AQA Chemistry 4: Chemical Changes

What do metals produce when they react with oxygen?

Metal oxides

What type of reaction is a metal reacting with oxygen?

Oxidation

The metal gains oxygen

What is the reactivity of a metal related to?

Its tendency to form positive ions

What type of ions do metals form when they react with other substances?

Positive (cations)

Which non metals are often included in the reactivity series?

Hydrogen and carbon

What can a more reactive metal do to a less reactive metal?

Displace it from a compound

Which metals react with dilute acids?

Metals above hydrogen

The more reactive the metal, the more vigorous the reaction

What is formed when metals react with acids?

Produce a salt and hydrogen gas

[HT only] What type of reaction is metals reacting with acids?

Redox reaction

Which metals react with water?

Potassium (violent)

Sodium (quick)

Calcium (less strongly)

What makes a metal more reactive?

The more readily it lose its outer electron(s) to form positive ions

Where are unreactive metals found?

Eg gold - found in the Earth as the metal itself

In what form are most metals found?

As compounds that require chemical reactions to extract the metal

How are metals less reactive than carbon extracted?

Reduction with carbon.

Reduction involves the less of oxygen

[HT only] In terms of electrons, what is reduction and oxidation?

Oxidation is the loss of electrons and reduction is the gain of electrons.

How are acids neutralised?

By alkalis and bases

What is produced in a neutralisation reaction?

Salt and water

What is produced when acids react with metal carbonates?

Salt, water, carbon dioxide

What does the salt produced in acid reactions depend on?

The acid used

The positive ions in the base, alkali or carbonate

Hydrochloric acid produces...

chlorides

Nitric acid produces...

nitrates

Sulfuric acid produces...

sulfates

How can soluble salts be made from acids?

-React them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates.

-The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.

-Salt solutions can be crystallised to produce solid salts

What do acids produce in aqueous solutions?

H⁺ ions (hydrogen)

Aqueous solutions of alkalis contain what ions?

OH⁻ ions (hydroxide)

The pH scale goes from ? to ?

0 to 14

What pH is neutral?

pH 7

Which pHs are acids?

Up to pH 7

Which pHs are alkalis?

Greater than pH 7

What happens in terms of ions during neutralisation reactions?

Hydrogen ions react with hydroxide ions to produce water

What is the symbol equation to show what happens in terms of ions during neutralisation reactions?

H⁺ (aq) + OH⁻ (aq) ---> H₂O (l)

What can we use to test pH and how does it work?

Universal indicator

Dropped on the substance (or UI paper dipped in)

Changes colour based on pH

Red-orange-yellow = acidic

Green = neutral

Blue-purple = alkaline

[Triple only] What does titration do?

Calculate volumes of acids and alkalis that react with each other

[Triple only] How is a titration carried out?

1. Use the pipette and pipette filler to add a measured volume of sodium hydroxide solution to a clean conical flask.

2. Add a few drops of indicator and put the conical flask on a white tile.

3. Fill the burette with hydrochloric acid and note the starting volume.

4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.

5. Stop adding the acid when the end-point is reached (when the indicator first permanently changes colour). Note the final volume reading.

6. Repeat steps 1 to 5 until concordant titres are obtained. More accurate results are obtained if acid is added drop by drop near to the end-point.

[Triple only] What is a strong acid?

A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids.

[Triple only] What is a weak acid?

A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids.

[Triple only] For a given concentration of an aqueous solution, how does strength of acid change pH?

Stronger acid = lower pH

[Triple only] As pH decreases by one unit, the hydrogen ion concentration of the solution...

...increases by a factor of 10

[Triple only] How can an acid be strong and dilute?

Strength = how much it ionises in an aqueous solution

Dilute = concentration, how many particles of acid are in the aqueous solution.

What is electrolysis?

The breaking down of a substance using electricity

What is an electrolyte?

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.

What happens in terms of ions during electrolysis?

Positively charged ions (cations) move to the negative electrode (cathode)

Negatively charged ions (anions) move to the positive electrode (anode)

Ions are discharged at the electrodes, which produces elements

What happens in electrolysis of molten simple ionic compounds?

The metal is produced at the cathode

The non metal is produced at the anode

Eg electrolysis of lead bromide:

Lead at the cathode

Bromine at the anode

When is electrolysis used to extract metals?

When the metal is too reactive to be extracted by carbon, or if the metal reacts with carbon

What is energy used for in electrolysis?

Large amounts!

To melt the compound

To produce the electrical current

How is aluminium manufactured?

Electrolysis of a molten mixture of aluminium oxide and cryolite

Why is cryolite used in the electrolysis of aluminium oxide?

Lowers the melting point of the mixture

Less energy is required

Why does the positive electrode in aluminium electrolysis need to be continually replaced?

The electrode is made of carbon

It reacts with oxygen to form carbon dioxide

It needs to be replaced as it is 'used up' by this reaction

What do the ions discharged in aqueous electrolysis depend on?

The relative reactivity of the elements in the electrolysis

What is the rule for products at the cathode, for aqueous electrolysis?

If the metal is more reactive than hydrogen: hydrogen is produced

If the metal is less reactive than hydrogen: the metal is produced

What is the rule for products at the anode, for aqueous electrolysis?

Oxygen is produced

UNLESS! The solution contains halide ions...

If the solution contains halide ions, the halogen is produced

What is the reasoning behind the anode and cathode rules for aqueous solutions?

In the aqueous solution:

-water molecules break down producing hydrogen ions -hydroxide ions that are discharged.

[HT only] What happens at the cathode in terms of ions?

Positively charged ions gain electrons

The reactions are reductions

[HT only] What happens at the anode in terms of ions?

Negatively charged ions lose electrons

The reactions are oxidations.

[HT only] How can we represent reactions at electrodes?

As half equations

These represent electrons as 'e