Title: Experiment 12 Electrolysis

Purpose

How can we break water down into the elements from which it is made? How do the properties of the compound (water) compare with those of the individual elements?

Introduction

In contrast to distillation (discussed in Chapter 2) electrolysis is used to break a compound down into the basic elements from which it was formed. The separation (which is really a decomposition), is carried out by passing an electric current through the compound to be broken down. In this case, unlike distillation, the identity of the original compound does change, so electrolysis is a chemical process.

In order to speed up the electrolysis, by improving the current flow through the solution, sodium sulfate, Na2SO4, has been added to the distilled water. The water also contains a small amount of bromothymol blue. Bromothymol blue is called an indicator, because it changes color in a base. Bromothymol blue is yellow in acid and blue in base.

The positive side of a battery is called the anode. The negative side is the cathode. At one side, hydrogen gas is made, and oxygen gas is made at the other.

Water + electrons  Hydrogen gas + hydroxide ions

Water  electrons + oxygen gas + hydrogen ions

Prelaboratory Assignment

  • Read the Introduction and Procedure before you begin.
  • Answer the Prelaboratory Questions.
    1.Write the formula of hydrogen gas, oxygen gas and water. Classify them as an atomic element, molecular element or compound.

2.How will you know which side hydrogen gas is produced? Oxygen gas?

Materials

  • U-tube
  • 9-volt battery
  • pencil-lead electrodes

Reagents

  • water
  • Na2SO4
  • bromothymol blue

Safety

1. While there is little danger of personal injury, due to the low level of electrical energy being used, to protect the life of the battery, you should be careful not to touch the two electrodes to each other.

Procedure

  1. Put 200 mL water in a 250 mL beaker.
  2. Add 2 scoops of Na2SO4. Stir with a stir rod.
  3. Add 15 drops bromothymol blue
  4. Assemble the battery and clip with graphite leads.
  5. Assemble the U-tube apparatus on a ring stand (Draw the setup).
  6. Put the graphite leads in the U-tube. Don’t put the alligators in the water! They can’t swim!
  7. Record your observations at the anode and cathode. *Remember: observations are things you see, hear, feel, smell (no tasting in lab), and measure. No thinking. Do not guess the identity of the chemicals made.
  8. Disconnect the clip from the battery.
  9. Clean up: liquid in U-tube pour in the sink with water. Broken pencil lead  garbage.
  10. Clean all equipment.

Data

Observations at anode: ______

Observations at cathode: ______

Analysis and Conclusions

1.Which side was oxygen gas produced (anode or cathode)? Hydrogen gas? How do you know?

2.Compare the physical properties of hydrogen gas and oxygen gas. How are they different? Alike?