Chapter 3 - Electrolysis Flashcards by Zheting Zhang

What are electrolytes?

Liquids that conduct electricity (presence of mobile ions that carry electrical charges)

Molten and solutions of ionic compounds
Some covalent compounds when dissolved in water

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What are electrodes?

Conductors that emit or take in electrons when immersed in an electrolyte

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Electrical conductivity of an aqueous solution ________ with increasing concentration due to ________ concentration of ions for carrying charges

increases

higher

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What is an electrochemical cell?

A system consisting of electrodes that dip into an electrolyte and in which a chemical reaction is either driven by or generates an electric current

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When is the cell an electrolytic cell?

If the chemical (redox) reaction is non-spontaneous, takes place by using an externally applied current (e.g. from a battery)

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When is the cell a galvanic cell or voltaic cell?

If the chemical (redox) reaction is spontaneous, current is generated

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Compare energy change in an electrolytic and galvanic cell

Electrolytic: from electrical energy to chemical energy
Galvanic: from chemical energy to electrical energy

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Compare anode of an electrolytic and galvanic cell

Electrolytic: positive electrode
Galvanic: negative electrode
- Oxidation occurs (loss of electrons)

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Compare cathode of an electrolytic and galvanic cell

Electrolytic: negative electrode
Galvanic: positive electrode
- Reduction occurs (gain of electrons)

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What is electrolysis?

The conduction of electricity by an electrolyte, leading to its decomposition

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What does an electrolytic cell consist of?

Electrodes
Electrolyte
Direct current (DC) source: such as battery

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The cations are attracted to the ________ electrode (________) and the anions are attracted to the ________ electrode (________).

negative (cathode)

positive (anode)

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What is discharge?

The loss or gain of electrons to the ions resulting in the formation of the products

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In general, what is discharged at the cathode?

Metal ions or hydrogen ion are discharged by receiving electrons from the cathode

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In general, what is discharged at the anode?

Non-metal ions (except hydrogen ion) are discharged by releasing electrons to the anode

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Electrolysis of molten sodium chloride

What happens at the cathode?

Na+ ions are discharged. They take in electrons to form Na atoms (reduction).
Na+(l) + e‒ → Na(l)

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Electrolysis of molten sodium chloride

What happens at the anode?

Cl‒ ions are discharged. They give away electrons to become Cl2 (oxidation).
2Cl‒(l) → Cl2(l) + 2e‒

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Electrolysis of molten sodium chloride

Overall Equation

2NaCl(l) → 2Na(l) + Cl2(g)

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Electrolysis of molten lead(II) bromide

What happens at the cathode?

Pb2+ ions are discharged. They take in electrons to form Pb atoms (reduction).
Pb2+(l) + 2e‒ → Pb(l)
Lead is deposited

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Electrolysis of molten lead(II) bromide

What happens at the anode?

Br‒ ions are discharged. They give away electrons to become Br2 (oxidation).
2Br‒(l) → Br2(g) + 2e‒
Brown fumes of bromine are observed

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Electrolysis of molten lead(II) bromide

Overall Equation

PbBr2(l) → Pb(l) + Br2(g)

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When a molten binary ionic compound is electrolysed, the ________ is always produced at the cathode while the ________ is always produced at the anode

metal

non-metal

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When does the selective discharge of ions occur?

When one ion, which gains or loses electrons easiest is discharged, while the other ions, which are harder to discharge remains in solution

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What are the factors which affect the selective discharge of ions at the electrode?

Position of ions in the electrochemical series
Concentration of ions in the electrolytes
Types of electrodes

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How does the position of ions in the electrochemical series affect the selective discharge of ions?

The lower the position of the ion in the electrochemical series, the more likely that it will be discharged

Only cations such as ____, ____ and ____ which are less reactive than H+ will be discharged in aqueous solution Otherwise, ____ will be reduced to ____

Cu2+, Ag+ and Au+ H+ H2

____ and ____ are anions with very low priority or tendency of being discharged (usually not discharged even if solution is concentrated)

NO3- and SO42‒

Electrochemical series: Cations

``` K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe2+ Sn2+ Pb2+ H+ Cu2+ Ag+ Au+ ```

Electrochemical series: Anions

``` F‒ SO42‒ NO3‒ Cl‒ Br‒ I‒ OH‒ ```

How does the concentration of ions in the electrolytes affect the selective discharge of ions?

The higher the concentration of the ion, the more likely that it will be discharged at the electrode

Compare the influence of: (1) Position of ions in the electrochemical series (2) Concentration of ions in the electrolytes

The influence of (2) is usually secondary to that of (1), and is effective only when the two rival ions are closely positioned in the series

How does the types of electrodes affect the selective discharge of ions?

Reactive electrodes that can influence the ionic discharge due to their strong affinity for certain ions

What are inert electrodes?

