Changes at the electrodes

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Changes at the electrodes
6.2 Redox Reactions Changes at the electrodes EXAM TIPS: You will be expected to write half equations for the reactions at the electrodes. In half equations, the number of electrons MUST balance the number of charges on the ion.

Objectives: I will understand
• That electrons are transferred during electrolysis. • That electrolysis can be represented in half equations. • That water affects electrolysis. Outcomes: I will demonstrate I understand at Grade C by • Recall the transfer of electrons at the anode and cathode. • Recognise oxidation and reduction at electrodes. • Predict the products of electrolysis. Outcomes: I will demonstrate I understand at Grade A by • Explain the transfer of electrons in electrolysis. • Construct half equations • Explain how water affects the products of electrolysis.

Do you remember what goes where?
heat Negative ions move to the positive electrode (the anode) and lose electrons (Ox) Positive ions move to the negative electrode (cathode) and gain electrons (Red)

What is Oxidation?

Oxidation What is reduction? Oxidation is the gain of oxygen
Rust is an example of oxidation What is reduction?

Reduction Metal Displacement reactions are an example of reduction. +
Reduction is the loss of oxygen Metal Displacement reactions are an example of reduction. oxygen removed reduction lead oxide + carbon lead carbon monoxide oxygen added oxidation

Why is this type of reaction called a redox reaction?
Reduction and oxidation always take place together. Why is this type of reaction called a redox reaction? redox = reduction and oxidation YOU SHOULD ALWAYS WRITE THIS NEXT TO THE QUESTION OILRIG Oxidation Is Loss of electrons Reduction Is Gain of electrons

 + magnesium + oxygen magnesium oxide 2Mg(s) O2(g) 2MgO(s)
Magnesium burns in oxygen to form Magnesium oxide. The Magnesium obviously is oxidised but what happens to the oxygen? magnesium + oxygen magnesium oxide 2Mg(s) O2(g) 2MgO(s) +

Mg is oxidised (lost electrons) O is reduced (gained electrons)
Redox Reactions can also be explained in the terms of losing or gaining electrons oxidized (electrons lost) reduced (electrons gained) Mg Mg2+ O2- O + Mg is oxidised (lost electrons) O is reduced (gained electrons)

In terms of oxygen In terms of electrons
Copy and Complete the table... In terms of oxygen In terms of electrons Oxidation Loss of electrons Reduction Loss of oxygen

Half Equations  + magnesium + oxygen magnesium oxide 2Mg (s) O2 (g)
Redox reactions involve the transfer of electrons. Equations written to show what happens to the electrons during oxidation and reduction are called half-equations. magnesium + oxygen magnesium oxide 2Mg (s) O2 (g) 2MgO (s) + oxidation: Mg  Mg e- reduction: O2 + 4e-  2O2-

Na+ + e- → _____ 2. Pb2+ + ____ e- → Pb Cu2+ + _____e- → ____
Oxidation or Reduction? Na+ + e- → _____ 2. Pb2+ + ____ e- → Pb Cu2+ + _____e- → ____ 4. ___H+ + _____e- → H2 Na REDUCTION 2 REDUCTION 2 Cu REDUCTION 2 2 REDUCTION

2K+ + ____e- → 2K 2Cl- - ___e- → Cl2 REDUCTION 2 2 OXIDATION Oxidation
or Reduction?

Draw a flow diagram Boardworks GCSE Additional Science: Chemistry
Electrochemistry Draw a flow diagram 15

Electrolysis of molten PbBr2 – redox equations
Boardworks GCSE Additional Science: Chemistry Electrochemistry 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 with state symbols? lead bromide  lead + bromine PbBr2 (l)  Pb (l) + Br2 (g) 16

Can you role-play what happens at the electrodes?
We need an Anode (to take electrons) Cathode (to give out electrons) Pb and Br ions. Can you role-play what happens at the electrodes?

Your Turn – Complete these equations with states
2FeCl3(aq) Anode(+)? Cathode(-)? 2Fe(s) + 3Cl2(g) 2Cl-(aq) Cl2(g) + 2e- OXIDATION Fe3+ (aq) + 3e Fe (s) REDUCTION

How does water effect the products of electrolysis?
Water contains hydrogen and hydroxide ions. When solutions of ions in water are electrolysed, hydrogen may be produced at the positive electrode. This happens if the other positive ions are metals more reactive than hydrogen. Whats the name of the positive electrode?

Summary notes When positively charged ions reach the negative electrode they gain electrons to become neutral atoms. Gaining electrons is called reduction, so the positive ions have been reduced. Ions with a single positive charge gain one electron and those with a 2+ charge gain 2 electrons. At the positive electrode, negative ions lose electrons to become neutral atoms. This is oxidation. Some non-metal atoms combine to form molecules, for example bromine forms Br2. We can represent the changes at the electrodes by half equations. For lead bromide. At the negative electrode: Pb2+ + 2e- Pb At the positive electrode: 2Br- Br2 + 2e- Water contains hydrogen and hydroxide ions. When solutions of ions in water are electrolysed, hydrogen may be produced at the positive electrode. This happens if the other positive ions are of metals more reactive than hydrogen. Oxidation is loss (OIL) reduction is gain (RIG). In half equations the number of electrons must balance the number of charges on the ion.

What happens in electrolysis to copper ions, Cu2+, at the negative electrode? What happens in electrolysis to chloride ions, Cl-, at the positive electrode? Why are ions of metals always reduced in electrolysis? CLICK AGAIN FOR THE ANSWERS ANSWERS: They are discharged, gain electrons and are reduced to form copper atoms. They are discharged, lose electrons and are oxidised to form chlorine atoms which then bond to form chlorine molecules (diatomic molecules) All metals form positive ions so they are attracted to the negative electrode where they gain electrons.

Extra Notes

Electrochemistry Teacher notes Appropriately coloured voting cards could be used with this classification activity to increase class participation. 23

Electrochemistry Teacher notes This completing sentences activity provides the opportunity for some informal assessment of students’ understanding of redox reactions. 24

Electrochemistry Teacher notes This drag and drop activity provides the opportunity for informal assessment of students’ understanding of the products of electrolysis. 25