Introduction to Acids and Alkalis

What are Acids?

Acids are substances that have a sour taste and can turn blue litmus paper red. They contain hydrogen ions ($H^+$) when dissolved in water. Common examples of acids include:

  • Hydrochloric acid (HCl): Found in your stomach to help digest food.
  • Sulphuric acid (H₂SO₄): Used in car batteries.
  • Citric acid: Found in citrus fruits like lemons and oranges.

What are Alkalis?

Alkalis are substances that have a bitter taste and feel slippery. They turn red litmus paper blue and contain hydroxide ions ($OH^-$) when dissolved in water. Common examples of alkalis include:

  • Sodium hydroxide (NaOH): Used in drain cleaners.
  • Potassium hydroxide (KOH): Found in some soaps.
  • Ammonium hydroxide: Used in cleaning products.

pH Scale

The pH scale measures how acidic or alkaline a solution is. It ranges from 0 to 14:

  • pH 0-6: Acidic
  • pH 7: Neutral (pure water)
  • pH 8-14: Alkaline

Each whole number change in pH represents a tenfold change in acidity or alkalinity. For example, a solution with pH 3 is ten times more acidic than one with pH 4.

Neutralisation

What is Neutralisation?

Neutralisation is a chemical reaction between an acid and an alkali that produces water and a salt. For example:

\text{HCl (acid) + NaOH (alkali) } \rightarrow \text{ NaCl (salt) + H}_2\text{O (water)}

Key Points on Neutralisation:

  • The end product of a neutralisation reaction is a salt and water.
  • The pH of the solution after neutralisation will be closer to 7 (neutral).

Titrations

What is a Titration?

A titration is a method used to determine the concentration of an acid or alkali. It involves slowly adding one solution to another until the reaction is complete.

How to Perform a Titration:

  1. Prepare the equipment: You will need a burette, pipette, conical flask, and a white tile.
  2. Fill the burette: Add the titrant (usually an alkali) to the burette.
  3. Measure the acid: Use a pipette to measure a specific volume of the acid into the conical flask.
  4. Add an indicator: This will show when the reaction is complete. For example, phenolphthalein changes from pink (alkaline) to colourless (acidic).
  5. Perform the titration: Slowly release the alkali from the burette into the acid while swirling the flask. Stop when the indicator changes colour.
  6. Record the volume: Note how much alkali you used to reach the endpoint.

Key Tips for Titrations:

  • Use a white tile under the flask to see the colour change clearly.
  • Be patient and add the titrant slowly as you approach the endpoint.
  • Repeat the titration several times to get an average result for accuracy.

Questions

Easy Level Questions

  1. What does an acid taste like?
  2. What colour does blue litmus paper turn in an acid?
  3. Name one common acid.
  4. What is the pH of a neutral solution?
  5. What do alkalis feel like?
  6. What is produced when an acid reacts with an alkali?
  7. Name one common alkali.
  8. What colour does red litmus paper turn in an alkali?
  9. What does pH stand for?
  10. What is the general formula for an acid?
  11. What happens to the pH when an acid is added to water?
  12. What is an indicator?
  13. Can you name a natural indicator?
  14. What is the endpoint in a titration?
  15. What equipment is needed for a titration?
  16. What do you use to measure the acid in a titration?
  17. What colour does phenolphthalein turn in an acid?
  18. What is formed at the end of a neutralisation reaction?
  19. What is the purpose of a titration?
  20. Why is it important to repeat titrations?

Medium Level Questions

  1. Explain the pH scale.
  2. Describe how to perform a titration step by step.
  3. What is the role of an indicator in a titration?
  4. What are the products of a neutralisation reaction?
  5. Why do we use a white tile during titration?
  6. How does the concentration of an acid affect its pH?
  7. What is the difference between strong and weak acids?
  8. Describe what happens to the pH of a solution as you add an alkali to an acid.
  9. Why is it important to record the volume of titrant used in a titration?
  10. What safety precautions should you take when working with acids and alkalis?
  11. How can you tell when the titration is complete?
  12. What is a burette used for?
  13. What happens to the colour of phenolphthalein in a neutral solution?
  14. How do you find the concentration of an unknown solution using titration?
  15. What is the formula for calculating the concentration after a titration?
  16. How does temperature affect pH?
  17. What is the difference between a strong alkali and a weak alkali?
  18. What is the significance of a pH of 7?
  19. How do you ensure accurate readings when performing a titration?
  20. What is a common use of titration in the real world?

