4.4 - Chemical Changes

What is the equation for concentration

concentration = moles / volume (mol/dm³)

concentration = mass/ volume (g/dm³)

Which 2 non-metals are also included in the reactivity series

Carbon and Hydrogen

What is the definition of reactivity in metals

The tendency of a metal to form positive ions

What is a displacement reaction

The occurrence of a more reactive element displacing a less reactive element in a compound

e.g. Iron Sulphate + Magnesium → Magnesium Sulphate + Iron

or Lithium Bromide + Chlorine → Lithium Chloride + Bromine

Define Oxidation

The loss of electrons/ the gain of oxygen

Define Reduction

The gain of electrons/ the gain of hydrogen

What is the term for when both oxidation and reduction occur in a reaction

Redox

What does OILRIG stand for

Oxidation

Is

Loss (of electrons)

Reduction

Is

Gain (of electrons)

What type of reaction is the reaction between oxygen and metals

Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen.

In this reaction what has been oxidised, and what has been reduced:

Mg + CuSO₄ → MgSO₄ + Cu

Magnesium has lost electrons to form Mg²⁺ - it has been oxidised

Copper/Copper sulphate has lost electrons to form Cu- it has been reduced

Write an ionic equation for this reaction, as well as the half equations for the redox reactions

Mg + CuSO₄ → MgSO₄ + Cu

Mg + Cu²⁺ + SO₄^2- → Mg²⁺ + SO₄^2- + Cu

SO₄^2- appears on both sides, and doesn't change in the reaction, so we can remove it for the ionic equation

↓

Mg + Cu²⁺ → Mg²⁺ + Cu

We can write this as 2 half equations

Mg → Mg²⁺ + 2e^- Oxidation

Cu²⁺ + 2e^- → Cu Reduction

How are most metals found in the earth

Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal.

What is the definition of an ore

A rock that contains a high enough proportion of metal to be financially viable to extract

How can metals less reactive than carbon, such as iron or copper, be extracted from ores

Using carbon reduction

e.g. 2Fe₂O₃ + 3C → 3CO₂ + 4Fe

Why can’t metals more reactive than carbon be extracted using carbon reduction

They won’t be displaced by carbon in a compound, so they have to be extracted using electrolysis

What is the formula for hydrochloric acid

HCl

What is the formula for nitric acid

HNO₃

What is the formula for sulphuric acid

H₂SO₄

What is the general equation for a metal and acid reaction

metal + acid → salt + hydrogen

What is the general equation for metal oxide and acid reaction

metal oxide + acid → salt + water

What is the general formula for an alkali (like a metal hydroxide) and acid reaction

alkaline + acid → salt + water

What is the general equation for a metal carbonate and acid reaction

metal carbonate + acid → salt + water + carbon dioxide

What is the difference between a base and an alkali

Any substance that can neutralise an acid to produce salt and water, like a metal oxide or a metal hydroxide

An alkali is any base which is soluble in water

A copper oxide is a base, but is not an alkali as it isn’t soluble in water

Describe a method to produce a soluble salt from an insoluble base

• Measure 20cm³ of acid using a measuring cylinder, and pour into a beaker. Gently heat the beaker using a tripod, gauze and bunsen burner, don’t boil.

• While still heating, add spatulas of insoluble base to the acid and stir, until it no longer dissolves and is in excess; this ensures the reaction is complete, and no acid remains

• We add excess base instead of acid as it can be easily filtered out, whereas the acid would be harder to remove

• Filter out the excess insoluble salt using filter paper and a funnel into a conical flask

• Pour the salt solution into an evaporating basin, and place on a beaker of water on the bunsen apparatus. Heat gently until crystals begin to form at the edges, then remove from the heat and leave the solution to evaporate on a windowsill

• When all the water has evaporated, there will be a pure, dry sample of soluble salt crystals

What makes something an acid

When dissolved in water it releases H⁺ ions

e.g. HCl (g) → H⁺ (aq) + Cl^- (aq)

They have pHs from 0 to (not including) 7

What makes something an alkali

When dissolved in water it releases OH^- ions

e.g. NaOH (s) → Na⁺ (aq) + OH^- (aq)

What is the difference between a strong an weak acid

A strong acid fully ionises/dissociates in water

e.g. HCl (g) → H⁺ (aq) + Cl^- (aq)

While a weak acid partially ionises/dissociates in water

e.g CH₃COOH ⇌ H⁺ + CH₃COO^-

What is the difference between a concentrated and a dilute acid

A concentrated acid has a large amount of acid particles per unit volume of water, while a dilute acid has a small amount of acid particles per unit of volume

What is the equation for neutralisation

OH^- (aq) + H⁺ (aq) → H₂O (l)

What are some ways to calculate pH

• Litmus paper → Red = acid, Blue = alkali

• Universal indicator → Red-Yellow = Acid, Green = Neutral, Cyan-Purple = Alkali

• pH probe

• Phenolphthalein → Colourless = Acid, Pink = Alkali

Describe how to find the concentration of an unknown alkali solution (in this case sodium hydroxide)→ titration

