Introduction to Titrations and Neutralisation
Welcome! In this chapter, we are going to learn how scientists act like "chemical detectives." We already know that acids and alkalis can cancel each other out in a reaction called neutralisation. But how do we know exactly how much acid is needed to neutralise a specific amount of alkali? This is where titration comes in. It is a precise laboratory technique used to find out the concentration of a solution.
Whether you are aiming for a grade 9 or just trying to get through your first lab report, don't worry! We will break this down step-by-step.
1. What is Neutralisation?
In simple terms, neutralisation happens when an acid reacts with a base (or alkali) to produce a salt and water. You might remember this general word equation:
\(\text{acid} + \text{alkali} \rightarrow \text{salt} + \text{water}\)
Neutralisation in Ionic Terms
To understand what is happening at the atomic level, we look at the ions involved. Acids produce hydrogen ions \((H^{+})\) in solution, and alkalis produce hydroxide ions \((OH^{-})\). When they meet, they form water, which is neutral.
The Ionic Equation:
\(H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)\)
Memory Tip: Think of \(H^{+}\) and \(OH^{-}\) as two puzzle pieces that fit together perfectly to make a stable drop of water (\(H_{2}O\)).
2. The Core Practical: Carrying Out a Titration (5.9C)
A titration is used to measure the volume of one solution that reacts exactly with a known volume of another solution. Usually, we use this to find an unknown concentration. Here is how you perform Core Practical 5.9C safely and accurately.
The Equipment You Need:
1. Burette: A long, thin glass tube with a tap at the bottom, used to add acid drop by drop.
2. Pipette: Used to measure an exact, fixed volume of alkali (usually \(25 \text{ cm}^{3}\)).
3. Conical Flask: Holds the alkali and indicator; its shape allows for easy swirling.
4. White Tile: Placed under the flask so you can see the colour change clearly.
5. Indicator: A chemical that changes colour at the exact moment neutralisation occurs.
Step-by-Step Procedure:
1. Use the pipette and a pipette filler to add a set volume of alkali to the conical flask.
2. Add a few drops of a suitable indicator (like phenolphthalein) to the alkali.
3. Fill the burette with acid and record the starting volume.
4. Slowly add the acid from the burette to the alkali, swirling the flask constantly.
5. When the colour starts to change slowly, add the acid drop by drop.
6. Stop as soon as there is a permanent colour change. This is called the end-point.
7. Record the final volume on the burette. The volume of acid added is called the titre.
8. Repeat the experiment until you get concordant results (results within \(0.10 \text{ cm}^{3}\) of each other) to ensure accuracy.
Quick Review: Why use a white tile? It makes the "end-point" colour change much easier to see, preventing you from adding too much acid!
3. Choosing the Right Indicator
For a titration, we need an indicator that changes colour sharply at the end-point. We don't usually use Universal Indicator because its colour changes are too gradual.
Common Indicators to Know:
1. Phenolphthalein:
- In alkali: Pink
- In acid/neutral: Colourless
2. Methyl Orange:
- In alkali: Yellow
- In acid: Red
- (At neutral end-point: Orange)
3. Litmus:
- In alkali: Blue
- In acid: Red
4. Titration Calculations (Higher Tier Only)
If you are taking the Higher Tier paper, you will need to use your titration results to calculate the concentration of the unknown solution. Concentrations are measured in \(g \text{ dm}^{-3}\) or \(mol \text{ dm}^{-3}\).
The Key Formula:
\(moles = concentration \times volume\)
Crucial Step: Converting Units
Volumes in chemistry experiments are usually measured in \(cm^{3}\), but concentrations use \(dm^{3}\).
To convert \(cm^{3}\) to \(dm^{3}\), you must divide by 1000.
\(1000 \text{ cm}^{3} = 1 \text{ dm}^{3}\)
Calculation Checklist:
1. Write out the balanced chemical equation for the reaction.
2. Calculate the moles of the "known" solution (the one where you have both volume and concentration).
3. Use the reacting ratio from the equation to find the moles of the "unknown" solution.
4. Calculate the concentration of the unknown solution using \(concentration = \frac{moles}{volume}\).
5. Preparing Pure, Dry Soluble Salts
Titration isn't just for measuring; it's also used to make salts. When you react a soluble acid with a soluble alkali (like sodium hydroxide), you can't just filter out excess solid because there isn't any! Everything is liquid.
The Process:
1. Carry out the titration as described above to find the exact volume of acid needed.
2. Repeat the process without the indicator using the exact same volumes. This ensures the salt isn't contaminated with indicator dye.
3. Evaporate the water from the resulting solution using a water bath or Bunsen burner to leave behind pure, dry crystals of the salt.
Summary: Key Takeaways
• Neutralisation is the reaction between \(H^{+}\) (from acid) and \(OH^{-}\) (from alkali) to form \(H_{2}O\).
• Titrations use a burette and pipette to find the exact volume of acid needed to neutralise an alkali.
• Phenolphthalein and methyl orange are the preferred indicators for sharp end-points.
• Concordant results (within \(0.10 \text{ cm}^{3}\)) are necessary for a reliable experiment.
• (H) Always convert your volumes from \(cm^{3}\) to \(dm^{3}\) before calculating concentration!