Chemistry - chemical changes

What is the reactivity series?

A list of elements ordered by their reactivity

The reactivity series

Potassium

sodium

calcium

magnesium

aluminium

carbon

zinc

iron

lead

hydrogen

copper

silver

gold

Which elements in the reactivity series react with oxygen?

everyone except gold

Reaction with water (K - Ca)

fizz, give off hydrogen gas and leave behind an alkaline solution of metal hydroxide

Reaction with water (Mg - Fe)

very slow reaction

Reaction with water (Sn and Pb)

slight reaction with steam

Reaction with water (Cu - Au)

no reaction even with steam

Reaction with dilute acid (K - Li)

explodes

Reaction with dilute acid (Ca - Fe)

fizzing, giving off hydrogen gas and forming a salt

Reaction with dilute acid (Sn and Pb)

reacts slowly with warm acid

Reaction with dilute acid (Cu - Au)

no reaction

What are displacement reactions?

A reaction in which one substance replaces another. The more reactive element "pushes out" a less reactive element from a compound.

What is oxidation in terms of oxygen?

gain of oxygen.

What is reduction in terms of oxygen?

loss of oxygen

What is oxidation in terms of electrons?

loss of electrons

What is reduction in electrons?

gain of electrons

What are redox reactions?

chemical reactions that transfer electrons between reactants

Example of redox reactions

Zn + CuSO₄→ZnSO₄ + Cu

Zn is oxidised, and copper is reduced

What are ionic equations?

equations that only show the atom and ions that change and take part in a chemical reaction

What are spectator ions?

The ions that don't change in the reaction. they don't change form and appear on both sides of the equation

Ionic equation for neutralisation

H⁺(aq) + OH⁻(aq) --> H2O(l)

How are the metals above carbon extracted?

by electrolysis because of their high reactivity

How are the metals below carbon to copper extracted?

reduction with carbon in a blast furnace to cause a replacement reaction, removing the oxygen

How are metals like silver or gold extracted?

They are found as pure metals because they don't form ores

What are acids?

- Substances that release hydrogen ions in a solution

- They have a Ph less than 7

What makes solutions acidic?

A solution is acidic when it has a high concentration of hydrogen ions

Examples of acids

- hydrochloric acid (HCl)

- Nitric acid (HNO₃)

- Sulfuric acid (H₂SO₄)

What are buffers?

Chemicals that resist changes to pH (keep pH constant)

What is the pH scale?

a scale that measures how acidic or alkaline a solution is.

What is ph?

a scale used to measure the concentration of H⁺ ions

What does a low pH mean?

The solution is acidic

What does a high pH mean?

The solution is alkaline

What does a pH of 7 mean

The solution is neutral

What does a high concentration of H+ ions mean?

A solution is highly acidic and has a low pH

What does a low concentration of H+ ions mean?

It indicates a weak acid, a highly diluted acid, or an alkali

How does the pH scale measure acids?

- by the concentration of H+ ions

- the higher the concentration, the lower the pH

What does pH stand for?

potential of hydrogen

What does a decrease mean on the pH scale?

- Every decrease of 1 on the scale is an increase in H+ by a factor of 10

- e.g, 0.1 mol/dm³ of H⁺ ions = pH 1

- 0.01 mol/dm³ of H⁺ ions = pH 2

- 0.001 mol/dm³ of H⁺ ions = pH 3

What is the universal solvent?

H₂O

What is the pH of a substance with a concentration of 0.1 mol/dm³ of H⁺ ions?

pH 1

What is the pH of a substance with a concentration of 0.01 mol/dm³ of H⁺ ions?

pH 2

What is the pH of a substance with a concentration of 10⁻⁵ mol/dm³ of H⁺ ions?

pH 5

What is the pH of a substance with a concentration of 10⁻⁹ mol/dm³ of H⁺ ions?

pH 9

What is a base?

- any substance that neutralises an acid to form only salt and water

- They are typically metal oxides, metal hydroxides, or metal carbonates.

- they have a pH greater than 7

How do bases neutralise acids?

They are hydrogen ion acceptors. When they react, they accept H+ions.

What is an alkali?

- a soluble base that dissolves in water to produce hydroxide ions

- they have a pH greater than 7

What is the difference between bases and alkalis?

A base is any substance that neutralises acids to form water and salt, while an alkali is specifically a base that is soluble in water

Examples of bases

- Sodium hydroxide (NaOH)

- Lime water Ca(OH)2

- Ammonia NH3

What makes solutions alkaline?

A higher concentration of OH⁻ ions than H⁺ ions

What are alkalis used for?

cleaning, industrial manufacturing, and neutralising acids

Everyday examples of alkalis

drain cleaner, oven cleaner, and bleach use strong alkalis to break down grease and fats.

Acid + base

→salt + water

Metal + Acid

→ salt + hydrogen

Acid + alkali

→salt + water

Acid + Carbonate

→salt + water + CO₂

metal oxide + acid

→salt + water

Neutralisation ionic equation

H⁺(aq) + OH⁻→H₂O

Acid + ammonia

→ ammonium salt

What is a salt?

an ionic compound made from any acid-base reaction

What does concentrated mean?

