Chemical changes

Question 1

pH scale

Answer 1

The pH scale goes from 0 – 14 (extremely acidic substances can have values of below 0).
All acids have pH values of below 7, all alkalis have pH values of above 7.
The lower the pH then the more acidic the solution is.
The higher the pH then the more alkaline the solution is.
A solution of pH 7 is described as being neutral.

Question 2

Indicator

Answer 2

An indicator is a substance that is one colour in acids and another colour in alkalis. universal indicator is a mixture of several different indicators. Whereas most indicators can just show whether a substance is acid or alkali, universal indicator can indicate the strength. The strength is measured on the PH scale.

Question 3

Hydrogen ions and pH

Answer 3

The more hydrogen ions the stronger the acid, but the lower the pH.
The higher the concentration of hydroxide ions in a solution the higher the pH.
The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10.
Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4.

Question 4

Acid solutions

Answer 4

Solutions of acid are made by dissolving certain covalent molecules in water. The water causes the covalent molecules to break apart in a way that generates hydrogen ions.

Question 5

Acid concentration

Answer 5

A solution is formed when a solute is dissolved in a solvent.
A concentrated solution of an acid is one that contains a high number of acid molecules per dm3 of solution.
A dilute acid solution is therefore one that has much fewer acid molecules per dm3 of solution.

Question 6

Acid strength

Answer 6

Acids can be either strong or weak, depending on how many ions they produce when they dissolve in water.
When added to water, acids ionise or dissociate to produce H+ ions:
Hydrochloric acid: HCl ⟶ H+ + Cl–

Strong acids such as HCl and H2SO4 dissociate completely in water, producing solutions with a high concentration of H+ ions and thus a very low pH.
Weak acids such as ethanoic acid, CH3COOH and hydrofluoric acid, HF only partially ionize in water, producing solutions of pH values between 4 – 6.
For weak acids there is an equilibrium set-up between the molecules and their ions once they have been added to water.
Ethanoic acids: CH3COOH ⇌ H+ + CH3COO–
The ⇌ symbol indicates that the process is reversible, as the products can react together forming the original reactants.

Question 7

Phenolphthalein

Answer 7

Acid - no colour
Neutral - no colour
Alkaline - pink

Question 8

Methyl orange

Answer 8

Acid - pink
Neutral - orange
Alkaline - yellow

Question 9

Bases

Answer 9

Bases are substances which can neutralise an acid, forming a salt and water.
A base which is water-soluble is referred to as an alkali.
Bases are usually oxides, hydroxides or carbonates of metals.

Question 10

Alkalis

Answer 10

Alkalis have pH values of above 7.
The presence of the OH- ions is what makes the aqueous solution an alkali.
Like acids, alkalis can be concentrated or dilute, weak or strong.
Strong alkalis like sodium hydroxide will completely dissolve In water to give a high concentration of OH- ions.
Weak alkalis like ammonia will only partially react with water to give a low concentration of OH- ions.

Question 11

Reaction of acids with metals

Answer 11

Only metals above hydrogen in the reactivity series will react with dilute acids.
The more reactive the metal then the more vigorous the reaction will be.
Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react explosively with acids.
Reactive metal + acid ——- metal salt + hydrogen

Question 12

Reaction of acids with oxides & hydroxides

Answer 12

When an acid reacts with an oxide or hydroxide, a neutralisation reaction occurs.
Metal oxides and metal hydroxides act as bases.
The identity of the salt produced depends on the acid used and the positive ions in the base.
Hydrochloric acid produces chlorides, sulfuric acid produces sulfate salts and nitric acid produces nitrates.
Base (alkali) + acid ——- metal salt + water

Question 13

Reactions of Acids with Metal Carbonates

Answer 13

These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the carbon dioxide gas.
Metal carbonate + acid ——– metal salt + water + carbon dioxide

Question 14

Ammonia and acids

Answer 14

Ammonia reacts with acids to form ammonium salts - which contain the ammonium ion (NH4)+.
Ammonia + acid —- ammonium salt

