Chemistry Reversible Reactions and Electrolysis

Reversible reaction

Reaction where the products can react with each other and convert back to the original reactants.

Dynamic equilibrium

When the rate of the forward reaction is equal to the rate of the backward reaction in a closed system (so no change in concentration of reactants or products)

Increase in temperature effect on dynamic equilibrium

Shifts equilibrium in the direction of the endothermic reaction

Decrease in temperature effect on dynamic equilibrium

Shifts equilibrium in the direction of the exothermic reaction

Increase in pressure effect on dynamic equilibrium

Shifts equilibrium to the side where there are fewer moles of gas

Decrease in pressure effect on dynamic equilibrium

Shifts equilibrium to the side where there are more moles of gas

Catalyst effect on dynamic equilibrium

No effect because it speeds up the forward and backward reaction by the same amount

Electrolysis

Using electricity to decompose aqueous or molten ionic compounds by pasing an electric current thorugh an electrolyte, causing it to decompose.

Electrolyte

Ionic compound that is molten or dissolved

Cathode

Negatively charged electrode that attracts positive ions (cations)

Anode

Positively charged electrode that attracts negative ions (anions)

What happens at the cathode?

Reduction (gain of electrons)

What happens at the anode?

Oxidation (loss of electrons)

What kind of material are electrodes made of

Inert material so that they do not take part in the reaction

How to set up an electrolysis experiment?

Get two inert electrodes and place them in a breaker filled with the electrolyte. Connect the electrodes to a power supply using crocodile clips and wires, and turn on the power supply.

What can electrodes be made of? (example)

platinum or graphite

Ions in water

H⁺ and OH^-

Which ion is discharged at the cathode? + state the reactivity series

The less reactive ion in the reactivity series is discharged

Which ion is discharged at the anode?

Halide ions (forms halogen gas) or otherwise hydroxide ions (forms oxygen gas)

Half equation if hydroxide ion is discharged

Se4OH^- → O₂ + 2H₂O + 4e^-

Setup for purification of copper using electrolysis

Pure copper as the cathode (negatively charged) and impure copper as the anode (positively charged)

What is the negative electrode in the purification of copper experiment? (with half equation)

Pure copper, reduction occurs Cu²⁺ + 2e^- → Cu

What is the postive electrode in the purification of copper experiment? (with half equation)

Impure copper, oxidation occurs Cu → Cu²⁺ + 2e^-

Describe the purification of copper process (5 steps)

1. Negative electrode is pure copper, positive electrode is impure copper

2. Positive electrode gets smaller, negative electrode gets bigger

3. Copper ions are attracted to negative electrode and reduction occurs

4. Copper atoms at positive electrode lose two electronss to form copper ions which dissolve into the electrolyte

5. Atoms of impurities around the copper atoms have turned into ions and are released also become free, collecting as ‘mud’ beneath the electrode.

What is ‘mud’/sludge in purification of copper?

Atoms of impurities that become free when copper atoms are released into the electrolyte, containing silver and gold.

Why might a reaction not reach equilibrium?

It is not a closed system

Why might a small amount of acid be added to water before it is electrolyzed?

To increase electrical conductivity

How does catalyst affect yield?

NO EFFECT!! (in the end the amount produced is same, only sped up)

Final step of bromine ions becoming bromine gas in electrolysis

The bromine atoms join in pairs to form bromine molecules