C3: Chemical Changes

Question 1

Which ions make aqueous solutions acidic?

Answer 1

Hydrogen ions (H+)

Question 2

Which ions make aqueous solutions alkaline?

Answer 2

Hydroxide ions (OH-)

Question 3

What is the pH scale?

Answer 3

The pH scale ranges from pH 0 to pH 14 and measures the acidity or alkalinity of a solution.

Question 4

What are pH ranges for acids and alkalis? What the pH of a neutral solution?

Answer 4

Acid - Less than pH 7 (pH 1 is strongest).
Neutral - pH 7.
Alkali - Greater than pH 7 (pH 14 is strongest).

Question 5

What can be used to measure pH?

Answer 5

Universal indicator
pH probe

Question 6

What colour is phenolphthalein in acid and alkali?

Answer 6

Acid - Colourless
Alkali - Pink

Question 7

What colour is methyl orange in an acid and an alkali?

Answer 7

Acid - Red
Alkali - Yellow

Question 8

What colour is red litmus paper in an acid and an alkali?

Answer 8

Acid- stays red
Alkaline- turns blue

Question 9

What colour is blue litmus paper in an acid and an alkali?

Answer 9

Acid- turns red
Alkaline- stays blue

Question 10

Suggest a problem with using universal indicator to test the ph of a solution?

Answer 10

The colour of the solution is matched to a ph colour chart. This is quite subjective as people may disagree with which colour the solution matches.
It doesn’t provide an exact pH value.

Question 11

Acid X has a pH of 1. What can you say about the concentration of hydrogen ions in acid X?

Answer 11

There is a high concentration of hydrogen ions in the acid, making it a strong acid.
The lower the pH of the acid, the higher the concentration of Ht ions.

Question 12

Alkali Y has a pH of 8.5. What can you say about the concentration of hydroxide ions in alkali Y?

Answer 12

There is a low concentration of hydroxide ions in the alkali, making it a weak alkali.
The lower the pH of the alkali, the lower the concentration of OH ions.

Question 13

If pH decreases by one unit, what happens to the concentration hydrogen ions?

Answer 13

The hydrogen ion concentration increases by a factor of 10.

Question 14

What is a neutralisation reaction? During an acid-alkali neutralisation reaction, what happens?

Answer 14

A neutralisation reaction is a reaction between an acid and a base.
In an acid-alkali neutralisation reaction, H+ ions from the acid react with OH ions from the alkali to form water.

Question 15

What do the terms concentrated and dilute mean when talking about acid? Is this the same as strong/ weak acids?

Answer 15

Concentrated acids have more moles of acid per unit volume of water than dilute acids.
The concentration of an acid is not the same as strength.
Strength refers to whether the acid has completely dissociates in water or not.

Question 16

An acid only partially dissociates in water. What can be said about the strength of the acid?

Answer 16

Weak acid

Question 17

What is a base?

Answer 17

Any substance that reacts with an acid to form salt and water only.

Question 18

True or false?
‘Alkalis are insoluble bases’

Answer 18

FALSE
Alkalis are soluble bases.

Question 19

Why are metal oxides normally bases rather than alkalis?

Answer 19

Metal oxides are normally insoluble.
Alkalis are soluble.

Question 20

What is the name of the salt formed from magnesium and sulfuric acid?

Answer 20

Magnesium sulfate

Question 21

What is the name of the salt formed from zinc oxide and nitric acid?

Answer 21

Zinc nitrate

Question 22

What is the name of the salt formed from calcium carbonate and hydrochloric acid?

Answer 22

Calcium chloride

Question 23

When a soluble salt is prepared from an acid and an insoluble reactant, why is excess of the insoluble reactant added?

Answer 23

To ensure all the acid reacts.

Question 24

When a soluble salt is prepared from an acid and an insoluble reactant, how and why is the excess reactant removed?

Answer 24

By filtration.
It is removed to leave a pure solution of the salt.

Question 25

What method must be used to prepare a salt from an acid and a soluble reactant? Why?

Answer 25

Titration. Since both the reactants are soluble, a titration allows you to combine the reactants exactly and avoid adding an excess of either reactant as this would be hard to remove.

Question 26

Name the method that could be used to prepare a sample of soluble copper sulfate from insoluble copper oxide and sulfuric acid?

