to form sodium at the negative electrode.

Instead, hydrogen gas is produced here. This is because sodium chloride solution also contains H+ ions from some of the water: H2O (l) H+ (aq) + OH- (aq). At the negative electrode, the H+ ions compete with the Na+ ions. The H+ ions gain electrons; the Na+ ions stay in solution. For all ionic compounds containing a metal that is more reactive than hydrogen, electrolysis of a solution of the compound will produce hydrogen rather than the metal. How does the hydrogen form? In the electrolysis of NaCl solution, the positive hydrogen ions

(H+) are attracted to the negative electrode. Here, the H+ ions gain electrons to make hydrogen atoms, which then form hydrogen molecules (H2). Are the H+ ions oxidized or reduced? reduced How many electrons are gained by each H+ ion? one How many H+ ions join to make a H2 molecule? two What is the half-equation for this redox process? 2H+ + 2e- H2 (reduction)

How does the sodium hydroxide form? Sodium chloride solution has four types of ions: Na+ and Cl- ions from the sodium chloride H+ and OH- ions from the water. The Cl- ions form chlorine at the positive electrode and the H+ ions form hydrogen at the negative electrode. So, whats left? Na+ and OH- ions are left behind and so a solution of sodium hydroxide (NaOH) is formed. What is the overall equation for the electrolysis of a sodium chloride solution? 2NaCl (aq) + 2H2O (l) H2 (g) + Cl2 (g) + 2NaOH (aq)

increasing reactivity Hydrogen or metal? potassium sodium calcium magnesium aluminium (carbon) zinc iron lead

(hydrogen) copper silver gold platinum If an ionic compound contains a metal that is more reactive than hydrogen, electrolysis of a solution of the compound will produce hydrogen, not the metal. Complete the table for these compounds. Ionic compound

Product at the negative electrode potassium chloride hydrogen copper sulphate copper sodium bromide

hydrogen silver nitrate silver zinc chloride hydrogen Electrolysis of dilute sulfuric acid Electrolysis can be used to split water (H2O) into its elements, hydrogen and oxygen.

This is how hydrogen for fuel cells can be made and how oxygen can be produced from water on spacecraft. Water is a covalent compound and so is a poor conductor of electricity. However, it does contain a few free H+ and OH- ions: H2O (l) H+ (aq) + OH- (aq). The conductivity of water can be improved by adding dilute sulfuric acid. This releases more ions so that more current flows during electrolysis, which creates hydrogen and oxygen. Which product will form at each electrode? Electrolysis of dilute sulfuric acid

Electrolysis of dilute H2SO4 redox equations What happens at the electrodes during the electrolysis of dilute sulfuric acid? At the negative electrode: 2H+ + 2e- H2 (reduction) At the positive electrode: 4OH- 2H2O + O2 + 4e- (oxidation) What is the overall equation for the electrolysis of dilute sulfuric acid? 2H2O (l) 2H2 (g) + O2 (g)

Twice as much hydrogen forms as oxygen. Why is this? In water, there are 2 hydrogen atoms for every oxygen atom, so the ratio by volume, of H2 to O2, is 2:1. Glossary electrode A solid conductor of electricity, which is used to make electrical contact with an electrolyte. electrolysis The process which uses electricity to split up compounds. electrolyte A substance which conducts electricity and can be split up by a current when molten or in solution. ions Charged particles formed when atoms lose or gain

electrons. oxidation A type of reaction involving the gain of oxygen or the loss of electrons. redox A type of reaction in which oxidation and reduction take place at the same time. reduction A type of reaction involving the loss of oxygen or the gain of electrons. What is involved in electrolysis?

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