SC8 - Acids and Alkalis

Question 1

SC8a
What do the following hazard symbols mean:
1) Exclamation mark
2) Person with white chest
3) Skull and crossbones
4) Chemical on hand

Answer 1

1) Exclamation mark: moderate health hazard
2) Person with white chest: serious health hazard
3) Skull and crossbones: toxic
4) Chemical on hand: corrosive

Question 2

SC8a
1) Name the ions present in all acidic and all alkaline solutions.
2) State the pH values associated with acidic, alkaline and neutral solutions.
3) Explain the link between pH and the concentration of ions in acids and alkalis.

Answer 2

1) If a solution is acidic, then it will contain hydrogen ions, H+. If a solution is alkaline, then it will contain hydroxide ions, OH-.
2) Acidic solutions have a pH less than 7 (pH 1-6). Neutral solutions have a pH of 7. Alkaline solutions have a pH more than 7 (pH 8 - 14).
3) If a solution is acidic, then it will contain hydrogen ions, H+. The higher the concentration of H+ ions in an acidic solution, the lower the pH.
If a solution is alkaline, then it will contain hydroxide ions, OH-. The higher the concentration of OH- ions in an alkaline solution, the higher the pH.

Question 3

SC8b
1) Describe the relationship between hydrogen ion concentration and pH.
2) Explain the difference between a dilute and concentrated solution (in terms of the amount of solute present).
3) Explain the difference between strong and weak acids (in terms of the degree of dissociation of the acid molecules).

Answer 3

1) If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1. So multiplying the concentration by 10 decreases the pH by 1, and multiplying the concentration by 100 decreases the pH by 2.
2) A concentrated solution is one that has a relatively large amount of dissolved solute for a certain volume.
A dilute solution is one that has a relatively small amount of dissolved solute for a certain volume.
3) In a weak acid, only a small proportion of molecules dissociate to release hydrogen ions.
In a strong acid, all of the molecules dissociate to release hydrogen ions.

Question 4

SC8b
1) Explain how the pH and reactivity of an acid depend on the concentration and the strength of the acid.
2) What does the chemical equation for a weak acid look like?

Answer 4

1) The higher the concentration of H+ ions, the lower the pH and the stronger the acid. A stronger acid will react more than a weak acid of the same concentration of solvent. A more concentrated acid will have a stronger reaction than a more dilute acid with the same concentration of H+ ions.
2) On the products side, write the same formula, but remove the hydrogen and write it separately as a H+ ion. The end of the new formula will have a - sign on the oxygen (O-).
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
The ⇌ symbol is used in the equation to show that the reaction is a reversible reaction, and does not go to completion.

Question 5

SC8c
1) Describe how a base reacts in a neutralisation reaction, and how the pH changes in the acid
2) Describe what happens when an acid reacts with a metal oxide
3) What is the word and symbol equations (with magnesium and hydrochloric acid) for the reactions of acids and metal?

Answer 5

1) During neutralisation, adding the base to the acid increases the pH of the acid until it is above 7. The hydroxide ions in the base react with the hydrogen ions in the acid. Therefore, the hydrogen ion concentration is reduced.
2) A salt and water are produced when acids react with metal oxides. Metal oxides are bases, because they neutralise acids.
3) acid + metal → salt + hydrogen
Eg. (With magnesium and hydrochloric acid, already balanced) 2HCl + Mg → MgCl2 + H2

Question 6

SC8c
1) Explain what happens during a neutralisation reaction, including the products
2) Why is an excess of insoluble reactant is used when preparing a soluble salt?
3) Why is the excess reactant is removed when preparing a soluble salt?

Answer 6

1) Acids can react with bases. When an acid reacts with a base, this forms a salt and water. This reaction is known as neutralisation.
Salt formation can occur. One of the products of a neutralisation reaction is a salt. This compound is formed from the acid and the metal (or positive ion) in the base.
2) Excess of the solid reactant is added to make sure that all of the acid has reacted. When the solid reactant remains (does not react), this shows that all the acid has been used up.
3) This ensures that the soluble salt is not contaminated with insoluble reactant once it has dried.