There is lots of solute in a particular volume

What does dilute mean?

There is little solute in a particular volume

What is a concentrated acid?

an aqueous solution containing a high proportion of acid molecules and very little water

What is the pH of a concentrated acid?

0-4

What is a dilute acid?

an aqueous solution containing a low proportion of acid molecules and lots water

What is the pH of a dilute acid?

4-6

Why don't acids form ionic bonds?

because the water molecules get in the way

What is a strong acid?

- an acid that fully dissociates in aqueous solution

- they produce a higher concentration of H⁺ ions so they have a lower pH (0-4)

Why are strong acids more reactive than weak acids at the same concentration?

They have a higher concentration of H⁺ ions, which means there are more reactive particles available, leading to faster, more vigorous reactions with substances like metals, bases, or carbonates compared to weak acids.

Examples of strong acids

- hydrochloric acid (HCl)

- sulfuric acid (H₂SO₄)

- nitirc acid (HNO)

Equation for full dissociation

HCL→H⁺ + Cl⁻

What is a weak acid?

- An acid that does not completely ionise/dissociate in water

-They have a higher pH than stronger acids because they release fewer H⁺ ions (4-6)

Examples of weak acids

- Ethanoic acid,CH₃COOH

- Citric acid,C₆H₈O₇

- Carbonic acid, H₂CO₃

Equation for dissociation of weak acids

CH₃COOH ⇌ H⁺ + CH₃COO⁻

Why is the dissociation of weak acids reversible?

they only partially ionise in water, creating a dynamic equilibrium between the molecules and their ions

What are strong bases?

compounds that dissociate completely into hydroxide ions in aqueous solution, resulting in a high pH. (11-14)

Examples of strong bases

- hydroxides of alkali metals (Group 1) and alkaline earth metals (Group 2)

- LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂

What is a weak base?

- a substance that only partially ionizes or dissociates in water, producing a low concentration of hydroxide ions

- they have a lower pH than strong acids (8-11)

Why does diluting acids increase the pH?

Because it reduces the concentration of H⁺ ions

What is electrolysis?

Electrolysis is the breakdown of a substance by passing an electric current through it.

What is a buffer?

- A solution which resists a pH change when small amounts of acid or alkali are added

- they either release H⁺ to lower the pH or take up H⁺ to raise the pH

What is an electrolyte?

An ionic compound that dissociates into ions when placed in water.

What kind of compounds are used in electrolysis?

ionic compounds, either molten or dissolved in water (aqueous solution)

What are electrodes made from?

inert substances such as graphite or platinum

What is the cathode in electrolysis?

- The negatively charged electrode, where reduction (gain of electrons) occurs

- it is connected to the negative terminal of the power supply

What is the anode in electrolysis?

- the positively charged electrode where oxidation (loss) of electrons occurs

- it is connected to the positive terminal

What does PANIC stand for?

Positive

Anode

Negative

Is

Cathode

What are anions in electrolysis?

negatively charged ions in electrolysis that are oxidised by the positive electrode (anode)

What are cations in electrolysis?

positively charged ions that are reduced by the negative electrode (cathode) during electrolysis

How does electrolysis work?

- One electrode is connected to the positive terminal of the power supply. This is called the anode

- The other electrode is connected to the negative terminal and is called the cathode

When current flows:

- positive ions (cations) move to the negative electrode (cathode) and gain electrons (reduction)

- negative ions (anions) move to the positive electrode (anode) and lose electrons (oxidise)

- When the ions reach the electrodes, they lose their charge and become elements

Half equation of oxygen

4OH⁻ →O₂ + 2H₂O +4e⁻

Half equation for chlorine

2Cl(l)⁻→ Cl₂(g) + 2e⁻

Half equation for zinc

Zn²⁺(aq) + 2e⁻→ Zn(s)

Half equation for hydrogen

2H⁺ + 2e⁻ →H₂

Why are electrodes made from graphite?

- it is a good conductor of electricity

- it is inert (non-reactive)

- it is capable of withstanding extreme temperatures

- it is cost-effective

What are the rules of electrolysis in solution?

- if the metal ion is more reactive than hydrogen, hydrogen will form at the cathode

- if there is a halogen involved, the halogen will form; if not, then oxygen will form

What happens when there are two elements that can be formed at an electrode?

the more reactive metal will form

Why do electrodes need to be constantly replaced?

- Positive carbon anodes must be constantly replaced because they burn away

- The oxygen produced at the anode reacts with the hot carbon, forming carbon dioxide gas, which consumes the electrode

What is cryolite?

a mineral used as a solvent in the electrolytic extraction of aluminium from molten aluminium oxide

Why do electrodes need to be inert?

to conduct electrons without chemically reacting with the electrolyte or products

Why is cryolite used in the electrolysis of aluminium oxide?

- to lower its melting point from over to around. This significantly reduces energy consumption and costs

- acts as a solvent for the aluminium oxide

- increases electrical conductivity.