Chem Electrolysis Notes

Secondary 4 Express Pure Chemistry Page 1 Name: ( Class: 4CYA ) Date: Introduction ▪ We have learnt that ionic compounds conduct electricity only when molten or aqueous. ▪ When an electric current passes through such compounds, they become decomposed in a chemical reaction. ▪ The process of decomposing a compound into its constituent elements by electricity is known as . Electrolytic Cell ▪ A simple electrolytic cell is made up of : - - - - a cell or battery two electrodes an electrolyte other electrical devices ~ bulb, switch, resistor, voltmeter, ammeter. 1 ElectricityElectricity andand ChemistryChemistry

Secondary 4 Express Pure Chemistry Page 2 Electrodes ▪ The plates which carry the electricity into the electrolyte are called electrodes. ▪ They are usually made of inert materials such as carbon (graphite), platinum and mercury. ▪ The electrode connected to the - negative terminal of the battery is called the . - positive terminal of the battery is called the . ▪ The electrodes are also the terminals through which electrons enter and leaves the electrolyte. ▪ Electrons enter via the and leave via the . Electrolyte ▪ An electrolyte is a compound which conducts an electric current when molten or dissolved in water, and is decomposed by the electric current. ▪ Electrolytes are usually : (i) (ii) ▪ Example: ▪ Non- electrolytes are the that do not contain ions . Examples are distilled water, alcohol, oil and other organic solvent. 2 Weak Electrolytes - Weak acids and alkalis Strong Electrolytes - Strong acids, alkalis and salt solution

Secondary 4 Express Pure Chemistry Page 3 Why Can’t Solid Ionic Compounds Conduct Electricity? The ions in solid ionic compounds are held in fixed positions in a lattice. They are unable to move to conduct electricity. Elements and Compounds Conduct Electricity Differently Element - By mobile electrons and element remains unchanged as electricity passes Compound - By mobile ions and Decomposes and forms new substances 3

Secondary 4 Express Pure Chemistry Page 4 Electrolysis When electricity is passed through an electrolyte, chemical decomposition occurs. ▪ This involves the splitting up of the electrolyte into positive and negative ions.▪ - - Positive ions are called . Negative ions are called . During electrolysis, the are attracted to the while are attracted to the . ▪ anions cations anode cathode During electrolysis, electrons flow from the negative terminal to the positive terminal of the battery. On reaching the respective electrodes, these ions are , The process of gaining or losing electrons at the electrodes is called discharge. i.e. they lose their charges and form neutral atoms or molecules. ▪ ▪ ▪ - - At the anode, anions are discharged by (oxidation). At the cathode, cations are discharged by (reduction). 4

Secondary 4 Express Pure Chemistry Page 5 Example 1: Molten lead (II) bromide Molten lead (II) bromide At the anode (positive electrode): - - migrate to the anode and are discharged. Each bromide ion loses one electron and is . - is produced. At the cathode (negative electrode): - - migrate to the cathode and are discharged. Each lead (II) ion gains two electrons and is . - are deposited. 5 Pb2+ (l) + 2e- 2Br- (l) Electrolyte: Electrodes: Ions produced: (cations) (anions)

Secondary 4 Express Pure Chemistry Page 6 Example 2: Molten lithium chloride At the anode: - - migrate to the anode and are discharged. each chloride ion loses one electron and is . - is produced. At the cathode: - - migrate to the cathode and are discharged. each lithium ion gains one electrons and is . - are deposited. Exercise: Complete the table below. 6 Molten electrolyte Product at cathode Product at anode Calcium chloride, CaCl2 Sodium iodide, NaI Magnesium oxide, MgO Lead (II) oxide, PbO Li+ (l) + 1 e- 2Cl- (l) Electrolyte: Electrodes: Ions produced: (cations) (anions)

Secondary 4 Express Pure Chemistry Page 7 Electrolysis of Aqueous Electrolytes ▪ Aqueous electrolytes are mixtures of two electrolytes - the compound itself and water. ▪ During electrolysis, water will ionise to produce and . ▪ With the presence of more than one type of anions and cations, we must now determine what ions will be discharged. ▪ There are 3 things we need to consider: - - - The electrochemical series The electrolytic cell used for electrolysis of aqueous electrolyte is shown below. - Any gases produced in the process can be collected in the test-tubes and then identified by simple tests. 7 Ion lowest in the series will be discharged first if present. anions cations K+ (aq) Na+ (aq) Ca2+ (aq) Mg2+ (aq) Zn2+ (aq) Fe2+ (aq) Pb2+ (aq) H+ (aq) Cu2+ (aq) Ag+ (aq) F- (aq) SO4 2- (aq) NO3 - (aq) Cl- (aq) Br- (aq) I- (aq) OH- (aq)

Secondary 4 Express Pure Chemistry Page 8 Example 1: Aqueous sodium chloride solution Aqueous sodium chloride solution At the anode: - - migrate to the anode and are discharged. each hydroxyl ion loses one electron and is . - are liberated. At the cathode: - - migrate to the cathode and are discharged. each hydrogen ion gains one electrons and is . - is produced ( hydrogen gas burns with a pop sound ). 8 2H+ (aq) + 2e- 4OH- (aq) Electrolyte: Electrodes: Ions produced: (cations) (anions)

Secondary 4 Express Pure Chemistry Page 9 Example 2: Aqueous sulphuric acid, H 2 SO 4 Sulphuric acid At the anode: - - migrate to the anode and are discharged. each hydroxyl ion loses one electron and is . 4OH- (aq) are liberated.- At the cathode: - - migrate to the cathode and are discharged. each hydrogen ion gains one electrons and is