Electrolysis

Breaking and forming bonds

When methane, CH₄ burns in oxygen gas, O₂, bonds must first be broken in both molecules before new bonds forming the products can be made.

Energy is measured in kilojoules or kJ.

When bonds break, energy must be absorbed from their surroundings. Taking in energy reduces the temperature of the surroundings - this is called an endothermic reaction. This value is always given a positive sign, for example, +345kJ.

When bonds are made, energy is released to the surroundings. Energy that is released to the surroundings is called an exothermic reaction. This value is always given a negative value, for example, -345kJ.

Remember, when a reaction takes place bonds break (endothermic) then bonds are made (exothermic).

Overall, the reaction will be exothermic if more energy is released into the surroundings than was absorbed.

An endothermic reaction will occur overall if, more energy is absorbed from the surroundings than is released.

Bond energy: This is the energy required to break one mole of bonds. The bond energy is also the energy given out when a mole of bonds is formed.

Activation energy: This is the minimum amount of energy required to break bonds to start the reaction off.

Electrolysis is the decomposition of a compound using electricity:

The decomposition of molten lead bromide occurs using the apparatus above. A current is passed through graphite rods called electrodes.

The negative terminal is attached to one rod, which becomes the negative electrode, the cathode.

The positive terminal is attached to the other rod. This becomes a positive electrode, the anode.

Note: The compound must be molten to allow the charged ions to flow. You cannot carry out electrolysis on solid lead bromide.

How does lead bromide decompose?

The diagram above shows how the oppositely charged ions are attracted to oppositely charged electrodes.

Cations (positive ions - metal ions and hydrogen) travel to the negative electrode, the cathode.

Anions (negative ions - non-metal ions) travel to the positive electrode, the anode.

Cations are positive so the go to the negative electrode, the cathode.

Anions are negative so go to the positive electrode, the anode.

Summary of electrolysis:

1. All ionic compounds when molten can be decomposed when electricity is passed through using electrolysis.
2. The metal and hydrogen always forms at the cathode.
3. Non-metal always forms at the anode.
4. Cations travel to the cathode.
5. Anions travel to the anode.
6. The electrodes are made from inert material such as graphite, so that they do not involve themselves with the reaction.
7. The molten substance been electrolysed is called the electrolyte.

Examples:

At the cathode:

At the anode:

At the cathode:

At the anode:

When a salt is dissolved in water, its ions become mobile.

Hence, the solution can be electrolysed. However, the products from the salt solution will be different to the molten solution because of the presence of the water, which itself produces ions.

During electrolysis, these ions compete with the metal and non-metal ions from the dissolved salts, to receive or give up electrons.

So who wins?

At the cathode:

The more reactive a metal is the more it prefers being ions.

Therefore, if a reactive metal such as zinc or magnesium is present it will remain as the ions. The H⁺ ions will accept the electrons and hydrogen gas will be given off at the cathode.

If a less reactive metal, such as copper or silver is present it would rather accept the electrons than H⁺.

Hence, the metal forms at the cathode.

At the anode:

If halide ions are present, Cl^-, Br^-, I^-, they will give up there electrons to become molecules of Cl₂, Br₂ and I₂ respectively.

If no halogen is present, OH^- will give up electrons more readily than other non-metal ions, and oxygen forms.

Examples:

Potassium bromide solution (aq):

At the cathode:

At the anode:

Copper (II) nitrate solution (aq):

At the cathode:

At the anode:

Depositing Copper:

When a solution of copper (II) sulphate is electrolysed using copper electrodes the following reactions occur:

At the cathode:

Copper ions become copper atoms:

The copper atoms deposit themselves on the cathode.

At the anode:

The copper anode dissolves, forming copper ions:

mass of copper lost at anode = mass of copper gained at cathode

This method is used to purify copper in industry. By placing the impure copper at the anode, pure copper is formed at the cathode, as the copper ions migrate from the impure copper anode.

You can use electrolysis to coat one metal with another. This is called electroplating. Electroplating is used a great deal in industry, for example; chrome-plating car bumpers.

If you wanted to coat a nickel vase with silver, you would set the vase as the cathode and the silver as the anode.

At the anode: Silver dissolves forming silver ions.

At the cathode: Silver ions receive electrons and form a layer of silver on the vase.

The electrolysis of salt water:

This industry has been based around the electrolysis of brine, salty water!

