5. ELECTRICITY AND CHEMISTRY -

ELECTROLYSISWhat is “electrolysis”?

Definition:

What is electricity? What is an electric current?

Copy the labelled diagram (6.1.1) from your text book…(p74)

We need to define the following terms:

Electrode

Anode

Cathode

Electrolyte

Points to note:

! Ionic substances only conduct electricity when molten because charged ions need to be mobile for current to flow.

! Melting ionic substances needs a lot of heat energy

Which ions are present in the electrolysis of Lead Bromide?

What happens to these ions?

Pb2+ ions move to the cathode and lead atoms (metal) form at the electrode

EQN: Observations?

Br- ions move to the anode and bromine atoms (gas) forms at the electrode.

EQN: Observations?

These equations are called “HALF EQUATIONS”

If we combine them we get the full equation for this process…

The electrolysis of MOLTEN IONIC COMPOUNDS - PbBr2(l)

What are ANIONS?

What are CATIONS?

In this electrolysis, the electrodes are usually made of graphite. Why?

NB: When ions lose or gain electrons to produce neutral atoms or molecules we say that they have been “DISCHARGED”.

In most of the cases that you will come across the electrodes will not take part in the chemical reactions taking place. They are called “inert” or “passive” electrodes. GRAPHITE AND PLATINUM are the 2 classic examples.

We can use the above electrolysis as a model to be able to predict the products of electrolysis of other molten ionic substances…

EC p75 Q1-3

Compound Product at the CATHODE Product at the ANODE

Aluminium Oxide

Zinc Bromide

Potassium Chloride

Lithium Iodide

The electrolysis of SOLUTIONS OF IONIC SUBSTANCESWe can deduce which ions are in solution by looking at the formula for the compound in question, e.g..

Sodium Chloride solution will definitely contain: …

But we must also take into account that a very small number of water molecules can break up to give hydrogen ions and hydroxide ions…

EQN:

So in aqueous Sodium Chloride solution we have the following ions:

ANIONS CATIONS

What happens and what do we see when we pass an electric current through concentrated aqueous sodium chloride solution?

Let’s try and work out the half equations for what it going on…

At the ANODE…

At the CATHODE…

Which ions remain in solution?

How do we know which ion will discharge if we have a choice of 2 ions at the anode and 2 ions at the cathode?

We use the discharge series:

NB1. The cations discharge according to the reactivity series IN REVERSE!

NB2. When a species gains electrons we say that it has been “REDUCED”

When a species loses electrons we say that it has been “OXIDISED”.

We can remember this by the pneumonic….OiLRiG

Anions Cations

Most likely to discharge I- Cu2+

Br- H+

Cl- Al3+

OH- Mg2+

NO3- Na+

Least likely to discharge SO42- K+

The electrolysis of brine

What is brine?

This takes place in the “diaphragm cell”…

What are the 3 important products in this process and what are they used for?

1.

2.

3.

Points to note:• When electrolysing molten ionic substances, the metal will be formed at the cathode• When electrolysing aqueous ionic substances, hydrogen is usually formed at the

cathode. • Compound or polyatomic ions are not discharged• Non-metals are formed at the anode

EC p77 Q1-3

The electrolysis of AcidsConcentrated Hydrochloric Acid

Ions present?

Anode

Cathode

Dilute Sulphuric Acid

Ions present

Anode

Cathode

NB. OH- ions will discharge when they are with nitrate or sulphate ions OR when they are mixed with a DILUTE solution of halide ions (Cl-, Br-, I-).

EC p79 Q1-3

Purifying or Refining Copper

Copper is used in lots of different contexts due to its excellent electrical conductivity. Before we can use it, though, it must be purified (impurities affect the properties of a metal!).

We do this in the following way…

What are the half equations at the..

ANODE

NB. The copper anode takes part in the reaction here and so is an ACTIVE electrode.

CATHODE

The anode “mud” or “sludge” is full of precious metals such as platinum and silver.

What would happen if we set up the same electrolysis but with inert electrodes? (graphite)

What are the half equations at the..

ANODE

CATHODE

EC p81 Q1-2

ElectroplatingElectrolysis is used to put a thin layer of one metal on top of another metal…

Look at pp 82-83 of your text book.

But why do this?

Examples…

Electroplating with copper

The 3 conditions:

1. The object to be coated is made the CATHODE.2. The metal that is being used to coat the object is the ANODE3. The electrolyte is a solution of the metal being used to coat the object.

At the Anode

At the Cathode

Electroplating with silver

EC p83 Q1-3

Extracting Aluminium using ElectrolysisRead p84 in your text book.

Aluminium is the most abundant metal in the earth’s crust but we have only be using it on a wide scale for not much more than 100 years.

Why?

What is the name of the ore of Aluminium?

