Topic 8 - Acids and Alkalis

Question 1

3.1 - What type of ions are in acids and alkalis?

Answer 1

Ions are atoms that have become charged by losing or gaining electrons.
Acids produce excess of hydrogen ions
Alkalis produce excess hydroxide ions

Question 2

3.2 - How does the pH of a solution vary based on its acidity?

Answer 2

A solution which is alkaline has a pH higher than 7.
A solution which is acidic has a pH higher lower 7.
A solution which is neutral has a pH of 7.

Question 3

3.3 - What are the effects of acid and alkalis on indicators?

Answer 3

Phenolphthalein - If acid the solution is pink, if alkaline the solution is colourless.
Methyl orange - If acid the solution is red, if alkaline the solution is yellow.
Litmus paper - If acid the solution is red, if alkaline the solution is blue.

Question 4

3.4 - What does the pH tell us about the ions in that solution?

Answer 4

Higher the amount of hydrogen ions, higher the concentration so the solution is more acidic and the pH is lower.
The higher the amount of hydroxide ions, higher the concentration so the solution is more more alkaline an pH is higher.
Neutral solutions with pH of 7 have equal and low amounts of hydrogen and hydroxide ions.

Question 5

3.5 - How do changes in concentration of hydrogen ions affect the pH?

Answer 5

When the concentration of hydrogen ions increase by a factor of 10, the pH decreases by 1.
When the concentration of hydrogen ions decreases by a factor of 10, the pH increases by 1.

Question 6

3.6 - Core Practical : Investigate the change in pH by adding alkaline salts to an acid

Answer 6

Method - add dilute HCl to the beaker and measure pH
add weighed mass of calcium hydroxide and stir then record pH
keep adding weighed masses of calcium hydroxide until there is no more
change to the pH
Recording - draw a line graph with mass added on the horizontal axis and with pH on the vertical axis, draw a line of best fit (ignore any anomalies).

Question 7

3.7 - Explain the difference between dilute and concentrated solutions

Answer 7

Concentrated solutions have a lot of dissolved solute per unit volume.
Dilute solutions have a small amount of solute per unit volume.

Question 8

3.8 - Explain the difference between strong and weak acids

Answer 8

Strong acids have molecules that dissociate completely into ions when they dissolve in water producing high concentration of hydrogen ions.
Weak acids have molecules that partially dissociate into ions in a solution, therefore there are less hydrogen ions so higher pH.
Strength of acid increases with dissociation as there are more hydrogen ions to react with, decreasing the pH.

Question 9

3.9 - What is a base?

Answer 9

They are substances the neutralise acids to form a salt and water only.

Question 10

3.10 - What type of solution is usually a base?

Answer 10

Alkaline solutions are soluble bases.

Question 11

3.11 - Explain the general reactions of acids with metals

Answer 11

Acid + metal –> hydrogen + salt
Acid + metal oxide –> water + salt
Acid + metal hydroxide –> water + salt
Acid + metal carbonate –> water + carbon dioxide + salt
Metal oxide make bases ( salt ) because they are insoluble.
Metal hydroxides make alkalis if soluble, or bases if insoluble.
HCL - chloride
H2SO4 - sulfate
HNO3 - nitrate

Question 12

3.12 - Describe the chemical tests for hydrogen and carbon dioxide

Answer 12

Hydrogen - burning splint over the open end of a test tube of hydrogen gas.
Creates a squeaky pop sound if present.
Carbon dioxide - Bubble the gas in limewater ( calcium hydroxide solution ), if present, turns milky.

Question 13

3.13 - What happens during neutralisation?

Answer 13

Reaction between an acid and a base.
Metal oxide + acid –> salt + water

Question 14

3.14 - Explain the reaction of ions for neutralisation

Answer 14

Metal oxide + acid –> salt + water
The hydrogen ions in the acid and the oxide ions in the metal oxide to form water. As this removes hydrogen ions the pH increases.
Salts are formed as there are no hydrogen ions

Question 15

3.15 - How can a soluble salt be prepared from an acid and insoluble base?

Answer 15

Add excess base to the acid so that all of the acid has reacted.
Excess reactant is removed in order to make sure the salt is pure by filtration to remove the residue of the unreacted metal oxide.
The solution remaining is salt and water, this is then made into solid sample of salt is obtained by letting the water evaporate so crystallisation happens, allowing the salt to form larger crystals.
If the salt was a carbonate you would still get salt + water as the CO2 would dissipate.

Question 16

3.16 - How do you make a soluble salt using titration?

Answer 16

Acid + Soluble reactant –> Soluble reactant + water
To obtain a soluble salt from that soluble solution, you have to have a neutral solution before evaporating the water, so that the salt isn’t contaminated by excess of one reactant.
To get this neutral solution, the amount of acid and alkali mixed must be in correct proportions to get a solution containing water and salt, this is done by titration.

Question 17

3.17 - Core Practical : Investigate the preparation of pure, dry, hydrated copper sulfate crystals from copper oxide

Answer 17

Measure sulfuric acid using measuring cylinder and pour into a flask.
Warm acid in a water bath at 50°, use thermometer to measure temperature.
Add copper oxide to acid and stir.
Continue adding copper oxide until it is in excess and all the acid has reacted.
Filter the mixture and transfer filtrate into an evaporating basin.
Heat the basin with a bunsen burner until crystal form around the edges.
Allow a few days for the solution to evaporate.

Question 18

3.18 - Describe how to carry out titration

Answer 18

Wash burette using water.
Use the pipette and pipette filler to add 25 cm3 of alkali to a clean conical flask.
Add a few drops of phenolpthalein and put the conical flask on a white tile.
Fill the burette with acid up to the meniscus and note the starting volume.
Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
Stop adding the acid when the there is a colour change from colourless to pink. Note the final volume.

Question 19

3.19 - What are the rules for solubility of common substances in water?

Answer 19

Soluble in water - All sodium, potassium and ammonium salts, carbonates and hydroxide.
All nitrates, most chlorides and sulfates.
Insoluble in water - Lead, barium and calcium sulfates.
Silver and lead chlorides.
Most carbonates and hydroxides.

Question 20

3.20 - How do you predict whether a precipitate will be formed in a reaction?

Answer 20

All salts are ionic so when two soluble solutions react the ions swap.
The precipitate can be predicted by knowing the solubility of both solutions.
If both the solutions are soluble then no precipitation is formed.
If one of the solutions are insoluble then a precipitation is formed.

Question 21

3.21 - How do you prepare a sample of pure, dry insoluble salt?

Answer 21

Insoluble reactant + Soluble reactant –> Insoluble salt + Soluble solution
Wear eye protection
Mix the two solutions in a beaker, then filter the mixture with the salt.
Rinse beaker with some distilled water then pour through the funnel.
Pour a little distilled water over the precipitate/residue to clean the soluble solution off of it.
Remove filter paper and dry in a warm oven.