Question 7

SC8d
1) Define alkali
2) What are the 4 state symbols?
3) How do you balance chemical equations
4) Describe the reactions of alkalis with acids, including what is visible

Answer 7

1) An alkali is a soluble base, and produces hydroxide ions, OH-(aq), when dissolved in water.
2) Solid (s), liquid (l), aqueous (aq) ie. dissolved in water, gas (g)
3) Change the big numbers in front of the formulae until there is the same number of each atom or ion on each side of the equation. It is easier to start with the least common element first.
4) It is a neutralisation reaction. Bubbles are formed, and a salt and water is produced.

Question 8

SC8e
1) Explain what happens to the ions from acids and alkalis during neutralisation
2) Explain why titration is used to prepare soluble salts

Answer 8

1) In a neutralisation reaction, hydrogen irons from the acid reacts with hydroxide ions from the alkali, the product of the reaction is water. The other ions from the acid and alkali stay in the solution as ions of the dissolved salt.
2) Titration is used to prepare soluble salts to ensure that a neutral solution is obtained. To obtain a neutral solution, you need to mix an acid and alkali in the correct proportions proportions that you end up with a solution that contains only water and the desired salt.

Question 9

SC8f
1) Explain the general reaction between an acid and a metal and what is visible
2) Explain the general reaction between an acid and a metal carbonate and what is visible

Answer 9

1) An acid and a metal reacts to produce a salt and hydrogen. The hydrogen causes bubbling during the reaction. In general, the more reactive the metal, the faster the reaction. This is indicated by more bubbles being given off per second from the metals with higher reactivity. Also, the reaction of metals with acids is exothermic (ie heat energy is given out).
2) An acid and a metal carbonate reacts to produce a salt, water and carbon dioxide. The carbon dioxide causes bubbling during the reaction, which is observed as fizzing. Also, the reaction of metal carbonates with acids is exothermic (ie. heat energy is given out).

Question 10

SC8f
1) Describe the test for hydrogen
2) Describe the test for carbon dioxide
3) How do you write balanced ionic equations?

Answer 10

1) Test for Hydrogen: Place a lighted splint in a test tube containing the gas. If the gas is hydrogen, the hydrogen gas will burn (there will be a squeaky pop sound heard).
2) Test for carbon dioxide: If you bubble carbon dioxide through limewater, the limewater will turn cloudy.
3) Step 1: Write out the full balanced equation:
2KI (aq) + Cl2 (aq) → 2KCl (aq) + I2 (aq)
Step 2: Identify the ionic substances and write down the ions separately
2K+ (aq) + 2I- (aq) + Cl2 (aq) → 2K+ (aq) + 2Cl- (aq) + I2 (aq)
Step 3: Rewrite the equation eliminating the ions which appear on both sides of the equation (spectator ions ) which in this case are the K+ ions:
2I- (aq) + Cl2 (aq) → 2Cl- (aq) + I2 (aq)

Question 11

SC8g
1) What ionic compounds are all soluble?
2) What are the solubility rules for chlorides and sulfates?
3) What are the solubility rules for carbonate and hydroxides?

Answer 11

1) All common sodium, potassium and ammonium salts, and all nitrates are soluble (SNAP acronym)
2) Most common chlorides are soluble except lead chloride (Pb), silver chloride (Ag) and mercury chloride (Hg), (PAH acronym)
Most common sulfates are soluble except barium sulfate, calcium sulfate, lead sulfate (BaCaP acronym)
3) Sodium carbonate, potassium carbonate, ammonium carbonate are soluble, but all other carbonates are insoluble
Sodium hydroxide, potassium hydroxide, ammonium hydroxide are soluble, but all other hydroxides are insoluble