Question 15

Neutralisation

Answer 15

When an acid reacts with an alkali - a neutral solution is formed. this type of reaction is called a neutralisation reaction.
Acid + base —– salt + water
Ionic equations are used to show only the particles that chemically participate in a reaction.
The other ions present are not involved and are called spectator ions.
HCl + NaOH ⟶ NaCl + H2O
H+ + Cl- + Na+ + OH- ⟶ Na+ + Cl- + H2O
The spectator ions are thus Na+ and Cl–.
Removing these from the previous equation leaves the overall net ionic equation:
H+ + OH- ⟶ H2O
The H+ ions come from the acid and the OH– ions come from the base, both combine to form the product water molecules.
This ionic equation is the same for all acid-base neutralisation.

Question 16

Test for hydrogen

Answer 16

The test for hydrogen consists of holding a burning splint held at the open end of a test tube of gas.
If the gas is hydrogen it burns with a loud “squeaky pop” which is the result of the rapid combustion of hydrogen with oxygen to produce water.
Be sure not to insert the splint right into the tube, just at the mouth, as the gas needs air to burn.

Question 17

Test for carbon dioxide

Answer 17

The test for carbon dioxide involves bubbling the gas through an aqueous solution of calcium hydroxide (limewater).
If the gas is carbon dioxide, the limewater turns milky or cloudy.

Question 18

Prepare a salt by titration

Answer 18

If salts are prepared from an acid and a soluble reactant then a titration technique must be used.
In a titration, the exact volume of acid and soluble reactant are mixed in the correct proportions so that all that remains is the salt and water.
Aim: To prepare a sample of a dry salt starting from an acid and an alkali.
Procedure: Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange).
Add the acid into the burette and note the starting volume.
Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour.
Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid - final volume of acid).
Add this same volume of acid into the same volume of alkali without the indicator.
Heat to partially evaporate, leaving a saturated solution.
Leave to crystallise decant excess solution and allow crystals to dry.

Question 19

Solubility rules

Answer 19

Compounds of sodium, potassium, and ammonium.
All nitrates.
All chlorides, except silver and lead (Il).
All sulphates except barium, calcium, and lead (Il)
Sodium, potassium, and ammonium carbonates.
Sodium, potassium, and ammonium hydroxides.

Question 20

Predicting precipitates

Answer 20

Some salts can be extracted by mining but others need to be prepared in the laboratory.
How the salt is made in the laboratory depends on whether the salt being formed is soluble or insoluble in water.
To do this the balanced equation is written down to determine the identify of the salt product.
Then check the solubility of the salt using the solubility table.
If it is soluble in water, then it can be prepared by titration.
If it is insoluble then it can be prepared by precipitation.

Question 21

Preparing an insoluble salt

Answer 21

Aim: To prepare a dry sample of an insoluble salt.
Procedure - Measure out 25 cm3 of 0.5 mol dm3 soluble salt1 solution and add it to a small beaker.
Measure out 25 cm3 of 0.5 mol dm3 of soluble salt2 add it to the beaker and mix together using a stirring rod.
Filter to remove the precipitate from mixture.
Wash the filtrate with distilled water to remove traces of other solutions
Leave in an oven to dry.

Question 22

Electrolysis

Answer 22

When an electric current is passed through a molten or aqueous ionic compound the compound decomposes or breaks down.
Liquids and solutions that are able to conduct electricity are called electrolytes.
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis.
During electrolysis the electrons move from the power supply towards the cathode.
Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode.
Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode.
Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode.

Question 23

Electrolytic cell

Answer 23

An electrolytic cell is the name given to the set-up used in electrolysis and which consists of the following:
Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte.
Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity.
Anode: the positive electrode of an electrolysis cell.
Anion: negatively charged ion which is attracted to the anode.
Cathode: the negative electrode of an electrolysis cell.
Cation: positively charged ion which is attracted to the cathode.

Question 24

Positive electrode

Answer 24

Negatively charged OH– ions and non-metal ions are attracted to the positive electrode.
If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine).
If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen gas.
In both cases, the other negative ion remains in solution.

