Chemical changes

Question 1

Describe a method to find the position of an unknown metal in the reactivity series and how you would make it valid

Answer 1

-Pour dilute acid into a beaker
-Add unknown metal into the beaker and record the temperature change with a thermometer
-Repeat with other metals
-Place metals in order of temperature change: highest temperature change is most reactive
-Control the initial temperature for validity

Question 2

Why may there be a difference in the volume of gas in a situation where no gas escapes from the apparatus?

Answer 2

The element may react with water in the solution/ dissolve

Question 3

RPA 2: describe what indicator could be used and how to record the volume of acid needed.

Answer 3

Add indicator ie phenolphthalein to a conical flask. Read initial volume from bottom of meniscus base. Add acid from burette and swirl flask and add dropwise from burette towards end point. Until indicator colour change, from pink to colourless.

Question 4

RPA 2: roughly explain what equipment is used and its purpose.

Answer 4

• pipette - accurately measures fixed volume
• burette - accurately measures variable volume and can be added dropwise towards end point
• conical flask - prevents splashing so can easily swirl
• white tile - clearly see colour change
• few drops of single indicator - see a clear sudden colour change

Question 5

RPA 1: explain why the solid at the bottom of the flask is filtered.

Answer 5

To remove as an excess solid insoluble base

Question 6

RPA 1: how do you gently heat?

Answer 6

Using a water bath.

Question 7

RPA 1: explain why acid is initially heated.

Answer 7

To speed up the reaction.

Question 8

RPA 1: explain why a base metal is filtered rather than the acid.

Answer 8

Can be filtered as it is insoluble solid

Question 9

Explain what carbon displacing a metal says about the metals position in the reactivity series.

Answer 9

Below carbon in the reactivity series as the metal is less reactive than carbon so carbon displaces the metal.

Question 10

Explain why aluminium is more expensive than carbon.

Answer 10

Aluminium requires more energy to be extracted by electrolysis.

Question 11

What does excess mean in a reaction?

Answer 11

Some will still be present AFTER the reaction.

Question 12

Explain why conductivity is 0 when sulfuric acid is just neutralised and a solid ionic compound has formed.

Answer 12

Neutralisation means water is produced. Small molecule so no ions or electrons present in water to move through structure and carry a charge(no overall charge). Solid ionic compound has ions fixed in place so can’t move and carry a charge through the structure.

Question 13

What is the colour of water when an indicator is added?

Answer 13

Green.

Question 14

What is the state symbol of acids?

Answer 14

(Aq) - aqueous

Question 15

What are two observations when a base is added to acid until in excess?

Answer 15

1. Gas bubbles given out/effervescence
2. Solid remains at the bottom of the beaker

Question 16

Explain how oxygen gas is produced in the electrolysis of sodium sulphate solution.

Answer 16

The aqueous solution means dissolved in water so hydroxide ions are present as water molecules break down to produce hydroxide ions. The OH- ions are strongly attracted to the positive electrode and lose one electron at the anode to be discharged to form oxygen gas.

Question 17

RPA 2: give three improvements to improve accuracy

Answer 17

• use a pH probe - not subjective
• use a white tile and swirl flask
• repeat and calculate a mean(of concordant results)

Question 18

Explain the difference between the processes in electrolysis and in a chemical cell

Answer 18

Electrolysis used electricity to produce a chemical reaction whereas a chemical cell uses a chemical reaction to produce electricity.

Question 19

Describe how a solid like copper forms at the negative electrode in aqueous solution

Answer 19

Copper ions less reactive than hydrogen ions so are displaced by hydrogen, they are attracted to the negative electrode and gain two electrons to be discharged to form (solid) copper atoms

Question 20

Explain why a mixture is used as the electrolyte instead of using only aluminium oxide

Answer 20

Cryolite lowers the melting point of the mixture so lowers energy needed (and so costs)

Question 21

How do you know something is a mixture?

Answer 21

Molecules are not chemically combined and are not in fixed proportions

Question 22

Explain why the positive electrode must be continually replaced when splitting aluminium oxide.

Answer 22

The electrode is made of carbon. It reacts with oxygen which burns away to produce carbon dioxide over time

Question 23

Explain why electrolysis will not occur in a solid ionic compound.

Answer 23

The ions are fixed in place and so cannot move through the structure. This means they can’t conduct electricity (as ions can’t carry a charge through the structure).

Question 24

Explain how an alkaline (not acidic) solution is produced in aqueous electrolysis.

Answer 24

Halide/hydroxide ions are discharged at the positive electrode (anode) and hydrogen/less reactive metal ions are discharged at the negative electrode (cathode) which determined acidity before. Ions that weren’t discharged remain in the solution and so react.

Question 25

Explain why pH of an acid depends on the strength of the acid and the concentration of the acid.

Answer 25

• Stronger acid means a greater ionisation strength of H+ ions so pH is lower
• Higher H+ concentration means a lower pH (as pH decreases by one H+ concentration increases by a factor of 10)
• Greater concentration means more acid per unit volume so lower pH

Question 26

Explain why sodium chloride solution cannot be used as electrolyte to produce sodium metal.

Answer 26

Sodium is more reactive than hydrogen so displaces hydrogen ions and so hydrogen is discharged at the negative electrode forming hydrogen gas

Question 27

RPA 3: what are two control variables?

Answer 27

• Concentration and volume of electrolyte
• (Initial) temperature

Question 28

RPA 3: how do you know if electrodes are most/least reactive?

Answer 28

Highest (positive) voltage means that electrode is more reactive than the one it is being compared to. If less reactive then will be a negative voltage.

Question 29

Give two examples of a base

Answer 29

Insoluble metal hydroxides and metal oxides

Question 30

Give an example of an alkali metal

Answer 30

Soluble metal hydroxides

Question 31

What happens as pH decreases by one unit?

Answer 31

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

Question 32

Describe the process of electrolysis

Answer 32

Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements.

Question 33

RPA: plan a method to make pure, dry crystals of copper chloride from copper carbonate and dilute acid

Answer 33

• Measure dilute HCl in a measuring cylinder and pour into a beaker
• Add copper carbonate (insoluble base) into beaker with HCl until it is in excess (solid remains at the bottom of the beaker) and stir solution with a glass rod
• Filter excess with filter paper and filter funnel
• Pour salt solution into an evaporating basin and gently heat with a water bath/bunsen burner until point of crystallisation
• Leave solution to cool and crystals to form then filter and pat dry with filter paper or using a drying oven

Question 34

Give two reasons why a solution may become cloudy

Answer 34

• Solid sulphur may have formed which is a precipitate
• Carbon dioxide is present (in lime water)