Question 12

SC8g
1) How to predict whether or not a precipitate will form from two solutions
2) Describe how to prepare a pure, dry sample of an insoluble salt
3) Why is the beaker rinsed and then added to the filter paper?
4) Suggest why deionised/ distilled water is used instead of tap water

Answer 12

1) You can predict whether a precipitate will form by checking the solubilities of the products. If both products are soluble, no precipitate will form. For example, potassium sulfate does not form a precipitate because all potassium salts are soluble. However, copper carbonate is not soluble, so it should form a precipitate.
2) A pure, dry sample of an insoluble salt can be prepared from two soluble salts in this way:
• Wear eye protection.
• Mix the two solutions in a beaker, then filter the mixture.
• Rinse the beaker with a little distilled water and pour this through the funnel.
• Pour a little distilled water over the precipitate in the funnel.
• Carefully remove the filter paper containing the precipitate and dry it in a warm oven in order to obtain the solid.
3) To make sure that all of the precipitate is transferred from the beaker to the funnel, and make sure that she doesn’t lose any of the product.
4) Deionised water doesn’t contain any other ions which might contaminate the pure salt.

Question 13

SC8
1) What does a metal and acid react to produce?
2) What does a base and acid react to produce?
3) What does an alkali and acid react to produce
4) What does a carbonate and acid react to produce?

Answer 13

1) Metal + acid → salt + hydrogen
2) Base + acid → salt + water
3) Alkali + acid → salt + water
4) Carbonate + acid → salt + water + carbon dioxide

Question 14

SC8c - Core practical
1) Why is the acid heated during the preparing copper sulfate core practical?
2) Why is the acid heated with a water bath instead of a Bunsen burner?
3) What are the safety considerations for the practical?

Answer 14

1) The sulfric acid is heated to speed up the reaction.
2) The water bath heats the acid to a fixed lower temperature. It is safer than a Bunsen burner which will heat the acid and could make it boil.
3) Wear eye protection/safety glasses (to reduce the risk of splashes damaging eyes.)
Clean up all spills to reduce the risk of accidental damage to skin or other materials.

Question 15

SC8c - Core practical
1) Suggests a reason why different sized crystals may be produced.
2) How do you know a chemical reaction has occurred when the copper is mixed with an acid
3) When given the choice, what compound should be used to react with an acid to prepare crystals?

Answer 15

1) Small crystals are produced by fast evaporation of the water in the solution. Large crystals are produced by slow evaporation of the water in the solution.
2) A chemical reaction always forms a new substance so the change of colour (to blue) indicates a new substance being formed.
3) An insoluble solid, for example most hydroxides (eg. magnesium hydroxide) is insoluble.

Question 16

SC8c - Core practical
Describe the method for the preparing copper sulfate core practical

Answer 16

Aim: investigate the preparation of pure, dry hydrated copper sulfate crystals starting from copper sulfate including the use of a water bath.
A) Measure 20 cm3 of dilute sulfuric acid using a measuring cylinder and pour it into a small conical flask.
B) Warm the acid in a water bath set at 50 °C. Use a thermometer to measure the temperature.
C) Add a little copper oxide powder to the acid and stir.
D) If all the copper oxide reacts, and disappears, add a little more. Stop when the copper oxide is in excess and no longer reacts.
E) Filter the mixture and transfer the filtrate to an evaporating basin.
F) Heat the evaporating basin by placing it over a beaker of water heated with a Bunsen burner as shown in diagram D on the previous page. Stop heating when crystals start to form.
G) Pour the solution into a watch glass and leave for a few days to allow all the water to evaporate.