Hard Level Questions

  1. Write the balanced chemical equation for the neutralisation of hydrochloric acid with sodium hydroxide.
  2. How would you determine the concentration of a hydrochloric acid solution using a sodium hydroxide solution of known concentration?
  3. Explain the concept of titration curves.
  4. Why may phenolphthalein not be suitable for all titrations?
  5. Calculate the pH of a solution with a concentration of $10^{-4}$ moles of hydrogen ions.
  6. Describe how you would prepare a standard solution for titration.
  7. Discuss the importance of indicators in determining the endpoints of titrations.
  8. How can you use titrations to determine the purity of a sample?
  9. What is the role of a catalyst in acid-base reactions?
  10. Describe how to carry out a titration using a color change indicator.
  11. Calculate the molarity of an acid if 25 ml requires 20 ml of 0.1 M sodium hydroxide for neutralisation.
  12. Discuss the safety measures needed when performing titrations with strong acids.
  13. Explain the difference between a monoprotic and diprotic acid.
  14. How does dilution affect the pH of a solution?
  15. Discuss how to handle a spill of hydrochloric acid in a laboratory.
  16. Explain why it is important to swirl the flask during titration.
  17. What are the common errors made during titrations?
  18. How can the endpoint of a titration be affected by the choice of indicator?
  19. Calculate the total volume of solution after adding 25 ml of a base to 25 ml of an acid.
  20. How can you tell if a reaction is exothermic or endothermic when performing a titration?

Answers

Easy Level Answers

  1. Sour.
  2. Turns red.
  3. Hydrochloric acid.
  5. Slippery.
  6. Water and salt.
  7. Sodium hydroxide.
  8. Turns blue.
  9. Potential of Hydrogen.
  10. HA (where A is an anion).
  11. The pH decreases.
  12. To show pH changes.
  13. Red cabbage.
  14. The point at which the reaction is complete.
  15. Burette, pipette, conical flask, tile.
  16. Pipette.
  17. Colourless.
  18. Water and salt.
  19. To get accurate results.
  20. To ensure reliability.

Medium Level Answers

  1. A scale from 0 (acidic) to 14 (alkaline).
  2. Prepare equipment. 2. Fill burette. 3. Measure acid. 4. Add indicator. 5. Titrate slowly. 6. Record volume.
  3. To show when the reaction is complete.
  4. Water and salt.
  5. To see the colour change clearly.
  6. Lower pH = higher concentration.
  7. Strong acids completely ionise; weak acids do not.
  8. The pH increases.
  9. To ensure the right amount for calculations.
  10. Wear gloves and goggles.
  11. The indicator colour changes.
  12. To dispense the titrant.
  13. Stays colourless.
  14. By using known solutions.
  15. \text{Molarity (M)} = \frac{\text{moles of solute}}{\text{litres of solution}}
  16. It can change the pH.
  17. Strong alkalis are fully ionised; weak are not.
  18. It is neutral.
  19. Use multiple trials.
  20. Cleaning products.

Hard Level Answers

  1. \text{HCl + NaOH} \rightarrow \text{NaCl + H}_2\text{O}
  2. Titrate with a known base and use C_1V_1 = C_2V_2 .
  3. It shows how pH changes with titrant addition.
  4. Some reactions require different pH levels.
  5. pH = 4.
  6. Weigh solute accurately and dilute to a known volume.
  7. To ensure accuracy.
  8. By comparing with a standard.
  9. To speed up reactions but not necessary for acid-base.
  10. To observe colour change.
  11. C = \frac{(0.1)(0.02)}{0.025} = 0.08 , M
  12. Use neutralising agents and absorbent materials.
  13. Monoprotic gives one proton; diprotic gives two.
  14. Dilution lowers pH.
  15. Use baking soda.
  16. To mix and ensure uniformity.
  17. Not using the right technique or measurements.
  18. Different indicators change at different pH levels.
  19. 50 ml.
  20. The temperature change can be measured.

Feel free to ask questions or request further clarification on any of the topics discussed!