At the cathode: Hydrogen bubbles off:

At the anode: Chlorine bubbles off:

Na⁺ and OH^- ions are left behind, which means a solution of sodium hydroxide forms.

The products from the electrolysis of brine are:

1. sodium hydroxide.
2. chlorine.
3. hydrogen.

These products are used for many purposes:

Sodium hydroxide is used for making, soaps, detergents and paper.

Chlorine is used for making, PVC, solvents, bleach, drugs, hydrochloric acid , paints and dyes.

Hydrogen is used for making fuel for rockets and nylon.

The reaction between hydrogen and nitrogen

Many of the reactions you observe only go one way - this means that reactants react to form products and eventually the reaction comes to a stop.

However, some reactions are never completed because there are two competing reactions occurring, a forward reaction and a backward reaction.

Take the reaction between nitrogen gas and hydrogen gas, the two competing reactions that occur are:

The forward reaction:

The backward reaction:

This is an example of a reversible reaction, and is often written as:

The symbol:

indicates a reversible reaction.

If the back and forward reaction rates become the same, we say that the reaction is in equilibrium.

If you want to increase the amount of ammonia you make, you have a problem when your reaction mixture reaches equilibrium as the amount of ammonia been produced remains constant.

An important fact about reversible reactions: it is in equilibrium, if you cause a change then it will oppose the change.

You cannot make a reversible reaction go to completion but you can change the conditions so that the equilibrium shifts to the right, producing more product, in this case ammonia.

Increasing temperature:

Since the forward reaction in the production of ammonia is exothermic - gives out heat - by increasing the temperature you are worse off.

This is because the back reaction being endothermic takes the extra energy i.e. it opposes the change. Hence, increasing temperature produces less ammonia!

Increasing pressure:

Pressure is caused by the collision of gas molecules and the container wall.

The fewer molecules you have the lower the pressure. By increasing pressure, the equilibrium mixture will oppose the change - more ammonia will form in order to reduce the pressure.

Adding a catalyst:

Iron acts as a catalyst for this reaction. The catalyst does not affect the equilibrium position since it increases the rate of the forward and back reactions equally, but it does speed up the speed at which equilibrium is reached.

Decomposition:

When a reactant breaks down to give two or more products, we call this type of reaction decomposition.

calcium carbonate → calcium oxide + carbon dioxide

Decomposition caused by heat is called thermal decomposition.

Decomposition can also be caused by light.

silver chloride → silver + chlorine

Combination:

The reverse to decomposition - combination involves often two reactants reacting to form just one product.

sodium + chlorine gas → sodium chloride

Neutralisation:

When acids react with bases, they neutralise each other the products of a neutralisation reaction are neither acids nor bases.

sodium hydroxide + hydrocholoric acid → sodium chloride + water

The products of neutralisation are a salt and water.

Electrolysis:

This reaction involves the decomposition of a compound by electricity.

lead bromide → lead + bromine gas

Fermentation:

Natural organisms, such as yeast can cause decomposition to occur. Yeast breaks down glucose, a sugar, into alcohol.

glucose → ethanol + carbon dioxide

This reaction is important to the yeast cells since it produces the energy they require to multiply. This reaction is used in the making of beer and wines.

This reaction is also used in breadmaking.

Precipitation:

When a reaction involving two solutions produces an insoluble product. The product appears as a precipitate. This reaction is known as precipitation.

barium nitrate + copper sulphate → barium sulphate + copper nitrate

In this reaction it is the barium sulphate that appears as the precipitate.

Combustion:

This reaction involves the reaction of a substance with oxygen in the air. Sometimes the word burning is used instead of combustion.

The substance that reacts with oxygen is said to be oxidised. The result is a product called an oxide.

This is an example of an exothermic reaction, one that gives out heat energy.

carbon + oxygen → carbon dioxide

iron + oxygen → iron oxide

Oxidation and reduction:

If a substance loses oxygen during a reaction it is reduced.

If a substance gains oxygen during a reaction it is oxidised.

Reduction and oxidation always take place at the same time.

For example: the reaction between black copper (II) oxide and hydrogen gas,

In the reaction above, copper (II) oxide is reduced as hydrogen takes oxygen away to form water. So the hydrogen gaining oxygen is oxidised. The copper (II) oxide is reduced to red/brown copper.