C2 - Chemical Reactions

Question 1

Describe how metals react with oxygen

Answer 1

When a metal reacts with oxygen, it forms the metal oxide. This is called an oxidation reaction.
When a metal oxide loses oxygen it is called a reduction reaction.

Question 2

Explain the reaction of metals with oxygen in terms of oxidation and reduction

Answer 2

When a metal gains oxygen through oxidation we say the metal has been oxidised and when it loses oxygen through reduction we say it has been reduced.

Question 3

Use the reactions between metals and water and metals and acids to construct a reactivity series

Answer 3

-Potassium, sodium and lithium react very rapidly with water at room temperature.
-Calcium reacts rapidly with water at room temperature.
-Magnesium, zinc, iron and copper have no reaction with water at room temperature.

-Potassium, sodium and lithium react dangerously fast with acids.
-Calcium reacts vigorously with acids.
-Magnesium reacts rapidly with acids.
-Zinc reacts quite fast with acids.
-Iron reacts quite slow with acids.
-Copper has no reaction with acids.

Question 4

Describe in terms of electrons what happens when metals react

Answer 4

When metals react, they lose electrons and form a positive ion.
The reactivity of a metal depends on its ability to lose electrons and form a positive ion.

Question 5

Use the reactivity series to explain the displacement of metals

Answer 5

A more reactive metal displaces a less reactive metal from its compound because it is easier for the more reactive metals at the top of the reactivity series to lose electrons and form positive ions than the less reactive metals below them.

Question 6

Describe how certain metals can be extracted using carbon

Answer 6

A more reactive element will displace a less reactive element from its compound. For example, when carbon reacts with iron oxide, carbon is more reactive so it displaces the iron to form carbon dioxide and iron.

Question 7

Explain what is meant by oxidation and reduction in terms of electrons

Answer 7

Oxidation is the loss of electrons and reduction is the gain of electrons.

Question 8

Describe what is meant by a base and an alkali

Answer 8

Bases are chemicals which can neutralise acids, producing a salt and water. They are usually metal oxides or metal hydroxides. Bases which are soluble in water are called alkalis. Copper oxide and iron hydroxide are insoluble in water so they are bases only.

However, sodium hydroxide is soluble in water meaning it is both a base and an alkali. In aqueous solutions, alkalis produce hydroxide ions (OH-).

Question 9

Describe how the pH scale gives information on acids and alkalis

Answer 9

-Acids have a pH between 0-6
-Solutions with a pH of 7 are neutral
-Alkalis have a pH between 8-14

To determine the pH of a solution you use a pH probe or a universal indicator.
A pH probe determines the pH electronically. Whereas the universal indicator changes colour depending on the pH of the solution. Green is neutral, yellow-red is acidic and blue-purple is alkaline.

Question 10

Describe the neutralisation reaction between acids and alkalis

Answer 10

Acids produce the hydrogen ion H+ and alkalis produce the hydroxide ion OH- . When an acid reacts with an alkali, the hydrogen ion reacts with the hydroxide ion to form water (H + OH -> H20) .

Question 11

Describe how acids react with metals

Answer 11

When acids react with metals they produce a salt and hydrogen because all acids contain hydrogen and any element above hydrogen in the reactivity series can displace it. For example, when sulfuric acid reacts with magnesium it forms magnesium sulfate and hydrogen.

When metals react with acids, the metal loses 2 electrons and the hydrogen gains 2 electrons.

Question 12

Describe the reaction between an acid and a base, alkali or metal carbonate

Answer 12

When an acid reacts with a base or alkali, it produces a salt and water.
-Salts contain a positive ion which comes from the base or alkali and also contain a negative ion which comes from the acid.
-For example, when lithium hydroxide reacts with nitric acid, it produces lithium nitrate and water.

When acids react with a metal carbonate, they make a salt, water and carbon dioxide.
-For example, when hydrochloric acid reacts with copper carbonate, it produces copper chloride, water and carbon dioxide.