1. Using a volumetric pipette and a pipette filler, measure 50cm³ of sodium hydroxide- ensure to hold the pipette near the top when attaching the pipette filler, to avoid it shattering and causing cuts- and empty it into a conical flask, and place the conical flask onto a white tile
2. Place the burette on a stand in a clamp and boss so that it is straight, and then place it on a chair, around eye level, to allow safer pouring of acid and avoid corrosive damage
3. Fill up the burette with hydrochloric acid of concentration 0.1 mol/dm³ to 0, or to another measurement on the scale, as long as that is factored in later
4. Put a few drops of phenolphthalein into the conical flask, and swirl it around- it should turn a deep pink
5. Place the burette over the conical flask, so that the tap will pour directly into it
6. Gradually release small amounts of hydrochloric acid from the burette, and continue swirling the conical flask, until the solution turns colourless
7. Minus the starting measurement from the end measurement, and record the result
8. Repeat steps 1-7 until concordant titres are achieved ( 3 results which are within 0.1 cm³ of each other) 9. Calculate the mean of the concordant results, and using the mean volume of hydrochloric acid, the concentration of hydrochloric acid, and the volume of sodium hydroxide, calculate the estimated concentration of sodium hydroxide

What is electrolysis

The separation of molten or dissolved ionic compounds substances using electricity.

Describe what happens to ions in electrolysis

• When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution.

• These liquids and solutions are able to conduct electricity and are called electrolytes.

• Passing an electric current through electrolytes causes the ions to move to the electrodes.

• Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode).

• Ions are discharged at the electrodes producing elements.

When would electrolysis be used

Electrolysis is used if the metal is too reactive to be extracted by reduction with carbon or if the metal reacts with carbon.

Why are substances which are extracted via electrolysis often more expensive than those that aren’t

Large amounts of energy are used in the extraction process to melt the compounds and to produce the electrical current.

Which two pieces of equipment are needed for electrolysis

• Graphite electrodes

• DC power supply (battery or cell)

What is an anion

A negative ion

(anion sounds like onion, onions make you cry, so they are negative)

What is a cation

A positive ion

(CATions are PAWSitive)

What is a cathode

A negative electrode

What is an anode

A positive electrode

What is an electrolyte

An ion containing medium, like salt water

What are the two forms of an ionic compound that can take part in electrolysis

Molten or solution

What takes place at the anode

Oxidation

What takes place at the cathode

Reduction

What are electrodes made of in electrolysis

Graphite

Why is cryolite added to molten aluminium oxide in electrolysis

To lower the melting point from about 2000^oC to about 950^oC, so less energy is required to melt it, so it costs less money

What happens at the anode during electrolysis of molten aluminium oxide, including a half equation

The O^2- anions are attracted to the anode, as it is positive, and opposite charges attract. They each lose 2 electrons to form oxygen atoms, which then bond to form O₂ molecules. This is oxidation

2O^2- → O₂ + 4e^-

What happens at the cathode during electrolysis of molten aluminium oxide, including a half equation

The Al³⁺ cations are attracted to the negative cathode, because opposite charges attract. They each gain 3 electrons to form Al atoms. This is reduction

Al³⁺ + 3e^- → Al

Why does the anode need to be replaced during electrolysis of molten aluminium oxide

Due to the high temperature, the oxygen produced at the anode reacts with the carbon (graphite) anode to produce carbon dioxide, causing it to wear away, so will need to be continuously replaced, about every month.

Which ions are present in brine (salt water)

• Na⁺

• Cl^-

• H⁺

• OH^-

Which ions in brine (salt water) will go to the anode

Cl^- and OH^-

Which ions in brine (salt water) will go to the cathode

Na⁺ and H⁺

Why can oxygen and hydrogen be produced during the electrolysis of aqueous solutions

• When the current is passed through the solution, the water molecules break down to produce H⁺ and OH^- ions

• These are then attracted to the cathode and anode, respectively, where they are discharged

What happens at the anode during electrolysis of brine (salt water)

• There are halide ions present (Cl^-), so chlorine (Cl₂) is produced.

• Each Cl^- ion loses 1 electron to form chlorine gas which bubbles away- chlorine is oxidised

• 2Cl^- (aq) → Cl₂ (g) + 2e^-

• The OH^- ions remain in the solution

What happens at the cathode during electrolysis of brine (salt water)

• Sodium is higher up on the reactivity series than hydrogen, so hydrogen (H₂) is produced

• Each H⁺ ion gains 1 electron to form hydrogen gas which bubbles away- hydrogen is reduced

• 2H⁺ (aq) + 2e^- → H₂ (g)

• The Na⁺ ions remain in the solution

What is left at then end of electrolysis of brine (salt water)

The Na⁺ ions and OH^- ions are left, so the remaining solution is a sodium hydroxide solution

What is the rule for what is produced at the cathode during electrolysis of solutions

• Whichever is the least reactive is the one which is reduced and produced, metal or hydrogen

• The other ion remains in the solution

What is the rules for what is produced at the anode during electrolysis of solutions

• If halide ions are present, then they are oxidised and halogen molecules are produced

• Otherwise, oxygen is produced

How are hydrogen ions reduced

2H⁺ + 2e^- → H₂

How are hydroxide ions oxidised

4OH^- → O₂ + 2H₂O + 4e^-