At the Anode

At the Cathode

The ore is purified using sodium hydroxide solution. The aluminium oxide reacts with the NaOH(aq) in the following way…

EQN:

Aluminium oxide’s melting point is very high (over 2000ºC) so the Al2O3 is dissolved in CRYOLITE - Na3AlF6 which melts at about 1000ºC

The overall equation for the electrolysis is…

EQN:

At the ANODE:

The graphite anode has to be replaced periodically. Why?

At the CATHODE:

EC p85 Q1-3

Conductors and InsulatorsRead pp86-87

Define and give examples:

Conductors:

Insulators:

Why are steel cored aluminium cables used for overhead power cables?

Why Aluminium?

Why Steel?

Electrolysis of Aqueous solutions

Aim: to relate the products of electrolysis to the electrolyte and electrodes used, Describe electrolysis in terms of the ions present and reactions at the electrodes Predict the products of electrolysis of a specified halide in dilute or concentrated aqueous solution

Introduction

In general, the electrolysis of any concentrated salt solution produces a metal or hydrogen at the cathode and oxygen or a halide at the anode. Dilute or concentrated solutions of carbonates, sulfates, hydroxides and nitrates usually produce oxygen at the anode whereas concentrated halides produce halogens at the anode.

1. How can you predict what will be produce at the cathode ?

……………………………………………………………………………………………………………………………………………………………

……………………………………………………………………………………………………………………………………………….………(2)

Hypothesis 2. For the following solutions predict what will be formed at i) the cathode and ii)

the anode.

a) dilute sulphuric acid i) ……………………………….…………ii) …………………………………….…………

b) potassium hydroxide solution i) ………………………….…………ii) …………………………………….

c) sodium bromide solution i) …………………………….…………ii) ……………………………………

d) copper sulphate solution i) …………………………….…………ii) ……………………………………

e) Sodium chloride solution* i) …………………………………….…………ii) ……………………………………

Method

Draw a labelled diagram of the experimental set-up.

• Carry out electrolysis in the order shown below in the results table. • Fill the cell and test tubes with the electrolyte. • Set the voltage at 6V. • Pass the electricity through the solution until the test-tube (ignition tube) is

full of gas. • Record all colour changes, bubbles, smells and other observations. • Test any gas formed at the cathode with a lit splint and damp litmus paper. • Wash out the cell. • Repeat for the next electrolyte.

Safety precautions

• Wear safety goggles • Fill test-tubes/ ignition tubes carefully (you can use a pipette) • Make sure you are in a well-ventilated area and do not smell the gas produced

when undertaking the electrolysis of sodium chloride.

Results

4. Complete the following results table

Conclusion Explain your observations around the anode during the electrolysis of sodium iodide.

Explain the results of the litmus paper test on the gas produced at the anode during the electrolysis of sodium chloride.

Solution Observation at anode

Observation at Cathode

Test with damp red and Blue litmus

Test with lit splint

Dil H2SO4(aq)

KOH(aq)

NaI(aq)

NaCl(aq)

CuSO4 (aq)

Explain the colour change of the electrolyte during the electrolysis of copper sulphate solution.

For each electrolyte write the half equations for the reactions occurring at the anode and cathode and state what remains in solution.

Solution ½ equation at anode ½ equation at cathode electrolyte change (from___to___)

Dil H2SO4(aq)

KOH(aq)

NaI(aq)

NaCl(aq)

CuSO4 (aq)

Refining Copper and Electroplating (DEMO)

Relate the products of electrolysis to the electrolyte and electrodes used using carbon electrodes and using copper electrodes (as used in the refining of copper)

Introduction

Active electrodes are electrodes which participate in the reaction.

We use active electrodes for:

- electroplating (non-inert electrode is the anode); - refining impure metals (impure metal as anode);

We will set up an experiment that demonstrates how an pure sample of copper can be refined from an impure sample of copper.

Hypothesis

What will you expect to happen to the mass of the cathode ?

What would you except to observe underneath the impure copper anode ?

Method

Write a method to demonstrate how impure copper can be refined using electrolysis, the first step and last steps have been done for you.

• Use a balance to weigh and record the mass of the impure copper electrode.

• Wash the impure copper electrode with ethanol and dry. • Re-weigh the copper electrode.

Results

What did you observe at the cathode, anode and in the electrolyte ?

Conclusion

Explain your observations

Write an ionic ½ equation for the reaction happening at the anode.

Write an ionic ½ equation for the reaction happening at the cathode.

Why did you wash and dry the electrode using ethanol?

At start At the end Difference

Mass of Anode (g)

Mass of Cathode (g)

Electroplating (demo)

We will set up the following arrangement:

Explain what happen including observations and equations

Electroplating with Silver…

Explain what happens…

Electrolysis (3)

5. Electrolysis

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