Question 13

Describe what is meant by a strong and weak acids in aqueous solutions with examples

Answer 13

All acids produce hydrogen ions (H+) in aqueous solutions. For example, in aqueous solutions, acid molecules ionise (split) and release H+.

-Strong acids fully ionise in aqueous solutions, every acid molecule ionises to release H+.
-Examples of strong acids are hydrochloric acid, sulfuric acid and nitric acid.

-Weak acids partially ionise in aqueous solutions meaning only some acid molecules ionise.
-Examples of weak acids are carbonic acid, ethanoic acid and citric acid.

Question 14

Describe the effect on pH of the strength of an acid

Answer 14

The pH scale gives an idea of the concentration of hydrogen ions.
Strong acids have a lower pH than weak acids for a given concentration because strong acids fully ionise, producing a greater concentration of hydrogen ions than weak acids.

As the pH scale decreases by one unit, the concentration of hydrogen ions increases by ten times.

Question 15

Describe what is meant by a dilute and a concentrated acid

Answer 15

The concentration of an acid tells us the amount of acid molecules in a given volume of solution.

A dilute acid will have fewer acid molecules in a given volume than a concentrated acid even if the strength of the acid is the same.

Dilute acid - less acid molecules in a given volume than a concentrated acid.

Question 16

Explain why ionic compounds can conduct electricity when molten or dissolved in water

Answer 16

Solid ionic compounds cannot conduct electricity because the ions are locked in place and are not free to move. This is because the ions are held in place by strong forces of attraction.

However, when an ionic compound is melted or dissolved in water, the forces of attraction are broken and the ions are free to move so it can conduct electricity. These liquids or solutions are known as electrolytes.

Question 17

Describe the reactions taking place at the positive and negative electrodes during electrolysis

Answer 17

In electrolysis the two electrodes are made of a conducting material such as graphite or a metal.
-The negative electrode is called a cathode which is covered with electrons and attached to the negative terminal of a power pack.
-The positive electrode is called the anode and lacks electrons, it is connected to the positive terminal of the power pack.

-The positive ions are attracted to the negative electrode because opposite charges attract. The positive ions then gain electrons to form atoms. As they are gaining electrons, it is an example of a reduction reaction.
-The negative ions are attracted to the positive electrode where they lose electrons to form an atom. As they are losing electrons, it is an example of an oxidation reaction.

Question 18

Describe why electrolysis is used to extract some metals

Answer 18

Metals which are less reactive than carbon are extracted by reduction with carbon and metals which are more reactive than carbon are extracted using electrolysis.

Question 19

Describe how aluminium is extracted by electrolysis

Answer 19

1) Aluminium oxide is mixed with cryolite which lowers the melting point as it has a very high melting point around 2000 degrees celsius. This reduces the amount of energy needed and saves money.
2) Now an electric current is applied to the molten aluminium oxide. The cathode and anode are made of graphite which is a form of carbon. Graphite is a good conductor of electricity and has a very high melting point so it can be used at high temperatures without melting.
3) The aluminium ions are attracted to the cathode where each ions gain three electrons and form an aluminium atom.
4) The oxide ions are attracted to the anode where each ion loses 2 electrons to form an oxygen atom.

Question 20

Write half-equations for the reactions at the cathode and anode

Answer 20

When aluminium is extracted from aluminium oxide during electrolysis, the half equation at the anode is:
2O2- —> O2 + 4e- (this is an oxidation reaction)
The half equation for the reaction at the cathode is:
Al3+ + 3e- —> Al

Question 21

Describe the reactions that take place during electrolysis of an aqueous solution

Answer 21

Aqueous solutions are dissolved in water. Water molecules ionise (split) forming hydrogen ions and hydroxide ions.

When an aqueous solution reacts during electrolysis, the negative cathode attracts 2 positive ions. The hydrogen ion and the metal ion. If the metal is more reactive than hydrogen, then hydrogen is produced at the cathode. At the anode, oxygen gas is made. However, if the aqueous solution contains halide ions, then the halogen will be produced at the anode.

It is important that the electrodes do not react with the chemicals that are being made in electrolysis. Platinum is often used as inert (unreactive) electrodes.