2014/01/01 Difference between galvanic & electrolytic cells Galvanic cells consist of self sustaining electrode reactions converting chemical energy into electrical Galvanic – no batteries energy. GALVANIC AND Galvanic & electrolytic cells They produce electricity ELECTROLYTIC CELLS Electrolytic cells are sustained by a supply of electrical energy from a current source, converting electrical energy into chemical energy. They are used to electroplate items. Electrolytic – batteries required Lemon battery ZnO - Redox reaction REDOX REACTIONS Mg → Mg2+ + 2e- oxidation reaction OXIDATION REDUCTION reducing agent (donates electrons and so can cause reduction) - - Cl2 + 2e → 2Cl reduction reaction A reaction in A reaction in which a which a oxidising agent (accepts electrons and substance substance gains so can cause oxidation) Redox agents loses electrons electrons Mg → Mg2+ + 2e- Mg is oxidised (1) - - Cl2 + 2e → 2Cl Cl2 is reduced (2) Mg + Cl2 → MgCl2 Redox reaction Mg in Cl2 The electrons cancel each other out. Redox reactions 2+ - MgCl2 is an ionic compound (Mg 2Cl ) Gain & loss of electrons The equation shows 2 half reactions (1 and Redox examples 2) that add to give the full redox reaction. 1 2014/01/01 DIRECT ELECTRON TRANSFER From the observations we can infer that: Cu in AgNO3 Cu → Cu2+ + 2e- A coil of copper was Ag+ + e- → Ag placed in a silver nitrate solution Electrons are transferred The solution became blue from the copper atoms on because copper ions the piece of copper, to the were formed. silver ions in the silver nitrate solution. Solid silver deposited on This is a redox reaction. Cu in AgNO3 the copper wire. This is a spontaneous reaction. A GALVANIC CELL The following observations were made: • the zinc plate decreases in mass. Cu/Zn cell THE ZINC-COPPER CELL Zn Cu Electrochemical cell • the copper plate increases in mass. Zn Cu cell • the voltmeter reading indicates that electrons flow from the zinc plate to the copper plate. Zn/Cu cell From these observations we can infer that Salt bridge The salt bridge: The redox reaction for the cell: • is a gel solution that connects the two 2+ - Zn/Cu cell electrolyte solutions that the metals dip into. Zn(s) → Zn(aq) + 2e 2+ - • acts as a transfer medium that allows ions to Cu(aq) + 2e → Cu(s) flow through but prevents the two solutions Zn + Cu 2+ → Zn 2+ + Cu mixing. (s) (aq) (aq) (s) • completes the The zinc reacts & dissolves – the mass of the circuit allowing zinc plate decreases. electrons to flow The copper plate increases in mass because through the copper ions deposit as copper metal. connecting wire. 2 2014/01/01 The electrodes: Cell notation: Each metal strip is an electrode. Reactions at electrodes Zn / Zn2+ // Cu2+ / Cu Each electrode with its associated electrolyte Write the anode half reaction first solution, is called a half cell. – salt bridge (//) – then the cathode half reaction The electrode where oxidation occurs is the ANODE. It has a negative charge. Zinc atoms are oxidised to Zn2+ ions The electrode where reduction (anode). occurs is the CATHODE. It has Cu2+ ions are reduced to copper atoms a positive charge. (cathode). Reduction written on the Right Cu/Zn cell Cu/Zn cell ELECTROLYTIC CELLS An electrolytic cell Source of energy Positive electrode Negative electrode Refining copper A battery is always required in this type of cell. Chemical reactions occurring in the cell: Na+ ions are attracted to the negative Cl- ions are attracted to the positive electrode, where they gain electrons. electrode, where they lose electrons. + - - - - Na (aq) + e → Na(s) Cl (aq) → Cl (g) + e Reduction occurs at the Oxidation occurs at the negative electrode – positive electrode – the CATHODE. the ANODE. Electroplating with Cu Copperplating a key 3 2014/01/01 Comparison of galvanic and electrolytic cells. Galvanic cell Electrolytic cell Produces an emf from a Uses the emf of a cell to chemical reaction produce a chemical reaction Chemical potential Electrical potential energy converted to energy converted to electrical potential chemical potential energy energy Positive terminal - Positive terminal - anode cathode Two containers each with One container with both an electrodes electrodes dipping in What should you be able to do? • distinguish between galvanic and electrolytic cells. Cell architecture • write equations for the reactions occurring in galvanic and electrolytic cells. • explain the difference between oxidation and reduction and explain why the two types of reactions go together. • label diagrams of galvanic and electrolytic cells. 4 .