TOPIC 4 - CHEMICAL CHANGES

Question 1

What is the pH scale?

Answer 1

A measure of how acidic or alkaline a solution is.
The lower the pH, the more acidic.
The higher the pH, the more alkaline.

Question 2

How do you measure the pH of a solution?

Answer 2

By using an indicator. The dye changes colour depending on whether it is above or below a certain pH. Some indicators contain a mixtures of dyes which mean they gradually change colour over a scale of pH. These are called wide range indicators and they are useful for estimating the pH of a solution. A pH probe attached to a pH meter can also be used to measure pH electronically. The probe is placed in the solution you are measuring and the pH is given on a digital display as a numerical value, meaning its more accurate than an indicator.

Question 3

What is neutralisation?

Answer 3

Acids and Bases neutralise each other.
An acid is a substance that forms aqueous solutions with a pH of less than 7. Acids form H+ ions in water.
A base is a substance with a pH greater than 7.
An alkali is a base that dissolves in water to form a solution with a pH greater than 7. Alkalis form OH- ions in water.

The reaction between acids and bases is called neutralisation.

When an acid neutralises a base (or vice versa) the products are neutral so both have a pH of 7.

Question 4

What do acids form in water?

Answer 4

H+ ions.

Question 5

What do alkalis form in water?

Answer 5

OH- ions.

Question 6

What is the equation for the reaction between an acid and a base?

Answer 6

Acid + base —> salt + water.

H+ + OH- —> H2O

Question 7

Talk about titrations.

Answer 7

Titrations are a method of analysing the concentrations of solutions.
They allow you to find out exactly how much acid is needed to neutralise a quantity of alkali (or vice versa).
Say you want to find out the concentration of some alkali
- using a pipette and a pipette filler, add a set volume of alkali to a conical flask. Add two or three drops of indicator too.
- use a funnel to fill a burette with some acid of a known concentration. (Do this below eye level and wear safety goggles). Record the initial volume of the acid in the burette.
- using the burette, add the acid to the alkali a bit at a time, giving the conical a regular swirl. Go especially slowly when you think the end point (colour change) is about to be reached.
-the indicator changes colour when all the alkali has been neutralised.
- record the final volume of acid in the burette, and use it, along with the initial reading, to calculate the volume of acid used to neutralise the alkali.

Question 8

What type of indicator should you use in titrations?

Answer 8

Need to use a single indicator, such as litmus (blue in alkalis and red in acids). This is because you want to see a sudden colour change. This is instead of the variety of colour changes that happen in the universal indicator.

Question 9

What are strong acids?

Answer 9

The ionise completely in water. All acid particles dissociate to release H+ ions.

HCl —> H+ + Cl-

Strong acids are more reactive than weak acids because the concentration of H+ ions is higher.

Question 10

What are weak acids?

Answer 10

They do not fully ionise in solution. Only a small proportion of acid particles dissociate to release H+ ions.

The ionisation of weak acids are reversible reactions, which sets up equilibrium between undissociated and dissociated acid. Since only a few of the acid particles release H+ ions, the position of equilibrium lied to the left.

CH3COOH (reversible sign) H+ + CH3COO-
(the numbers of meant to be below the letters)

Question 11

How is pH measured?

Answer 11

The pH of an acid or alkali is a measure of the concentration of H+ ions in the solution.

For every DECREASE of 1 on the pH scale, the concentration of H+ ions INCREASES by a factor of 10. So, an acid that has a pH of 4 has 10 times the concentration of H+ ions of an acid that has a pH of 5. For a decrease of 2 on the pH scale, the concentration of H+ ions increases by a factor of 100.

Question 12

What is the general rule for pH in relation to H+ concentration?

Answer 12

Factor H+ ion concentration changes by = 10(to the power of -x)

X is the difference in pH. So if the pH falls from 7 to 4 the difference is -3. Therefore, it is 10 to the power of -(-3). So, it’s 10 to the power of three.

Therefore, the pH of a strong acid is always less than the pH of a weaker acid if they have the same concentration.

Question 13

What does the acid strength tell you?

Answer 13

What proportion of the acid molecules ionise in water.

Question 14

What is the reaction between an acid and a metal oxide?

Answer 14

Acid + metal oxide —> salt + water

Question 15

What is the reaction between an acid and a metal hydroxide?

Answer 15

Acid + metal hydroxide —> salt + water

Question 16

Give some example of acids reacting with oxide or hydroxide.

Answer 16

Hydrochloric acid + copper oxide —> copper chloride + water

Sulphuric acid + potassium hydroxide —> potassium sulphate

Nitric acid + sodium hydroxide —> sodium nitrate + water

Question 17

What is the reaction between an acid and metal carbonate?

