Question 1
In HCl + NaOH → NaCl + H₂O, do any oxidation numbers change?
Need a hint for Question 1?
Write the total required charge beside the formula before setting up the equation.
Check Question 1
No. This neutralisation is not a redox reaction.
STUDENT SPACE · TOPIC 2
Oxidation numbers
Use oxidation numbers to recognise redox and name compounds.
The essentials
- An uncombined element has oxidation number 0. A monatomic ion has the same oxidation number as its charge.
- In the compounds used here, O is −2 and H is +1. Group I metals are +1; Group II metals are +2.
- Iron(II) chloride is FeCl₂. Iron(III) chloride is FeCl₃.
Key vocabulary
| English | Chinese |
|---|---|
| oxidation number | 氧化数 |
| uncombined element | 游离态元素 |
| monatomic ion | 单原子离子 |
| Roman numeral | 罗马数字 |
Quick practice
Try each question before opening its hint or answer. Use paper for calculations and diagrams.
Question 2
Find the oxidation number of S in SO₂ and in SO₄²⁻.
Need a hint for Question 2?
Oxygen is −2; oxidation numbers sum to the overall charge.
Check Question 2
S is +4 in SO₂; +6 in SO₄²⁻.
Question 3
In 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻, identify what is oxidised and what is reduced.
Need a hint for Question 3?
Increasing oxidation number means oxidation.
Check Question 3
Fe²⁺ is oxidised: +2 to +3. Cl₂ is reduced: 0 to −1.
Use Lesson 2 in your worksheet for written practice. Your teacher will discuss those answers in class.