Redox Equilibria
Chem Factsheet
September 2002 Number 37
Redox Equilibria I: Standard Electrode Potentials and Cells
To succeed with this topic you need to: • be familiar with the concept of equilibrium (Factsheet 09); • be able to assign oxidation numbers to elements (Factsheet 11). After working through this Factsheet you will: • understand the link between cells and oxidising and reducing powers (redox reactions); • have met the Standard Hydrogen Electrode and why it is needed; • know the definition for Standard Electrode Potential (SEP) and its symbol E ; • be able to use SEP values to find values for different cells; • know how SEP values affect reducing and oxiding powers. Examination questions and techniques on SEPs and cells are covered separately in Factsheet 41. When a metal is placed into a solution of its own ions an equilibrium is set up. Cu Zn
Zn2+(aq) Zn2+(aq) + 2e Zn(s)
Cu2+(aq) Cu2+(aq) + 2e− Cu (s)
You will recognise the half equations – the diagrams show the half cells. NB: In this work, half equations are always shown with electrons on the left. If these two half cells are connected then the same reaction takes place as when a piece of zinc is put into a copper solution: wire Zn salt bridge Cu
Redox – revision
When a piece of zinc is placed in a copper salt solution the copper ions are displaced – zinc is more reactive than copper ions: Zn(s) + Cu (aq) ➝ Zn (aq) + Cu(s)
2+ 2+
You will recognise this from the reactivity series of metals at GCSE level. If we now apply oxidation numbers to the same equation: oxidation reduction Zn + 0 reducing agent Cu2+ ➝ +2 oxidising agent Zn2+ +2 + Cu 0 Zn2+(aq) Cu2+(aq)
O.N.
Electrons move from the zinc cell to the copper cell and so the zinc electrode will dissolve and the copper electrode will increase in size – this is called short-circuiting the cell. However, if a high-resistance voltmeter is connected so electrons cannot flow through, the voltmeter gives a reading – it measures the difference
September 2002 Number 37
Redox Equilibria I: Standard Electrode Potentials and Cells
To succeed with this topic you need to: • be familiar with the concept of equilibrium (Factsheet 09); • be able to assign oxidation numbers to elements (Factsheet 11). After working through this Factsheet you will: • understand the link between cells and oxidising and reducing powers (redox reactions); • have met the Standard Hydrogen Electrode and why it is needed; • know the definition for Standard Electrode Potential (SEP) and its symbol E ; • be able to use SEP values to find values for different cells; • know how SEP values affect reducing and oxiding powers. Examination questions and techniques on SEPs and cells are covered separately in Factsheet 41. When a metal is placed into a solution of its own ions an equilibrium is set up. Cu Zn
Zn2+(aq) Zn2+(aq) + 2e Zn(s)
Cu2+(aq) Cu2+(aq) + 2e− Cu (s)
You will recognise the half equations – the diagrams show the half cells. NB: In this work, half equations are always shown with electrons on the left. If these two half cells are connected then the same reaction takes place as when a piece of zinc is put into a copper solution: wire Zn salt bridge Cu
Redox – revision
When a piece of zinc is placed in a copper salt solution the copper ions are displaced – zinc is more reactive than copper ions: Zn(s) + Cu (aq) ➝ Zn (aq) + Cu(s)
2+ 2+
You will recognise this from the reactivity series of metals at GCSE level. If we now apply oxidation numbers to the same equation: oxidation reduction Zn + 0 reducing agent Cu2+ ➝ +2 oxidising agent Zn2+ +2 + Cu 0 Zn2+(aq) Cu2+(aq)
O.N.
Electrons move from the zinc cell to the copper cell and so the zinc electrode will dissolve and the copper electrode will increase in size – this is called short-circuiting the cell. However, if a high-resistance voltmeter is connected so electrons cannot flow through, the voltmeter gives a reading – it measures the difference