3.3.5Chemical Reactions
Effect of Hydrogen Ion Concentration on pH — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 16 of 20
Section 1 of 6pH and hydrogen ions (HT only)
What pH measures
- What pH measures pH is a measure of the concentration of hydrogen ions, H+, in a solution.
- The lower the pH, the higher the hydrogen ion concentration.
A logarithmic scale
- A logarithmic scale Each fall of 1 pH unit means the hydrogen ion concentration is ten times greater.
- A solution at pH 3 has ten times the H+ concentration of pH 4, and one at pH 2 has 100 times that of pH 4.
Section 2 of 6Strength and concentration (HT only)
| Idea | Meaning | Effect on pH |
|---|---|---|
| Concentrated | Many moles of acid or alkali per dm3 of solution | Very low (below about 4) for acids, very high (above about 10) for alkalis |
| Dilute | Few moles per dm3 | Closer to 7, around 5 to 9 |
| Strong acid | Ionises (dissociates) almost completely in water | Lower pH than a weak acid of the same concentration |
| Weak acid | Only partly ionises, so most molecules stay intact | Higher pH than a strong acid of the same concentration |
Section 3 of 6Why they are not the same thing (HT only)
Acid properties
Strength
- Strength Describes the proportion of acid molecules that ionise to release H+ ions.
Concentration
- Concentration Describes how much acid is dissolved in a given volume of solution.
Dilute strong acid
- Dilute strong acid Possible: few acid particles per dm3 but all of them ionised.
Concentrated weak acid
- Concentrated weak acid Possible: many acid particles per dm3 but only a small fraction ionised.
Section 4 of 6pH titration curves (HT only)
1Adding base to acid
- Adding base to acid The solution starts at low pH with a high H+ concentration.
2Neutralisation proceeds
- Neutralisation proceeds The base reacts with H+ ions, so their concentration falls and pH rises.
3Rapid change
- Rapid change Near the end point the pH rises steeply to above 7 as the last H+ ions are used up.
4Adding acid to alkali
- Adding acid to alkali The curve is the reverse: pH starts high and falls rapidly as H+ concentration increases.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| pH | A scale measuring the concentration of hydrogen ions in a solution. |
| Strong acid | An acid that is completely ionised in aqueous solution. |
| Weak acid | An acid that is only partly ionised in aqueous solution. |
| Concentration | The amount of substance dissolved per unit volume, for example mol/dm3. |
| Titration curve | A graph of pH against volume of solution added. |
Section 6 of 6Exam technique
1
Using strong and concentrated as if they mean the same. Strength is about ionisation, concentration is about quantity per volume.
2
Thinking a drop of 1 pH unit doubles the H+ concentration. It is ten times greater.
3
Saying a weak acid does not react. It reacts the same way but more slowly, because fewer H+ ions are available at a time.
4
Describing the titration curve as a gradual straight line. It changes slowly, then sharply near the end point.
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