CHEMISTRY · Stage 7 · Chapter 5: Acids and Alkalis
Chemistry 5.3 Neutralisation reactions — Acids Reacting with Alkalis
5 min read 8 parts Lesson 3 of 7
Lesson summary
5.3 Neutralisation reactions — Acids Reacting with Alkalis — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define neutralisation, describe how an acid is neutralised, write equations, name salts and describe applications such as soil pH and indigestion.Neutralisa…
- Everyday example
- Further addition
- Symbol equations
- Metal oxides
- Metal carbonates
- Excess solid
Section 1 of 8What neutralisation is
- Neutralisation = a type of chemical reaction in which an alkali reacts with an acid and the pH gets closer to 7.
- Acid and alkali cancel each other's effects.
- Word equation: acid + alkali → salt + water.
- Ions
- hydrogen ions from the acid react with hydroxide ions from the alkali to form water: H+ + OH− → H2O.
- Energy
- the reaction releases heat, so the temperature rises (exothermic).
- Everyday example
- lemon juice (citric acid) fizzes with baking soda, and one product, carbon dioxide, forms as a gas that makes bubbles.
- (sodium hydrogencarbonate)
- lemon juice (citric acid) fizzes with baking soda, and one product, carbon dioxide, forms as a gas that makes bubbles.
Section 2 of 8Neutralisation in the lab
- Add universal indicator to the acid, which turns red when the pH is 1.
- Add sodium hydroxide solution to the acid-indicator mixture, and the mixture turns orange (pH 3) when some of the acid is neutralised.
- Continue to add the alkali, a little at a time, until the colour is green (pH 7), which shows all the acid is neutralised.
- Record the volume of alkali used.
- Further addition
- more alkali turns the mixture blue and then purple.
- pH graph
- the pH rises slowly at first, steeply near the neutral point, then levels off in the alkaline range.
- The steep section is why drop-by-drop addition is needed near the end-point.
Section 3 of 8Naming salts
- Salt name
- the first part comes from the metal in the base or alkali and the second part comes from the acid.
| Acid | Salt family |
|---|---|
| Hydrochloric acid | Chlorides |
| Sulfuric acid | Sulfates |
| Nitric acid | Nitrates |
- Hydrochloric acid + sodium hydroxide → sodium chloride + water.
- Sulfuric acid + potassium hydroxide → potassium sulfate + water.
- Nitric acid + calcium hydroxide → calcium nitrate + water.
- Symbol equations
- HCl + NaOH → NaCl + H2O, and H2SO4 + 2NaOH → Na2SO4 + 2H2O.
Section 4 of 8Other neutralisation reactions
- Metal oxides
- copper oxide + sulfuric acid → copper sulfate + water.
- Metal carbonates
- calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
Section 5 of 8Making a salt
- Warm dilute sulfuric acid and add copper oxide a little at a time until no more dissolves.
- Filter to remove the excess solid.
- Heat the filtrate to evaporate about half the water.
- Leave to cool and crystallise, then dry the crystals.
- Excess solid
- an insoluble base added in excess to acid, then filtered, gives a pure salt solution.
Section 6 of 8Using neutralisation: soil pH
- Preferred pH
- every plant has its preferred soil pH.
- Carrots grow well at pH 6.0, tea at pH 4.0 to 5.5 and date palms at pH 6.5 to 8.0.
| Plant | Preferred soil pH |
|---|---|
| Cabbage | 6.0-7.0 |
| Onion | 6.0-6.5 |
| Maize | 5.5-7.0 |
| Sweet potato | 5.0-6.0 |
| Tomato | 5.5-6.8 |
1Testing2 points
- mix soil with pure water and add universal indicator.
- Soil of pH 5.0 is suitable for sweet potatoes.
2Adjusting2 points
- adding an alkali such as lime neutralises some acid and raises the pH so cabbage and onions can grow.
- If a soil is too alkaline, an acid is added.
Section 7 of 8Treating stomach ache
- Excess stomach acid
- it causes stomach ache and indigestion.
- Indigestion tablets
- they contain an alkali or a carbonate that neutralises the extra acid.
- Toothpaste
- it counters acid made by bacteria.
Section 8 of 8Safety and disposal
- Wear eye protection when handling acids and alkalis, even dilute solutions.
- Spills
- neutralise acid spills with plenty of water or sodium hydrogencarbonate.
- Never pour concentrated chemicals down a sink.
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