Chemistry: Topic 4: Chemical changes

What do metals react with oxygen to produce?

Metal oxides

What type of reaction is this and why?

Oxidation - the metals gain oxygen

What is reduction?

Loss of oxygen

What is oxidation?

Gain of oxygen

What happens when metals react with other substances?

The metal atoms form positive ions

What is the reactivity of a metal related to?

Its tendency to form positive ions

What is the reactivity series?

A way of rearranging metals in terms of their reactivity

What extra elements are sometimes included in the reactivity series?

Non-metals Hydrogen and Carbon

What is the order if the reactivity series?

• Potassium

• Sodium

• Lithium

• Calcium

• Magnesium

• Carbon

• Zinc

• Iron

• Hydrogen

• Copper

Reactivity series pneumonic

• Pop

• Stars

• Lives

• Can

• Make

• Ca

• Zillions

• In

• Happy

• Canada

What can a more reactive metal do?

Displace a less reactive metal from a compound

What is the reaction of potassium with water like?

Violent

What is the reaction of sodium with water like?

Very quick

What is the reaction of lithium with water like?

quick

What is the reaction of calcium with water like?

slower

What is the reaction of calcium with dilute acid like?

very quick

What is the reaction of magnesium with dilute acid like?

quick

What is the reaction of zinc with dilute acid like?

fairly slow

What is the reaction of iron with dilute acid like?

slower

What is the reaction of copper with dilute acid like?

very slow

What does the reactivity of a metal with water or dilute acid mean in terms of ion?

The more reactive the metal with water or dilute acids, the more likely the metal is to form its positive ion?

Give an example of an unreactive metal

Gold

Where are unreactive metals found and how?

In the Earth - as the metal itself, but most are found as compounds that require chemical reactions to extract the metal

How are metals less reactive than carbon extracted from their oxides?

By reduction with carbon (loss of oxygen)

What is oxidation in terms of electrons?

The loss of electrons

What is reduction in terms of electrons?

The gain of electrons

What do acids react with some metals to produce?

Salts and Hydrogen

What are acids neutralised by?

Alkalis and bases

Give an example of an alkali

Soluble metal hydroxides

Give 2 examples of bases

• Insoluble metal hydroxides

• Metal oxides

What does a neutralisation reaction (acid + alkali/ base) produce?

Salts + Water

What else can acid be neutralised by?

Metal carbonates

What does a neutralisation reaction (acid + metal carbonate) produce?

Salts + Water + Carbon Dioxide

What does the salt produced from any reaction between an acid and a base/ alkali depend on?

• The acid used

• The positive ions in the base/ alkali/ carbonate

What salt does a neutralisation with hydrochloric acid produce?

A chloride

What salt does a neutralisation with nitric acid produce?

A nitrate

What salt does a neutralisation with sulfuric acid produce?

A sulfate

How can soluble salts be made from acid?

By reacting them with solid insoluble substances

Give 4 examples of solid insoluble substances that can be used to make soluble salts

• Metals

• Metal Oxides

• Hydroxides

• Carbonates

When making salts, how much solid is added to the acid?

Until no more reacts

What happens after the solid and acid has reacted?

The excess solid is filtered off to produce a solution of the salt

What can you do to salt solutions?

Crystallise them to produce solid salts

What do acids produce in aqueous solutions?

hydrogen ions (H⁺)

What do aqueous solutions of alkalis contain?

Hydroxide ions (OH^-)

What does the pH scale range between?

0 - 14

What is the pH scale?

A measure of the acidity or alkalinity of a solution

How can pH be measured?

• Using UI (universal indicator)

• Using a pH probe

What is a pH does a neutral solution have?

7

What have pH values less than 7?

Aqueous solutions of acids

What have pH values greater than 7?

Aqueous solutions of alkalis

What is the word equation for a neutralisation reaction between an acid and alkali (in terms of H and O)?

Hydrogen ions + Hydroxide ions > Water

What is the equation for this?