Question 17

SC8d - Core Practical
Describe the method for the investigating neutralisation core practical

Answer 17

A) Use a measuring cylinder to add 50 cm3 of dilute hydrochloric acid to a beaker.
B) Estimate and record the pH of the contents of the beaker.
• Put a piece of universal indicator paper onto a white tile.
Dip the end of a glass rod into the liquid, then tap it onto the universal indicator paper.
• Wait 30 seconds, then match the colour to the appropriate pH on a pH colour chart.
Rinse the glass rod with water.
C) Measure out 0.3 g of calcium hydroxide powder onto a piece of paper or a ‘weighing boat’.
D) Add the calcium hydroxide powder to the beaker and stir. Then estimate and record the pH of the mixture.
E) Repeat steps B and C seven times so that you add a total of 2.4g of calcium hydroxide powder to the acid.
F) Plot a graph with pH on the vertical axis and mass of calcium hydroxide on the horizontal axis.

Question 18

SC8d - Core practical
1) What can be used to show that a solution is acidic or alkaline?
2) Why does adding hydroxide ions to an acid lead to an increase in pH?
3) Why must a pH metre be calibrated?
4) Why must eye protection be worn when handling hydrochloric acid?

Answer 18

1) A pH metre or an indicator.
2) Hydroxide ions react with hydrogen ions and reduce the hydrogen ion concentration therefore increase pH.
3) To make sure that it gives an accurate pH value.
4) To avoid damage to eyes because hydrochloric acid is irritant/corrosive.

Question 19

SC8g
1) How should you ensure accuracy when reading values from a burette during titration?
2) Describe how to carry out an acid–alkali titration
3) How would you obtain a pure, dry salt using titration?

Answer 19

1) Read to 2 decimal places, or the highest degree of accuracy that the burette provides.
Ensure that the test is done multiple times to find the mean, and any anomalous results are not used.
2) 1. Rinse pipette with alkali and burette with acid
2. Measure alkali using a pipette into suitable container e.g. flask/beaker and place flask on a white tile
3. Add a few drops of indicator/suitable named indicator (eg methyl orange/phenolphthalein)
4. Fill burette with acid and read volume of acid in burette
5. Add acid from burette to the flask slowly swirling the flask until {indicator just changes colour/correct colour change for named indicator (eg methyl orange yellow to peach/orange, phenolphthalein pink to colourless)/solution is neutral}
6. Read volume of acid in burette at end of titration
7. Repeat experiment until the results are consistent
8. Mix the same volume of alkali with the volume of acid determined from the titration but do not add indicator
9. Pour solution into an evaporating basin then {heat solution/leave the water to evaporate} until pure salt crystals are left
10. Dry crystals using absorbent paper
3) To make a pure, dry salt:
- Carry out a titration
- Note the exact volume of acid needed to neutralise the alkali
- Use the burette to add the correct volume of acid without the indicator
- Evaporate the water from the solution formed

Question 20

SC8a
What do the following hazard symbols mean:
1) Flame
2) Flame with circle
3) Tree and dead fish

Answer 20

1) Flame: flammable
2) Flame with circle: oxidising
3) Tree and dead fish: harmful to the environment

Question 21

SC8a
1) Describe the safety precautions that should be observed when handling different acids and alkalis.
2) Describe the effect of acids and alkalis of the 3 common indicators: blue litmus paper, methyl orange and phenolphthalein

Answer 21

1) Wearing eye protection and gloves to avoid contact with skin and eyes.
2) Blue litmus paper: acidic - red, neutral - purple, alkaline - blue
Methyl orange: acidic - red, neutral - yellow, alkaline - yellow
Phenolphthalein: acidic - colourless, neutral - colourless, alkaline - pink

Question 22

SC8d
What are the formulae for the following:
1) Hydroxide
2) Nitrate
3) Sulfate

Answer 22

1) Hydroxide: OH-
2) Nitrate: NO₃-
3) Sulfate: SO₄²⁻

Question 23

SC8d
What are the formulae for the following:
1) Carbonate
2) Ammonium
3) Hydrochloric acid

Answer 23

1) Carbonate: CO₃²⁻
2) Ammonium: NH₄+
3) Hydrochloric acid: HCl

Question 24

SC8d
What are the formulae for the following:
1) Sulfuric acid
2) Nitric acid

Answer 24

1) Sulfuric acid: H₂SO₄
2) Nitric acid: HNO₃