IGCSE CHEM CHAP 4 (CORRECT VERSION)

Hydrogen-oxygen fuel cells only use hydrogen and oxygen to produce a voltage (electricity) with water as its only chemical product

What is a hydrogen-oxygen fuel cell

Hydrogen + Oxygen -> Water; 2H₂ (g) + O₂ (g) → 2H₂O (l)

Give the overall word and symbol equation for the reaction in a hydrogen-oxygen fuel cell

Less carbon dioxide emissions than petrol/diesel engines since water is the only product; Reduces burning of fossil fuels; Hydrogen can be obtained from renewable energy sources

Describe the advantages of hydrogen-oxygen fuel cells (extended only)

Not as durable and long-lasting as petrol/diesel engines; Production of hydrogen can release CO₂; Storage of hydrogen is difficult, dangerous and expensive to maintain; More expensive than petrol/diesel engines

Describe the disadvantages of hydrogen-oxygen fuel cells (extended only)

The breakdown of a molten or aqueous ionic compound using electricity (an electric current being passed through)

What does the term electrolysis mean?

Cathode - negatively charged electrode; Anode - positively charged electrode

What is the cathode and anode?

The electrolyte is an ionic compound in the molten or aqueous state that undergoes electrolysis; Ions in the electrolyte are able to move freely and are attracted towards the electrode with an opposite charge

What is an electrolyte?

If the metal is less reactive than hydrogen in the reactivity series then the metal will form; If the metal is more reactive than hydrogen then hydrogen will form

How can you predict whether a metal or hydrogen will form at the negative electrode during electrolysis of an aqueous compound?

Anode: Bromine; Cathode: Lead

What forms at each electrode during the electrolysis of molten lead(II) bromide?

Anode: Chlorine; Cathode: Hydrogen

What forms at each electrode during the electrolysis of concentrated hydrochloric acid?

Anode: Chlorine; Cathode: Hydrogen

What forms at each electrode during the electrolysis of concentrated sodium chloride solution NaCl (aq)?

Anode: Oxygen; Cathode: Hydrogen

What forms at each electrode during the electrolysis of dilute sulfuric acid H₂SO₄ (aq)?

Anode: non-metal element; Cathode: metal element

How can the products of electrolysis of molten electrolytes be predicted?

Positive electrode: Bromine; Negative electrode: Hydrogen

Predict the products of electrolysis of concentrated potassium bromide solution

Positive electrode: Iodine; Negative electrode: Copper

Predict the products of electrolysis of concentrated copper iodide solution

A process in which a metal is coated with a layer of another metal

What is electroplating?

To increase resistance to corrosion; To improve appearance (e.g. silver plated cutlery)

What are two reasons for using electroplating?

Similar setup to electrolysis: The metal being coated is the cathode; The metal that will form the exterior layer is the anode; The electrolyte solution must contain ions of the metal which will form the outer coating; A power supply is connected to both electrodes

Explain how the process of electroplating works

The direct current power supply is connected to the cathode, providing electrons, causing it to become negatively charged; At the cathode, cations from the electrolyte are attracted and gain electrons; At the anode, anions from the electrolyte are attracted and lose electrons; These electrons released at the anode transfer from the anode back to the direct current power supply

Describe the transfer of charge during electrolysis (extended only)

Electrons move through the metallic conductor meaning they move through the wire from the positive electrode to the negative electrode; Ions move through the electrolyte; Positive ions move to the negative electrode and negative ions move to the positive electrode

Compare the movement of electrons to the movement of ions in electrolysis (extended only)

Anode: Oxygen (from OH⁻ ions); Cathode: Copper

What is formed at each electrode during the electrolysis of copper sulfate solution using inert electrodes? (extended only)

Positive electrode: Copper ions (Cu²⁺); Negative electrode: Copper

What is formed at each electrode during the electrolysis of copper sulfate solution using copper electrodes? (extended only)

Place inert carbon electrodes into copper sulfate solution; Connect electrodes to a power supply; Copper ions gain electrons at the cathode to form pure copper; Oxygen ions lose electrons at the anode to form oxygen gas

Describe the electrolysis of copper(II) sulfate solution using carbon electrodes (extended only)

The anode is impure copper and the cathode is pure copper; Electrodes are placed in copper sulfate solution; Copper ions from the impure anode move to the cathode where they gain electrons and form pure copper; Impurities form as sludge below the anode

How can impure copper be purified using copper sulfate solution and copper electrodes? (extended only)

Anode: A halogen will be formed from the halide ion (Cl⁻ → Cl₂, Br⁻ → Br₂, I⁻ → I₂); Cathode: The metal from the ionic compound

What is formed at each electrode when an aqueous electrolyte contains a halide (extended only)

Oxidation is the loss of electrons; Oxidation occurs at the anode where anions lose electrons

What is oxidation? (extended only)

Reduction is the gain of electrons; Reduction occurs at the cathode where cations gain electrons

What is reduction? (extended only)

Oxidation Is Loss of electrons; Reduction Is Gain of electrons

What does OILRIG stand for? (extended only)

Half equations show what happens when ions gain or lose electrons; Electrons are written as e⁻; Overall charge must be equal on both sides; Number of atoms must be the same on both sides

Give the features of half-equations (extended only)

Pb²⁺ + 2e⁻ → Pb; 2Br⁻ → Br₂ + 2e⁻

Write half equations for the reactions that occur at the electrodes when molten lead(II) bromide is electrolysed

Reduction (gain of electrons) occurs

What reaction occurs at the cathode in electrolysis?

Oxidation (loss of electrons) occurs

What reaction occurs at the anode in electrolysis?

The cathode is the negative electrode

What is the charge of the cathode in electrolysis?

The anode is the positive electrode

What is the charge of the anode in electrolysis?

If the metal is less reactive than hydrogen, the metal is formed

What happens at the cathode if the metal ion is less reactive than hydrogen?

If the metal is more reactive than hydrogen, hydrogen gas is formed

What happens at the cathode if the metal ion is more reactive than hydrogen?

A metal is formed at the cathode

What forms at the cathode during electrolysis of molten electrolytes?

A non-metal is formed at the anode

What forms at the anode during electrolysis of molten electrolytes?

Halogen gas forms (Cl₂, Br₂, I₂)

What forms at the anode when a halide ion is present in aqueous electrolysis?

Oxygen gas forms

What forms at the anode when no halide ions are present in aqueous electrolysis?

Positive ions move to the cathode

Where do cations move during electrolysis?

Negative ions move to the anode

Where do anions move during electrolysis?

OILRIG stands for Oxidation Is Loss, Reduction Is Gain

What does OILRIG stand for in electrolysis?

4OH⁻ → O₂ + 2H₂O + 4e⁻

Write the half equation for the oxidation of hydroxide ions at the anode.

Lead forms at the cathode and bromine forms at the anode

What are the products of electrolysis of molten lead(II) bromide?

Cathode: Pb²⁺ + 2e⁻ → Pb

What is the cathode half equation for electrolysis of molten lead(II) bromide?

Anode: 2Br⁻ → Br₂ + 2e⁻

What is the anode half equation for electrolysis of molten lead(II) bromide?

Hydrogen forms at the cathode and oxygen forms at the anode

What are the products of electrolysis of dilute sulfuric acid?

Copper forms at the cathode and oxygen forms at the anode

What are the products of electrolysis of copper sulfate solution using inert electrodes?

Copper dissolves at the anode and copper is deposited at the cathode

What happens during electrolysis of copper sulfate using copper electrodes?

Oxygen gas is formed at the anode

What forms at the anode in electrolysis of dilute aqueous solutions without halides?