Chapter 4: Acids and Redox

Question 1

1. All acids contain hydrogen in their formulae. What happens when an acid is dissolved in water?
2. What is a strong acid? Give an example.
3. What is a weak acid? Give an example.
4. In an equation for a weak acid dissolving in water, what does the equilibrium sign indicate?

Answer 1

1. When dissolved in water, an acid releases hydrogen ions as protons, H⁺, into the solution.
2. A strong acid, such as hydrochloric acid (HCl), releases all its hydrogen atoms into solution as H⁺ ions and completely dissociates in aqueous solution.
3. A weak acid, such as ethanoic acid (CH₃COOH), only releases a small proportion of its available hydrogen atoms into solution as H⁺ ions. A weak acid partially dissociates in aqueous solution.
4. The equiibirum sign indicates that the forward reaction is incomplete.

Question 2

1. What is a base?
2. What types of substances are classified as bases?
3. What is an alkali?

Answer 2

1. A base neutralises an acid to form a salt.
2. Metal oxides, metal hydroxides, metal carbonates, and ammonia (NH₃) are classified as bases.
3. An alkali is a base that dissolves in water releasing hydroxide ions (OH^–) into the solution.

Question 3

1. Describe the neutralisation of an acid.
2. Describe the neutralisation reaction between acids and metal oxides/hydroxides.
3. Describe the neutralisation reaction between acids and alkalis.
4. What is the ionic equation for the neutralisation reaction between acids and alkalis?
5. Describe the neutralisation reaction betwee acids and metal carbonates.

Answer 3

1. In the neutralisation of an acid, H⁺ (aq) ions react with a base to form a salt and neutral water. The H⁺ ions are replaced by metal or ammonium ions from the base.
2. An acid is neutralised by a metal oxide or hydroxide to form salt and water only.
3. An acid is neutralised by an alkali to form salt and water only.
4. H⁺ (aq) + OH^– (aq) → H₂O (l)
5. An acid is neutralised by a metal carbonate to form salt, water and carbon dioxide gas only.

Question 4

1. What is a titration?
2. What can titrations be used for?

Answer 4

1. A titration is a technique used to accurately measure the volume of one solution that reacts exactly with another solution.
2. Titrations can be used for finding the concentration of a solution, identification of unknown chemicals, and finding the purity of a substance.

Question 5

1. What are the typical tolerances of a 100 cm³ and 250 cm³ volumetric flask?
2. Why is excess solution run through the burette tap before the titration?
3. Describe how a burette reading is taken and recorded.

Answer 5

1. A 100 cm³ volumetric flask is manufactured to a tolerance of ± 0.20 cm³. A 250 cm³ volumetric flask is manufactured to a tolerance of ± 0.30 cm³.
2. Excess solution is run through the burette tap before the titration to remove any air bubbles. If a bubble is left in the neck of the burette, the air could be released during the titration, leading to an error in the titre.
3. A burette reading is recorded to the nearest half division, with the bottom of the meniscus on a mark or between two marks. Each burette reading is measured to the nearest 0.05 cm³ so the reading always has two decimal places, the last place always being a 0 or a 5.

Question 6

Describe how a standard solution is prepared.

Answer 6

1. The solid is first weighed accurately.
2. The solid is dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark.
3. This solution will be transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water.
4. The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the bottom of the meniscus lines up exactly at the mark. You should view the graduation mark and meniscus at eye level for accuracy.
5. Finally, the volumetric flask is inverted several times to mix the solution thoroughly. This is to make sure the solution is consistent. You will be able to see the solution mixing when you invert the flask as the more dense original solution moves through the solution.

Question 7

Describe the procedure for an acid-base titration.

Answer 7

1. Add a measured volume of one solution to a conical flask using a pipette.
2. Add the other solution to a burette, and record the initial burette reading.
3. Add a few drops of indicator to the solution in the conical flask.
4. Run the solution in the burette into the solution in the conical flask, swirling the concical flask throughout to mix the two solutions. Eventually the indicator changes colour at the end point of the titration. The end point is used to indicate the volume of one solution that exactly reacts with the volume of the second solution.
5. Record the final burette reading. The volume of solution added from the burette is called the titre.
6. A quick, trial titration is carried out first to find the approximate titre.
7. The titration is then repeated accurately, adding the solution dropwise as the end point is approached. Further titrations are carried out until two accurate titres are concordant – agreeing to within 0.10 cm³.

Question 8

From the results of a titration, you will know the following:

• both the concentration and reacting volume for one of the solutions
• only the reacting volume (or only the concentration) for the other solution

Describe a method to determine the unknown value.

Answer 8

Step 1: Work out the amount, in mol, of the solute in the solution for which you know both the concentration and volume.

Step 2: Use the equation to work out the amount, in mol, of the solute in the other equation.

Step 3: Work out the unknown information for the other solution.

Question 9

1. What is an oxidation number.
2. What is the oxidation number rule for elements?

Answer 9

1. An oxidation number is a measure of the number of electrons that an atom uses to bond with atoms of another element. Oxidation numbers are derived from a set of rules.
2. The oxidation number is always zero for elements.

Question 10

What are the oxidation number rules in compounds and ions? Include examples and special cases.

Answer 10

Question 11

What does the sum of oxidation numbers represent?

Answer 11

sum of oxidation numbers = total charge

Question 12

1. What do roman numerals in compound naming represent?
2. When roman numerals are omitted, what is nitrate and sulfate assumed to be?

Answer 12

1. Roman numerals in a compound represent the oxidation number, without a sign.
2. Nitrate is assumed to be NO₃^–. Sulfate is assumed to be SO₄^2–.

Question 13

Describe redox reactions in terms of electrons and oxidation number.

Answer 13

In terms of electrons, reduction is the gain of electrons and oxidation is the loss of electrons.

In terms of oxidation number, reduction is a decrease in oxidation number and oxidation is an increase in oxidation number.

Question 14

Describe the reaction of dilute acids with metals. Include change in oxidation number.

Answer 14

Dilute acids undergo redox reactions with some metals to produce a salt and hydrogen gas.

In such reaction, the metal is oxidised and hence has an increase in oxidation number. The acid is reduced and hence has a decrease in oxidation number.