Student s Name: Date: Electrical conductivity of acids and bases Background Strong acids and bases are those that are essentially completely ionised or dissociated in aqueous solution. Weak acids and bases are those in which only a small proportion of the molecules or ions react with water to form hydrogen ions or hydroxide ions in aqueous solution. The electrical conductivities of aqueous solutions give an indication of the concentration of ions in the solutions. This experiment investigates the effect on electrical conductivity of acid and base strength and solution concentration. Equipment and chemicals D.C. power supply (0-12V) ammeter or globe ( 2.5V ) 4 x electrical leads 2 with alligator clips plate electrode system (see diagram next page) 2 x 100 ml beakers 1 M nitric acid solution HNO 3 1 M sodium hydroxide solution NaOH 1 M ethanoic acid solution CH 3 COOH (50mL) 1 M ammonia solution NH 4 OH 1 M hydrochloric acid HCl 0.1 M hydrochloric acid HCl 0.01 M hydrochloric acid HCl 0.001 M hydrochloric acid HCl STAWA40.DOC Adapted from the STAWA Chemistry Lab Manual by Rob Namestnik Page 1
International safety card information Use the website to find safety information for all the hazardous chemicals used in this experiment http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/ Electrode construction details Safety Information plate electrode system sodium hydroxide - NaOH is very corrosive and must NOT be allowed to come into contact with your skin or eyes. ammonia solution - NH 3 (aq) (although a weak alkali) is very corrosive and must NOT be allowed to come into contact with your skin or eyes. hydrochloric acid HCl is very corrosive and must NOT be allowed to come into contact with your skin or eyes. nitric acid - HNO 3 is very corrosive and must NOT be allowed to come into contact with your skin or eyes. ethanoic acid CH 3 COOH (although a weak acid) is corrosive and must NOT be allowed to come into contact with your skin or eyes. STAWA40.DOC Adapted from the STAWA Chemistry Lab Manual by Rob Namestnik Page 2
Procedure 1. Connect a 6 V DC power supply, ammeter and or globe and plate electrode system in series as shown in the diagram below. 2. Pour 50 ml of 1 mol L -1 HCl into a 100 ml beaker, put the electrodes into the solution and briefly close the circuit. 3. Record the reading on the ammeter or the brightness of the globe. 4. Wash out the beaker and electrodes with deionised water and then in sequence, test the following solutions: 0.1 mol L -1 HCl, 0.01 mol L -1 HCl, 0.001 mol L -1 HCl. Record your results on the data sheet. (If using a globe and a glow in NOT detected on the 6V setting, increase the voltage to 12V) 5. Wash out the beaker and electrodes with deionised water and then in sequence, test the following solutions: 1 mol L -1 HNO3, 1 mol L -1 NaOH, 1 mol L -1 CH 3 COOH and 1 mol L -1 NH 3. Record your results on the data sheet. (If using a globe and a glow in NOT detected on the 6V setting, increase the voltage to 12V) STAWA40.DOC Adapted from the STAWA Chemistry Lab Manual by Rob Namestnik Page 3
Data Sheet hydrochloric acid - HCl concentration (mol L -1 ) 1.0 Ammeter reading or globe brightness 0.1 0.01 0.001 Aqueous samples tested (1 mol L -1 ) nitric acid HNO 3 ethanoic acid CH 3 COOH sodium hydroxide NaOH Ammeter reading or globe brightness Electrolyte strength ammonia solution NH 3 STAWA40.DOC Adapted from the STAWA Chemistry Lab Manual by Rob Namestnik Page 4
Processing of results and questions Question 1 How do these solutions conduct electricity? Question 2 a) Draw a labelled graph of conductivity (ma) against concentration of HCl (mol L -1 ). b) Study the HCl results for varying concentrations and your graph from (2a). What conclusion can you draw concerning the conductivity of a solution and the concentrations of ions in it? STAWA40.DOC Adapted from the STAWA Chemistry Lab Manual by Rob Namestnik Page 5
Question 3 a) Were the conductivities of 1.0 mol L -1 HCl and 1.0 mol L -1 CH 3 COOH different? b) Account for the difference in conductivities. Question 4 Were the conductivities of 1.0 mol L -1 NaOH and 1.0 mol L -1 NH 3 different? Explain your observations. Question 5 Acid and bases are referred to as being strong or weak as distinct from being concentrated or dilute. Explain how these two variables, electrolyte strength and solution concentration, effect the conductivity of aqueous solutions. STAWA40.DOC Adapted from the STAWA Chemistry Lab Manual by Rob Namestnik Page 6