Chemistry - 5 Chemical Changes

Alkali

a soluble hydroxide

Base

substance that can neutralise acids

Acid

any compound that forms H⁺ ions in solution

Neutral

neither acidic nor alkaline

Dissolving sodium hydroxide in water

sodium hydroxide (water→) sodium ions(aq) + hydroxide ions(aq)

Indicators [4]:

- litmus paper
- universal indicator
- phenolphthalein
- methyl orange

Litmus paper [3]

acid: red
neutral: no change
basic: blue

Universal indicator

an indicator with a different colour for each pH value.

Phenolphthalein [3]

acid: colourless
neutral: colourless
basic: pink

Methyl orange [3]

acid: red-orange
neutral: yellow
basic: yellow

pH scale

measurement system used to indicate the concentration of hydrogen ions (H+) in solution; ranges from 0 to 14

Neutral solution + universal indicator

green

Acidic solution + universal indicator

red - yellow

Basic solution + universal indicator

blue - purple

pH meter

a device used to measure the pH of a solution

Equivalence point

the point in a titration where the number of moles of hydrogen ions equals the number of moles of hydroxide ions (middle of vertical drop)

Ionise

the reaction of a molecular substance with a solvent to form ions in solution.

Acids are classed as strong or weak depending on how they...

ionise in water

In aqueous solutions, acid molecules...

ionise and release H⁺ ions

Strong acids ... in aqueous solutions

fully ionise

How can we tell if an acid is weak?

it is a reversible reaction

Weak acids ... in aqueous solutions

partially ionise

Carbonic acid

H₂CO₃

Ethanoic acid

CH₃COOH

Citric acid

C₆H₈O₇

As the pH scale decreases by one unit...

the concentration of hydrogen ions increases by ten times

Concentration of acid

number of moles of acid molecules per unit of volume

Concentration vs. strength

concentration is the amount of solute dissolved in a given amount of solution while strength refers to the solute's tendency to form ions in water

Ethanoic acid dissolving in water

CH₃COOH ⇌ CH₃COO⁻ + H⁺

All acids contain...

hydrogen ions

Hydrochloric acid

HCl

Sulfuric acid

H₂SO₄

Nitric acid

HNO₃

What metals can react with acids?

metals more reactive than hydrogen (reactivity series)

What is the reaction between a metal and an acid?

a displacement reaction; the more reactive metal displaces the less reactive hydrogen in its acid

Salt

a compound formed when the hydrogen in an acid is wholly, or partially, replaced by metal (or ammonium) ions

Metal and acid

metal + acid → salt + hydrogen

Hydrochloric acid produces metal ... salts

chloride

Sulfuric acid produces metal ... salts

sulfate

Why does magnesium react rapidly with acid?

magnesium is more reactive than hydrogen; it has a stronger tendency to form positive ions

Chloride ions

Cl⁻

Sulfate ions

SO₄²⁻

Nitrate ions

NO₃⁻

The metal is ...

oxidised

The hydrogen ions are ...

reduced

The reaction of a metal and an acid is a ... reaction

redox

Base

compounds that can neutralise acids

An alkali is a base that...

is soluble in water

acid and base

acid + base → salt + water

Ammonium ion

NH₄⁺

Limiting reactant

a reactant that is totally consumed during a chemical reaction, limits the extent of the reaction, and determines the amount of product

Acid and alkali

acid + alkali → salt + water

In neutralisation, the ... reacts with the ... to form water

H⁺, OH⁻

Ionic equation for neutralisation

H⁺(aq) + OH⁻(aq) → H₂O(l)

Acid and ammonia solution

acid + ammonia solution → ammonium salt + water

Ammonia + water

NH₃ + H₂O → NH₄OH

How would you create a pure, dry sample of crystals from the reaction of an acid and an alkali? [3]

- carry out titration with indicator to find how much alkali completely reacts with the acid
- combine those volumes again, without indicator
- crystallise and dry crystals

Acid and metal carbonate

acid + metal carbonate → salt + water + carbon dioxide

Ore

a rock that contains a large enough concentration of a mineral making it profitable to mine

Oxidation

loss of electrons

Reduction

gain of electrons

Reactivity series

a list of metals which shows them in order of their reactivity, with the most reactive at the top

alkali metal and water

alkali metal + water → metal hydroxide + hydrogen

How can we test for orders of reactivity? [2]

- water (more reactive elements)
- dilute acid (less reactive elements)

Reactivity series:

potassium
sodium
lithium
calcium
magnesium
aluminium
CARBON
zinc
iron
tin
lead
HYDROGEN
copper
silver
gold

Reactivity series mnemonic

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Displacement reaction

a reaction in which a more reactive element displaces a less reactive element from an aqueous solution of the latter's ions.

A more reactive metal will...

displace a less reactive metal from an aqueous solution of one of its salts

Ionic equation

an equation in which ions are explicitly shown

Spectator ion

ion that does not participate in a reaction (not oxidised or reduced)

OILRIG

oxidation is loss, reduction is gain

Half equation

an equation for a redox reaction which considers just one of the species involved and shows explicitly the electrons transferred to or from it.

Redox reaction

an oxidation reaction paired with a reduction reaction

Displacement reactions are also known as...

redox reactions

Oxidising agent

substance that gains electrons

Reducing agent

substance that loses electrons

Where are metals from?

the Earth's crust

Whether it is worth extracting a particular metal depends on [3]:

- how easy it is to extract from its ore
- how much metal the ore contains
- the changing demands for a particular metal

How is copper extracted from copper ores? [5]

- ore is ground up into powder
- mixed with water and chemical that makes copper compound repel water
- air is bubbled through mixture
- copper compound floats to top as froth while rocky bits sink
- concentrated compound is scraped off to be extracted from

How are unreactive metals often found?

in their native state rather than as compounds

metal oxide and carbon

metal oxide + carbon → metal + carbon dioxide

What metals can be reduced using carbon?

metals less reactive than carbon

What metals can be reduced using hydrogen?

metals less reactive than hydrogen

How are metals more reactive than carbon extracted?

electrolysis of the molten metal compound