chem topic 4 (chemical changes)

Calcium reaction with water

Magnesium reaction with water

zinc, iron, copper w water

Slower reaction

Effervescence (hydrogen gas released)

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very slow reaction

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no reaction

Lithium, calcium, magnesium, zinc, iron reaction with dilute acids

Reactions become less vigorous moving along this group

Effervescence (hydrogen gas released)

Copper reaction with dilute acids

Does not react with dilute acids

Explain why reactions between acids and metals to form salts and hydrogen are examples of redox reactions

One substance is oxidised the other is reduced, due to a transfer of electrons

Give an example of an alkali

Soluble metal hydroxides

Give examples of bases

Insoluble metal hydroxides and metal oxides

Acids are neutralised by ______ and ______

Alkalis, bases

alkalis = soluble metal hydroxides // bases = insoluble metal hydroxides + metal oxides

Acid + base → ..

Acid + carbonate → ..

Salt + water

Salt + water + carbon dioxide

RP1 — method

Excess reagent

measure volume of acid (specify) using measuring cylinder, pour into beaker

heat dilute acid gently with Bunsen burner until warm (not boiling)

white tile under beaker, begin adding spatulas of base (specify) and stir continuously

until no more reacts as excess solid can be seen, which ensures all acid has reacted

use filter funnel and filter paper to remove excess solid, collect filtrate in separate beaker

pour filtrate into evaporating dish, heat gently with water bath to crystallisation point

leave to crystallise fully at room temperature

return later and pat dry with filter paper

In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water. Give the half equation 

H⁺_(aq)+ OH^-_(aq) → H₂O_(l)

RP2 — titration

Titration

measure fixed volume (state) of acid solution using pipette, transfer into conical flask

add few drops of suitable indicator (phenolpthalein used here) to acid in conical flask. should remain colourless

fill burette with alkali using filter funnel at top. remove funnel after filling

place conical flask on white tile, to see colour change clearly, and place under burette’s tap

take initial reading of burette volume at eye level, from bottom of meniscus and white tile behind

slowly open burette tap to add alkali to acid, swirling flask continuously

continue pouring until slight colour change (colourless to pink for phenolpthalein)

pour dropwise at endpoint until permanent colour change detected

take final reading of burette volume at eye level, same manner as before

record volume of alkali used (final - initial reading)

repeat several times until concordant results (within 0.1cm³ and calculate avg volume of titre used)

methyl orange and phenolpthalein in acid or alkali

methyl orange red in acid, yellow in alkali

phenolpthalein colourless in acid, pink in alkali

As the pH decreases by one unit..

The hydrogen ion concentration of the solution increases by a factor of 10

Define a dilute acid

Define a concentrated acid

Solution with small amount of acid dissolved in a given volume of water

Solution with large amount of acid dissolved in a given volume of water

Define a strong acid

Define a weak acid

Fully ionises in aqueous solutions, so all acid molecules release H+ ions

Only partially ionises in aqueous solutions, so only some molecules release H+ ions

What is meant by a dilute/concentrated, or a strong/weak acid?

Dilute/concentrated refers to amount of substance per unit volume, whereas strong/weak refers to the degree of ionisation affecting the acid’s pH

the basis of electrolysis

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

Define an electrolyte

Liquids and solutions which are able to conduct electricity due to the movement of ions

Passing a current through an electrolyte causes..

The ions to move to the electrodes

Positively charged ions move to the cathode

Negatively charged ions move to the anode

Ions are discharged at the electrodes, forming elements

When a simple ionic compound (eg. lead bromide) is electrolysed in the molten state using inert electrodes, what is formed at each electrode

The metal, lead, at the cathode

The nonmetal, bromine, at the anode

When will electrolysis be used instead of displacement with carbon (2)

If the metal is too reactive for displacement with carbon

If the metal reacts with carbon

Issue of using electrolysis to displace metals from compounds

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

Describe how aluminium is manufactured by electrolysis

Molten mixture of aluminium oxide and cryolite using carbon as the anode

Explain why, in the extraction of aluminium, a mixture containing cryolite is the electrolyte (3)

Aluminium has a very high melting point

Cryolite lowers melting point of mixture

So less energy used

Explain why the positive electrode must be continually replaced (4)

Made of carbon

Oxygen gas made at anode during electrolysis

Which reacts with electrode to form carbon dioxide

Which burns away the anode, so it needs to be replaced

For an aqueous solution’s electrolysis with inert electrodes, the ions discharged depend on..

The relative reactivity of the elements involved

What is formed at the cathode for electrolysis of an aqueous solution

What is formed at the anode for electrolysis of an aqueous solution

Hydrogen if the metal is more reactive than hydrogen (reactivity series), otherwise the metal is formed

Oxygen unless the solution contains halide ions, in which case the halogen is produced

(hydrogen produced unless metal more reactive than hydrogen) (oxygen produced unless halide ions present)

Explain why this happens

In aqueous solution, water molecules break down 

Producing hydrogen ions and hydroxide ions that are discharged

RP3 — method

Electrolysis of aqueous solution

hypothesis is that hydrogen is produced at the cathode (unless metal is more reactive than hydrogen) and oxygen is produced at the anode (unless the solution contains halide ions)

set up electrolysis circuit, by placing inert electrodes made of graphite into a beaker containing the aqueous solution to be tested (the electrolyte) and connecting them to the power supply

switch on the power supply and observe what happens at each electrode (eg gas bubbles, solid deposit, change of pH)

collect any gases produced at each electrode and use appropriate chemical test to identify (eg squeaky pop hydrogen)

repeat experiment with different aqueous solutions or compare results to the hypothesis

At the cathode, positively charged ions gain electrons so the reactions are ______

At the anode, negatively charged ions lose electrons so the reactions are ______

Reductions

Oxidations

half equations for the electrolysis of molten ionic compounds

cathode (reduction) eg. Na

Na⁺ + e^- → Na

anode (oxidation) eg. Cl

2Cl^- → Cl₂ + 2e^-

half equations for the electrolysis of aqueous ionic compounds — the cathode (reduction)

at the cathode (reduction) if the metal ions are less reactive than hydrogen eg. Cu²⁺

Cu²⁺ 2e^- → Cu

but if more reactive than hydrogen, hydrogen gas produced from water

2H₂O + 2e^- → H₂ ​+ 2OH^-

half equations for the electrolysis of aqueous ionic compounds — the anode (oxidation)

at the anode (oxidation) if halide ions are present, they are discharged to form chlorine gas

2Cl^- → Cl2₂ + 2e^-

if halide ions not present, water is oxidised to hydrogen gas and hydrogen ions

2H₂O → O₂ ​+4H⁺ +4e^-

4OH^- →O₂ + 2H₂O + 4e^-

The 2 harder half equations

4OH^- → O₂ + 2H₂O + 4e^-

4OH^- - 4e^- → O₂ + 2H₂O