Chemical Changes

Question 1

What is oxidation/reduction?

Answer 1

Oxidation - When a substance gains oxygen.
Reduction - When a substance loses oxygen

Question 2

What is the reactivity series of metals?

Answer 2

The series which arranges metals in the decreasing order of reactivity.

Question 3

What are the trends in reactivities of metals in reactions with acids/water?

Answer 3

• Metals above hydrogen in reactivity series react with acid to produce hydrogen. - The higher the metal’s reactivity, the quicker and more vigorous it reacts with acid.
• Metals below hydrogen don’t react with acids.
• Not all metals above hydrogen react with water - mostly Group I and II metals. Aluminium is the borderline case.

Question 4

What is a displacement reaction?

Answer 4

A reaction where a more reactive metal displaces a less reactive metal from a compound

Question 5

How are unreactive metals found in Earth?

Answer 5

• Unreactive metals (e.g., gold) are found in the Earth as the metal itself
• Most metals are found as compounds that require chemical reactions to extract the metal.

Question 6

How can metals less reactive than carbon be extracted? Explain your answer. (3)

Answer 6

Reduction with carbon. Carbon displaces the metal in a metal oxide (gets oxidised to carbon oxides) whilst the metal gets reduced to the pure metal.

Question 7

How are metals more reactive than carbon extracted? (1)

Answer 7

By electrolysis

Question 8

How are oxidation and reduction defined in terms of electron transfer? (1)

Answer 8

Oxidation – loss of electrons
Reduction – gain of electrons

Question 9

What is the general equation for a reaction between metals and acids? What type of reaction is this?

Answer 9

Metal + acid → salt + hydrogen
; Redox reaction, as well as a displacement reaction.

Question 10

Which metals in the reactivity series will react with acid?

Answer 10

Those above hydrogen

Question 11

What is the general equation for a neutralisation reaction?

Answer 11

Base + acid → salt + water

Question 12

What is the general equation for the reaction between metal carbonate and acid?

Answer 12

Metal carbonate + acid → salt + water + carbon dioxide

Question 13

What is the general equation for the reaction between metal oxides and acids?

Answer 13

Metal oxide + acid → a salt + water

Question 14

What is a redox reaction?

Answer 14

A reaction where both oxidation and reduction occurs.

Question 15

Explain in terms of gain or loss of electrons which one has been oxidised and reduced when magnesium reacts with hydrochloric acid.

Answer 15

• Magnesium has lost electrons and thus has been oxidised (Mg to Mg2+)
• The hydrogen in HCl has gained electrons and thus has been reduced (H+ to H2)

Question 16

How is a soluble salt formed in a short way? (3)

Answer 16

1. React the excess acid with some insoluble chemical (e.g. metal oxide)
2. Filter off the leftovers
3. Crystallise the product

Question 17

What do acids and alkalis produce in aqueous solutions?

Answer 17

Acids produce hydrogen ions, alkalis produce hydroxide ions

Question 18

What are bases, acids and alkalis? (3)

Answer 18

Bases: compounds that neutralise acids
Acids: substance that produces hydrogen ions in aqueous solutions
Alkalis: soluble base that produce hydroxide ions in aqueous solutions

Question 19

What is the pH scale and what does a pH of 7 show?

Answer 19

The measure of acidity/alkalinity of a solution; neutral solution

Question 20

State the general equation for a neutralisation reaction in a short, ionic form.

Answer 20

H⁺ + OH⁻ → H₂O

Question 21

What is a strong acid and weak acid?

Answer 21

Strong acid: an acid that is completely ionised (dissociated) in aqueous solution.
Weak acid: and acid that is only partially ionised (dissociated) in aqueous solution.

Question 22

What happens to pH as concentration of H⁺ increases?

Answer 22

The pH decreases.

Question 23

What is a concentrated acid and what is a diluted acid?

Answer 23

Concentrated acid: an acid that contains more moles of acid per unit volume in a solution.
Dilute acid: An acid that contains less mole of acid per unit volume in a solution.

Question 24

As the pH is increases by one unit, happens to the hydrogen ion concentration?

Answer 24

As the pH value increase by one unit, the hydrogen ion concentration decreases by a factor of 10.

Question 25

Name the following salts: LiNO3, K2CO3, MgBr2, BaSO4

Answer 25

- Lithium nitrate - Potassium carbonate - Magnesium bromide - Barium sulfate

Question 26

What is electrolysis?

Answer 26

The method used to separate ionic compounds into their pure elements.

Question 27

What is an electrolyte?

Answer 27

The liquid/solution which conducts electricity

Question 28

What is a cathode and what is an anode?

Answer 28

- Cathode is the negative electrode - Anode is the positive electrode

Question 29

What occurs at the cathode and what occurs at the anode during electrolysis?

Answer 29

- Reduction occurs at the cathode - Oxidation occurs at the anode

Question 30

What is the general equation for the reaction between metal hydroxides and acids?

Answer 30

Metal hydroxide + acid → salt + water

Question 31

Required Practical: How is a soluble salt formed with a base/metal/carbonate, include reasons for certain steps? (9)

Answer 31

1. Pour 40cm³ of dilute __ acid using a measuring cylinder into a beaker. 2. Place the gauze on the surface of the tripod with the Bunsen burner underneath. 3. Gently heat the acid until it is almost boiling to speed up the reaction and turn the bunsen burner off. 4. Add a spatula of metal oxide into the acid and stir with a glass rod until a reaction occurs. 5. Repeat step 4 until there is excess which shows that the acid and metal oxide have reacted. 6. Pour the solution in the filter paper and funnel inside the conical flask. 7. Repeat step 2 and place a beaker full of water and an evaporating basin with the solution on top of that. 8. Gently heat the solution until crystals start to form and turn off the Bunsen burner. 9. Set the solution in a cool area and when crystals have finished forming gently pat them dry using filter paper.

Question 32

Required Practical: How is a soluble salt formed with an alkali? (11)

Answer 32

1. Use the pipette and pipette filler to put the said volume of acid into the conical flask 2. Stand the flask on a white tile. 3. Clamp the burette using a clamp stand. 4. At low eye level, pour a certain volume of the alkali into the burette using a funnel. 5. Record the initial reading of alkali in the burette. 6. Add 5 drops of indicator (phenolphthalein) to the conical flask. 7. Slowly open the burette tap while swirling the conical flask. 8. Add the acid drop-by-drop when the colour starts to change slightly. 9. Close the burette when a permanent colour change occurs in phenolphthalein (from pink to colourless). 10. Record the final reading of acid in the burette and calculate the titre. 11. Repeat the process 2-3 times and calculate a mean removing any anomalies.