Electrodes that do not take part in the reactions and are not affected by the ions surrounding them

Examples of inert electrodes

Platinum and graphite electrodes

Electrolysis of aqueous sodium chloride with graphite electrodes CATHODE: Competing ions

Na+, H+

Electrolysis of aqueous sodium chloride with graphite electrodes CATHODE: Ions selectively discharged and why

H+ | H+ ion is lower than Na+ ion in the electrochemical series and is easier to discharge

Electrolysis of aqueous sodium chloride with graphite electrodes CATHODE: Half Equation

2H+(aq) + 2e‒ → H2(g)

Electrolysis of aqueous sodium chloride with graphite electrodes CATHODE: Observation

Effervescence observed. Colourless, odourless gas evolved which extinguished a lighted splint with a ‘pop’ sound. The gas is hydrogen.

Electrolysis of aqueous sodium chloride with graphite electrodes CATHODE: Products

Hydrogen gas

Electrolysis of aqueous sodium chloride with graphite electrodes ANODE: Competing ions

Cl‒, OH‒

Electrolysis of aqueous sodium chloride with graphite electrodes ANODE: Ions selectively discharged and why

OH‒ | OH‒ ion is lower than Cl‒ ion in the electrochemical series and is easier to discharge

Electrolysis of aqueous sodium chloride with graphite electrodes ANODE: Half Equation

4OH‒(aq) → O2(g) + 2H2O(l) + 4e‒

Electrolysis of aqueous sodium chloride with graphite electrodes ANODE: Observation

Effervescence observed. Colourless and odourless gas evolved which rekindled a glowing splint. The gas is oxygen.

Electrolysis of aqueous sodium chloride with graphite electrodes ANODE: Products

Oxygen gas and water

Electrolysis of aqueous copper(II) sulfate with graphite electrodes CATHODE: Competing ions

Cu2+, H+

Electrolysis of aqueous copper(II) sulfate with graphite electrodes CATHODE: Ions selectively discharged and why

Cu2+ | Cu2+ ion is lower than H+ ion in the electrochemical series and is easier to discharge

Electrolysis of aqueous copper(II) sulfate with graphite electrodes CATHODE: Half Equation

Cu2+(aq) + 2e‒ → Cu(s)

Electrolysis of aqueous copper(II) sulfate with graphite electrodes CATHODE: Observation

A reddish-brown solid is deposited on the cathode

Electrolysis of aqueous copper(II) sulfate with graphite electrodes CATHODE: Products

Copper

Electrolysis of aqueous copper(II) sulfate with graphite electrodes ANODE: Competing ions

SO42‒, OH‒

Electrolysis of aqueous copper(II) sulfate with graphite electrodes ANODE: Ions selectively discharged and why

OH‒ | OH‒ ion is lower than SO42‒ ion in the electrochemical series and is easier to discharge

Electrolysis of aqueous copper(II) sulfate with graphite electrodes ANODE: Half Equation

4OH‒(aq) → O2(g) + 2H2O(l) + 4e‒

Electrolysis of aqueous copper(II) sulfate with graphite electrodes ANODE: Observation

Effervescence observed. Colourless and odourless gas evolved which rekindled a glowing splint. The gas is oxygen.

Electrolysis of aqueous copper(II) sulfate with graphite electrodes ANODE: Products

Oxygen gas and water

Electrolysis of concentrated sodium chloride with graphite electrodes CATHODE: Competing ions

Na+, H+

Electrolysis of concentrated sodium chloride with graphite electrodes CATHODE: Ions selectively discharged and why

H+ | H+ ion is lower than Na+ ion in the electrochemical series and is easier to discharge

Electrolysis of concentrated sodium chloride with graphite electrodes CATHODE: Half Equation

2H+(aq) + 2e‒ → H2(g)

Electrolysis of concentrated sodium chloride with graphite electrodes CATHODE: Observation

Effervescence observed. Colourless, odourless gas evolved which extinguished a lighted splint with a ‘pop’ sound. The gas is hydrogen.

Electrolysis of concentrated sodium chloride with graphite electrodes CATHODE: Products

Hydrogen gas

Electrolysis of concentrated sodium chloride with graphite electrodes ANODE: Competing ions

Cl-, OH‒

Electrolysis of concentrated sodium chloride with graphite electrodes ANODE: Ions selectively discharged and why

Cl‒ | Concentration of Cl‒ ion is higher than that of OH‒ ion, hence it will be discharged

Electrolysis of concentrated sodium chloride with graphite electrodes ANODE: Half Equation

2Cl‒(aq) → Cl2(g) + 2e‒

Electrolysis of concentrated sodium chloride with graphite electrodes ANODE: Observation

Effervescence observed, greenish-yellow and pungent gas evolved/pale green solution observed On adding universal indicator to the solution after electrolysis, green universal indicator turns purple – pH is increased due to higher concentration of OH‒ ions when H+ ions are discharged.

