Investigating Neutralisation
Neutralisation occurs when acids and bases react to form salt and water. When you gradually add an alkali to an acid, the pH changes in a specific pattern - slowly at first, then rapidly, then slowly again.
A titration is a precise way to measure this neutralisation process. You use a burette (a tall glass tube with markings) to add alkali to acid and record the volume needed for neutralisation. This helps determine the exact amount needed to create a pure salt.
To make a pure dry salt, first perform a titration to find the exact volume of acid needed, then add that precise amount, and finally evaporate the water from the solution.
Top Tip: During a titration, the slower you add the reagent, the more accurately you can measure the pH at the neutralisation point!
Acid Reactions with Metals and Carbonates
When acids react with metals, they produce salt and hydrogen gas. You'll observe bubbles, the metal dissolving, and the solution warming up. The general equation is: Metal + Acid → Salt + Hydrogen
With carbonates, the reaction is: Carbonate + Acid → Salt + Water + Carbon Dioxide
These reactions can be written as ionic equations that show changes to ions. For example, when magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂
Bases, Alkalis and Balancing Equations
A base neutralises acids to form salt and water. An alkali is simply a base that dissolves in water. Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
When balancing equations for these reactions, remember that the numbers of atoms must be equal on both sides. For example: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Not all bases dissolve in water - insoluble bases like copper oxide can still neutralise acids, but they're not considered alkalis.
Acids, Alkalis and Indicators
The pH scale measures how acidic or alkaline a substance is. Acids have pH below 7, neutral substances have pH of 7, and alkalis have pH above 7. As the pH decreases by 1 unit, the concentration of H⁺ ions increases tenfold!
Different indicators change colour at different pH values:
- Litmus (red in acid, blue in alkali)
- Methyl orange (red in acid, orange in alkali)
- Phenolphthalein (colourless in acid, pink in alkali)
- Universal indicator (shows a range of colours from red to purple)
Acids produce excess hydrogen ions (H⁺) in water, while alkalis produce excess hydroxide ions (OH⁻).
Acids in Detail
The terms concentrated and dilute refer to the amount of acid dissolved in solution, while strong and weak refer to how completely the acid dissociates into ions.
Strong acids like hydrochloric (HCl), nitric (HNO₃), and sulfuric (H₂SO₄) dissociate completely in water, releasing many H⁺ ions. Weak acids like ethanoic acid (CH₃COOH) only partially dissociate.
Concentration is calculated as: amount dissolved ÷ volume of solution (dm³)
Bases and Salts
When an acid reacts with a base, it forms a salt and water. The type of salt produced depends on the acid and metal used:
- Hydrochloric acid forms chlorides
- Sulfuric acid forms sulfates
- Nitric acid forms nitrates
To prepare a soluble salt, warm some acid and add small amounts of the metal oxide until no more dissolves. Then filter to remove excess oxide and evaporate the water to obtain crystals.
Solubility and Precipitation
Not all salts dissolve in water. Most nitrates and sodium compounds are soluble, while most carbonates and hydroxides are insoluble.
A precipitate is an insoluble solid formed when two solutions mix, creating a cloudy appearance. This reaction is useful for preparing insoluble salts through precipitation reactions.
Practical Example: Preparing Copper Sulfate
To prepare copper sulfate crystals:
- Warm dilute sulfuric acid
- Add excess copper oxide (the solution turns blue)
- Filter to remove unreacted copper oxide
- Evaporate some water until crystallisation begins
- Leave to cool and form blue diamond-shaped crystals
Remember: Adding excess base ensures all the acid is neutralised, which is crucial for obtaining pure salt crystals!