H⁺ (aq) + OH^- (aq) > H₂O (l)

How can you measure the volumes of acid and alkali solutions that react with each other?

By titration using a suitable indicator

What happens to a strong acid in an aqueous solution?

It is completely ionised

Give 3 examples of strong acids

• Hydrochloric acid

• Nitric acid

• Sulfuric acid

What happens to a weak acid in an aqueous solution?

It is only partially ionised

Give 3 examples of weak acids

• Ethanoic acid

• Citric acid

• Carbonic acid

What is the relationship between the strength of an acid and its pH for a given concentration of aqueous solution?

The stronger acid, the lower the pH

As the pH decreases by one unit, what happens to the hydrogen ion concentration of the solution?

It increases by a FACTOR OF 10

What is the difference in terms of acids between weak/ strong and dilute/ concentrated?

• Weak/ strong = how well it ionises in an aqueous solution (degree of ionisation)

• Dilute/ concentrated = the amount of acid in a solution (amount of substance)

What happens to an acid’s pH value and hydrogen ion conc when it is neutralised?

• H⁺ conc - decreases

• pH value - increases

What happens to the ions when an ionic compound is melted or dissolved in water?

They are free to move about within the liquid/ solution

What are electrolytes?

Liquids and solutions of melted or dissolved ionic compounds which are able to conduct electricity

What happens when you pass an electric current through an electrolyte?

The ions will move to the electrodes

What electrode do positively charged ions move to?

Cathode - negative electrode

What electrode do negatively charged ions move to?

Anode - positive electrode

What happens to the ions once they are at the electrode?

They are discharged, producing elements

Describe the steps of electrolysis

• Electric current passed through electrolyte

• +ve ions - cathode, -ve ions - anode

• Ions discharged at electrodes, producing elements

What happens when a simple ionic compound is electrolysed in the molten state using inert electrodes?

• Metal is produced at cathode

• Non metal is produced at anode

Give an example of a simple ionic compound being electrolysed

• Lead bromide electrolysed

• Lead produced at cathode

• Bromine produced at anode

What can metals be extracted from molten compounds using?

Electrolysis

When is electrolysis used to extract a metal from a molten compound?

• If the metal is too reactive to be extracted by carbon reduction

• If the metal reacts with carbon

Why are large amounts of energy used in electrolysis extraction?

• To melt the compounds

• To produce the electrical current

Give an example of a metal manufactured by electrolysis

• Aluminium - from a molten mixture of Aluminium Oxide and cryolite

What is used as the anode in the electrolysis of molten AlO and cryolite?

Carbon

Why is a mixture of AlO and cryolite used as an electrolyte in its electrolysis?

• AlO has a high MP - too expensive to melt

• Adding cryolite reduces the MP and makes it cheaper to melt

Why must the positive electrode be continually replaced in the electrolysis of molten AlO and cryolite

• Oxygen is formed at the anode

• This reacts with the carbon anode to form CO₂

• This gradually burns away the anode

Why does the ions discharged when an aqueous solution is electrolysed depend on?

The relative reactivity of the elements involved

What is produced at the -ve (cathode) in aqueous electrolysis?

Hydrogen - if the metal is more reactive than hydrogen

less reactive goes to cathode

What is produced at the +ve (anode) in aqueous electrolysis?

Oxygen gas (from OH) - unless the solution contains halide ions

less complex ion goes to anode

Why are H and O sometimes produced at the electrodes in aqueous electrolysis?

• water molecules are breaking down - produces hydrogen + hydroxide ions which are discharged

What type of reaction happens at the cathode in electrolysis?

Reduction - positively charged ions gain electrons

What type of reaction happens at the anode in electrolysis?

Oxidation - negatively charged ions lsoe electrons

What is the half equation for hydrogen at the cathode in aqueous electrolysis?

2H⁺ + 2e^- > H₂

What is the half equation for hydroxide at the anode in aqueous electrolysis?

4OH^- > O₂ + 2H₂O + 4e^-