Answer 17

Acid + metal carbonate —> salt + water + carbon dioxide.

Question 18

Give some examples of reactions between acids and metal carbonates.

Answer 18

Hydrochloric acid + sodium carbonate —> sodium chloride + water + carbon dioxide

Sulphuric acid + calcium carbonate —> calcium sulphate + water + carbon dioxide.

Question 19

What is the practical for making soluble salts by using insoluble bases?

Answer 19

You must pick the right acid and insoluble base, such as an insoluble metal oxide, hydroxide or carbonate. Eg if you want to make copper chloride, you could mix hydrochloric acid and copper oxide.

Gently warm the dilute acid using a Bunsen burner, then turn off the Bunsen burner. Add the insoluble base to the acid a bit at a time, until no more reacts. You will know when it has all been neutralised because the excess solid will just sink to the bottom, even when stirred. Then filter out the excess solid to get the salt solution. To get pure, solid crystals of the salt, gently heat the solution using a water bath or electric heater to evaporate some of the water (to make it more concentrated). Leave the solution to cool after heating. Crystals of the salt should form which can then be filtered out of the solution and dried. This is called crystallisation.

Question 20

List the reactivity series.

Answer 20

Potassium 
Sodium 
Lithium 
Calcium 
Magnesium 
Carbon 
Zinc 
Iron 
Hydrogen 
Copper.

Question 21

How is the reactivity of a metal determined?

Answer 21

How easily they lose electrons, so forming positive ions. The higher up the list (reactivity series) they are, the more easily they form positive ions.
Also,
Higher up the list, the more easily they react with water or acid.

Question 22

What is the reaction between an acid and a metal?

Answer 22

Acid + metal —> salt + hydrogen.

Question 23

How can you see how reactive a metal is when reacting with an acid?

Answer 23

The rate at which hydrogen bubbles are given off.

Temperature change.

Question 24

What is the reaction between metal and water?

Answer 24

Metal + water —> metal hydroxide+hydrogen.

Question 25

How can some metals be extracted by reduction with carbon?

Answer 25

Some metals can be extracted from their ores chemically by reduction using carbon. The ore is reduced as oxygen is removed from it, and carbon gains oxygen so is oxidised.
Iron oxide + carbon —> iron + carbon dioxide.

Metals higher than carbon in the reactivity series can be extracted by reduction using carbon. For example, iron oxide is reduced in a blast furnace to make iron. This is because carbon can only take the oxygen away from metals which are less reactive than carbon itself.

Question 26

What are redox reactions?

Answer 26

If electrons are transferred, it is a redox reaction. Reduction and oxidation happen at the same time.
(Reduction is gain of electrons, oxidation is loss of electrons).

Question 27

What is electrolysis?

Answer 27

It meaning splitting up with electricity. During electrolysis, an electric current is passed through an electrolyte (a molten or dissolved ionic compound). The ions move towards the electrodes, where they react, and the compound decomposes.
The positive ions in the electrolyte will move towards the cathode (-ve electrode) and gain electrons (they are reduced)
The negative ions in the electrolyte will move towards the anode (+ve electrode) and lose electrons (they are oxidised).
This creates a flow of charge through the electrolyte as ions travel to the electrodes.
As ions gain or lose electrons, they form the uncharged element and are discharged from the electrolyte.

Question 28

What does electrolysis of molten ionic solids form?

Answer 28

Elements.
An ionic solid can’t be electrolysed because the ions are in fixed positions and can’t move.
Molten ionic compounds can be electrolysed because the ions can move freely and conduct electricity.
Molten ionic liquids eg lead bromine, are always broken up into their elements.
Positive metal ions are reduced to the element in the cathode.
Negative non-metal ions are oxidised to the element at the anode.

Question 29

How can metals be extracted from their ores using electrolysis?

Answer 29

If a metal is too reactive to be reduced with carbon or reacts with carbon, then electrolysis can be used to extract it. This method is very expensive as lots of energy is required to melt the ore and produce the required current.

Eg
Aluminium is extracted from the ore bauxite by electrolysis. Bauxite contains aluminium acids. Aluminium acids has a very high melting temperature so is mixed with cryolite to lower the melting point. The molten mixture contains free ions so it’ll conduct electricity. The positive Al3+ io s are attracted to the Heath e electrode where they each pick up three electrons and turn into neutral aluminium atoms. These then sink to the bottom of the electrolysis tank. The negative O2- ions are attracted to the posit e electrode where they each lose two electrons. The neutral oxygen atoms will then combine to form O2 molecules.