Question 25

Negative electrode

Answer 25

H+ ions and metal ions are attracted to the negative electrode but only one will gain electrons.
Either hydrogen or a metal will be produced.
If the metal is above hydrogen in reactivity series, hydrogen will be produced – bubbling will be seen at the cathode.

Question 26

Electrolysing copper chloride

Answer 26

Copper is below hydrogen so copper(II) ions are preferentially discharged at the cathode.
Chlorine is a halogen, so is preferentially discharged at the anode.

Question 27

Electrolysing sodium chloride

Answer 27

Sodium is above hydrogen so hydrogen ions are preferentially discharged at the cathode.
Chlorine is a halogen, so is preferentially discharged at the anode.

Question 28

Electrolysing sodium sulphate

Answer 28

Sodium is above hydrogen so hydrogen ions are preferentially discharged at the cathode.
Hydroxide ions are preferentially discharged over sulfate ions, so oxygen is produced at the anode.

Question 29

Electrolysing Acidified water (Dilute sulphuric acid)

Answer 29

Hydrogen ions are discharged at the cathode.
Oxygen from water molecules is preferentially discharged at the anode.

Question 30

Electrolysing lead bromide (molten)

Answer 30

Binary ionic compounds consists of just two elements joined together by ionic bonding.
When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge.
These compounds undergo electrolysis and decompose into their constituent elements.
Lead(II) bromide is an ionic solid with a relatively low melting point and can be used to illustrate the electrolysis of a molten compound.
Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off.
Positive lead ions move to the negative electrode (cathode) and gain electrons to form grey lead metal which deposits on the bottom of the electrode.

Question 31

Predicting the products of electrolysis

Answer 31

To predict the products of any binary molten compound first identify the ions present.
The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode.
Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal.

Question 32

Electrolysis and redox

Answer 32

Oxidation is when a substance loses electrons and reduction is when a substance gains electrons.
As the ions come into contact with the electrode, electrons are either lost or gained and they form neutral substances.
These are then discharged as products at the electrodes.
At the anode, negatively charged ions lose electrons and are thus oxidised.
At the cathode, the positively charged ions gain electrons and are thus reduced.

Question 33

Electrolysis of copper sulphate using inert (graphite) electrodes

Answer 33

A solution of copper sulphate (CuSO4) contains four different ions: Cu 2+, SO4 2-, H+ and OH-. When you electrolyse a solution of CuSO4 with inert electrodes:

Copper metal is less reactive than hydrogen. So, at the cathode, copper metal is produced and coats the electrode: Cu2+ + 2e- → Cu
There aren’t any halide ions present so at the anode oxygen and water are produced. The oxygen can be seen as bubbles: 4OH- → О₂ + 2H₂O + 4е-

Question 34

Electrolysis of copper sulphate using non-inert (copper) electrodes

Answer 34

As the reaction continues, the mass of the anode will decrease and the mass of the cathode will increase. This is because copper is transferred from the anode to the cathode. The reaction is quite slow, so the cell should be left for around 30 minutes in order to achieve a measurable change in mass.
You can measure how the mass of the electrodes has changed during the experiment by finding the difference between the masses of the electrodes before and after the experiment.
Anode – Cu → Cu 2+ + 2e-
Cathode – Cu 2+ + 2e- → Cu

Question 35

Purification of copper using electrolysis of copper sulfate using non-inert (copper) electrodes

Answer 35

Copper can be extracted from its ore by reduction with carbon, but copper made in this way is impure. Electrolysis can be used to purify the copper using an electrochemical cell with copper electrodes.
When copper is purified using electrolysis, the anode starts off as a big lump of impure copper and the cathode starts off as a thin piece of pure copper. The electrolyte is copper sulfate solution (which contains Cu²+ ions). Here’s what happens during the process:

Copper in the impure copper anode forms copper ions which dissolve into the electrolyte – Cu → Cu 2+ + 2e-
The copper ions move to the pure copper cathode, and react to form a layer of pure copper – Cu 2+ + 2e- → Cu
Any impurities from the impure copper anode sink to the bottom of the cell, forming a sludge.