Electrolysis of concentrated sodium chloride with graphite electrodes ANODE: Products

Chlorine gas

Electrolysis of aqueous copper(II) sulfate with copper electrodes CATHODE: Competing ions

Cu2+, H+

Electrolysis of aqueous copper(II) sulfate with copper electrodes CATHODE: Ions selectively discharged and why

Cu2+ | Cu2+ ion is lower than H+ ion in the electrochemical series and is easier to discharge

Electrolysis of aqueous copper(II) sulfate with copper electrodes CATHODE: Half Equation

Cu2+(aq) + 2e‒ → Cu(s)

Electrolysis of aqueous copper(II) sulfate with copper electrodes CATHODE: Observation

Cathode is coated with a reddish-brown deposit and increases in size

Electrolysis of aqueous copper(II) sulfate with copper electrodes CATHODE: Products

Copper

Electrolysis of aqueous copper(II) sulfate with copper electrodes ANODE: Competing ions

SO42‒, OH‒

Electrolysis of aqueous copper(II) sulfate with copper electrodes ANODE: Ions selectively discharged and why

Cu | Copper is a reactive electrode and will readily be oxidised to form Cu2+ ions

Electrolysis of aqueous copper(II) sulfate with copper electrodes ANODE: Half Equation

Cu(s) → Cu2+(aq) + 2e‒

Electrolysis of aqueous copper(II) sulfate with copper electrodes ANODE: Observation

Anode dissolves and reduces in size

Electrolysis of aqueous copper(II) sulfate with copper electrodes ANODE: Products

Cu2+ ions

What are the industrial applications of electrolysis?

1. Electrolytic purification of copper 2. Electroplating of metal 3. Extraction of metals from their ores

Electrolytic purification of copper | What is the anode?

Impure copper

Electrolytic purification of copper | What is the cathode?

Pure copper

Electrolytic purification of copper | What is the electrolyte?

An aqueous salt of the metal that is being purified | e.g. aqueous copper(II) sulfate

Electrolytic purification of copper | What happens at the cathode?

Cu2+ ions are discharged. They take in electrons to form Cu atoms (reduction). Cu2+(aq) + 2e‒ → Cu(s)

Electrolytic purification of copper | What happens at the anode?

Cu atoms become Cu2+ ions (oxidation). | Cu(s) → Cu2+(aq) + 2e‒

Electrolytic purification of copper | Observations

Impure copper anode decreases in mass Impurities from the anode fall to the bottom A layer of pure copper is deposited at the cathode

Why does copper need to be pure?

Copper is used in circuits because of its high electrical conductivity. Small quantities of impurities in the copper can significantly decrease its conductivity.

Electroplating of metal | What is the anode?

The plating metal

Electroplating of metal | What is the cathode?

The object to be plated

Electroplating of metal | What is the electrolyte?

An aqueous salt of the plating metal

Electroplating of metal | What happens at the cathode?

Ag+ ions are discharged. They take in electrons to form Ag atoms (reduction).

Electroplating of metal | What happens at the anode?

Ag atoms become Ag+ ions (oxidation). | Ag(s) → Ag+(aq) + e‒

Electrolysis of purified aluminium oxide | What is the anode?

Graphite anodes

Electrolysis of purified aluminium oxide | What is the cathode?

Graphite cell lining

Electrolysis of purified aluminium oxide | What is the electrolyte?

Aluminium oxide dissolved in molten cryolite, an aluminium compound with a lower melting point (to reduce some of the energy costs)

Electrolysis of purified aluminium oxide | What happens at the cathode?

Aluminium metal forms and sinks to the bottom of the tank, where it is tapped off (reduction). Al3+(l) + 3e‒ → Al(l)

Electrolysis of purified aluminium oxide | What happens at the anode?

Oxygen forms and reacts with the carbon to form carbon dioxide (oxidation). (The anode gradually burns away and has to be replaced frequently, adding to the cost) 2O2‒(l) → O2(g) + 4e‒

Electrolysis of purified aluminium oxide | Overall equation

2Al2O3(l) → 4Al(l) + 3O2(g)

What does a simple voltaic cell consist of?

1. Two different metal electrodes | 2. Electrolyte

Simple voltaic cell | Metals high up in the reactivity series give up electrons ________ readily

Simple voltaic cell | The anode is the ________ reactive metal while the cathode is the ________ reactive metal.

more | less

Simple voltaic cell | Electrons move from the ________ to the ________ through the ________

anode cathode wire

Simple voltaic cell | The further apart the metals are in the metal reactivity series, the ________ the voltage produced

higher

Why is the charge on the simple voltaic cell reversed from that of the electrolytic cell?

At the anode, electrons that are left behind from the oxidation reaction give the anode a slight negative charge. At the cathode, electrons are spontaneously taken in to produce neutral atoms but the ions at the cathode behave as though it is part of the cathode, hence it takes on a slight positive charge.

Chapter 3 - Electrolysis Flashcards

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