Answer 26

Filtration

Question 27

What 3 steps are required when producing a pure dry salt from an acid and alkali?

Answer 27

Complete a titration to find the volume of acid that reacts exactly with a set volume of alkali. Use the results from the titration to mix the acid and alkali in the correct proportions. Evaporate the water from the solution, leaving pure dry salt crystals.

Question 28

Most common chlorides are soluble. What are the two exceptions?

Answer 28

Silver chloride and lead chloride are insoluble.

Question 29

True or false? 'All nitrates are soluble'

Answer 29

TRUE

Question 30

Most common sulfates are soluble. What are the three exceptions?

Answer 30

Lead sulfate, calcium sulfate and barium sulfate are insoluble.

Question 31

Most common carbonates and hydroxides are insoluble. What are the three exceptions?

Answer 31

The carbonate / hydroxides of sodium potassium and ammonium are soluble.

Question 32

What salt is produced when lead reacts with sulfuric acid? Will a precipitate form?

Answer 32

Lead sulfate. A precipitate will form because lead sulfate is insoluble.

Question 33

What is an electrolyte?

Answer 33

An ionic compound in its molten or aqueous state. Aqueous - dissolved in water

Question 34

Why can an electrolyte carry charge?

Answer 34

An ionic compound in is molten or aqueous state has mobile ions which can carry charge.

Question 35

What is electrolysis?

Answer 35

A process which uses electrical energy (from a direct current supply) to decompose electrolytes.

Question 36

What is the cathode and anode?

Answer 36

Cathode - negative electrode Anode - positive electrode

Question 37

Where do charged ions in the electrolyte move to during electrolysis?

Answer 37

Cations (positive ions) move towards the cathode (negative electrode). Anions (negative ions) move towards the anode (positive electrode).

Question 38

What happens at the anode during electrolysis?

Answer 38

The anions (negatively charged ions) lose electrons to form their elements.

Question 39

What happens at the cathode during electrolysis?

Answer 39

Cations (positively charged ions) gain electrons to form their elements.

Question 40

Name the processes that occur at each electrode during electrolysis ?

Answer 40

Anode (positive) - oxidation. Cathode (negative) - reduction.

Question 41

What is formed at each electrode in electrolysis?

Answer 41

Positive electrode: Non metal. Negative electrode: Metal or hydrogen.

Question 42

How can you predict whether a metal or hydrogen will form at the negative electrode?

Answer 42

If hydrogen is above the metal in the reactivity series then the metal will form. If the metal is more reactive than hydrogen then hydrogen will form

Question 43

What is formed at each electrode during the electrolysis of copper chloride solution?

Answer 43

Positive electrode: Chlorine Negative electrode: Copper

Question 44

What is formed at each electrode during the electrolysis of sodium sulfate solution?

Answer 44

Positive electrode: Oxygen Negative electrode: Hydrogen

Question 45

What is formed at each electrode during the electrolysis of molten lead bromide?

Answer 45

Positive electrode: Bromine Negative electrode: Lead

Question 46

What is formed at each electrode during the electrolysis of sodium chloride solution?

Answer 46

Positive electrode: Chlorine Negative electrode: Hydrogen

Question 47

What is formed at each electrode during the electrolysis of water acidified with sulfuric acid?

Answer 47

Positive electrode: Oxygen Negative electrode: Hydrogen

Question 48

What does oxidation mean in terms of electrons?

Answer 48

Loss of electrons

Question 49

What does reduction mean in terms of electrons?

Answer 49

Gain of electrons

Question 50

What is OIL RIG in terms of electrolysis?

Answer 50

Oxidation is loss reduction is gain

Question 51

Describe how electrolysis of copper sulfate can be used to purify copper?

Answer 51

Place 2 copper electrodes into copper sulfate solution. The anode should be impure copper and cathode should be pure copper. Connect to a power supply. The copper in the impure anode is pulled towards the cathode to form pure copper. Impurities form as sludge below the anode. Cuz ions from copper sulfate remain in solution.

Question 52

How could you prepare a pure, dry sample of an insoluble salt?

Answer 52

1. Mix the two solutions required to form the salt. 2. Filter the mixture using filter paper. 3. The residue on the filter paper is the insoluble salt. 4. Wash the salt with distilled water and leave to dry.