0.8 Reactions in aqueous solutions  (Page 10/10)

Results:

• What did you observe when you dissolved each of the salts in water?
• What did you observe when you dissolved pairs of salts in the water?
• What did you observe when you dissolved sodium carbonate in hydrochloric acid?
• Why do you think we used bromothymol blue when mixing the hydrochloric acid and the sodium hydroxide? Think about the kind of reaction that occurred.
• What did you observe when you placed the zinc metal into the copper sulphate?
• Classify each reaction as either precipitation, gas forming, acid-base or redox.
• What makes each reaction happen (i.e. what is the driving force)? Is it the formation of a precipitate or something else?
• What criteria would you use to determine what kind of reaction occurs?
• Try to write balanced chemical equations for each reaction

Conclusion:

In the experiment above, you should have seen how each reaction type differs from the others. For example, a gas forming reaction leads to bubbles in the solution, a precipitation reaction leads to a precipitate forming, an acid-base reaction can be seen by adding a suitable indicator and a redox reaction can be seen by one metal disappearing and a deposit forming in the solution.

Summary

• The polar nature of water means that ionic compounds dissociate easily in aqueous solution into their component ions.
• Ions in solution play a number of roles. In the human body for example, ions help to regulate the internal environment (e.g. controlling muscle function, regulating blood pH). Ions in solution also determine water hardness and pH.
• Water hardness is a measure of the mineral content of water. Hard water has a high mineral concentration and generally also a high concentration of metal ions e.g. calcium and magnesium. The opposite is true for soft water.
• Conductivity is a measure of a solution's ability to conduct an electric current.
• An electrolyte is a substance that contains free ions and is therefore able to conduct an electric current. Electrolytes can be divided into strong and weak electrolytes, based on the extent to which the substance ionises in solution.
• A non-electrolyte cannot conduct an electric current because it dooes not contain free ions.
• The type of substance , the concentration of ions and the temperature of the solution affect its conductivity.
• There are three main types of reactions that occur in aqueous solutions. These are precipitation reactions, acid-base reactions and redox reactions.
• Precipitation and acid-base reactions are sometimes known as ion exchange reactions. Ion exchange reactions also include gas forming reactions.
• A precipitate is formed when ions in solution react with each other to form an insoluble product. Solubility 'rules' help to identify the precipitate that has been formed.
• A number of tests can be used to identify whether certain anions are present in a solution.
• An acid-base reaction is one in which an acid reacts with a base to form a salt and water.
• A redox reaction is one in which electrons are transferred from one substance to another.

End of chapter exercises

1. Give one word for each of the following descriptions:
   1. the change in phase of water from a gas to a liquid
   2. a charged atom
   3. a term used to describe the mineral content of water
   4. a gas that forms sulphuric acid when it reacts with water
2. Match the information in column A with the information in column B by writing only the letter (A to I) next to the question number (1 to 7)

   Column A	Column B
   1. A polar molecule	A. ${\mathrm{H}}_2{\mathrm{SO}}_4$
   2. molecular solution	B. ${\mathrm{CaCO}}_3$
   3. Mineral that increases water hardness	C. $\mathrm{NaOH}$
   4. Substance that increases the hydrogen ion concentration	D. salt water
   5. A strong electrolyte	E. calcium
   6. A white precipitate	F. carbon dioxide
   7. A non-conductor of electricity	G. potassium nitrate
   	H. sugar water
   	I. ${\mathrm{O}}_2$

3. For each of the following questions, choose the one correct answer from the list provided.
   1. Which one of the following substances does not conduct electricity in the solid phase but is an electrical conductor when molten?
      1. $\mathrm{Cu}$
      2. ${\mathrm{PbBr}}_2$
      3. ${\mathrm{H}}_2\mathrm{O}$
      4. ${\mathrm{I}}_2$
      (IEB Paper 2, 2003)
   2. The following substances are dissolved in water. Which one of the solutions is basic?
      1. sodium nitrate
      2. calcium sulphate
      3. ammonium chloride
      4. potassium carbonate
      (IEB Paper 2, 2005)
4. Explain the difference between a weak electrolyte and a strong electrolyte. Give a generalised equation for each.
   
5. What factors affect the conductivity of water? How do each of these affect the conductivity?
   
6. For each of the following substances state whether they are molecular or ionic. If they are ionic, give a balanced reaction for the dissociation in water.
   1. Methane ( ${\mathrm{CH}}_4$ )
   2. potassium bromide
   3. carbon dioxide
   4. hexane ( ${\mathrm{C}}_6{\mathrm{H}}_{14}$ )
   5. lithium fluoride ( $\mathrm{LiF}$ )
   6. magnesium chloride
   Click here for the solution
7. Three test tubes (X, Y and Z) each contain a solution of an unknown potassium salt. The following observations were made during a practical investigation to identify the solutions in the test tubes: A: A white precipitate formed when silver nitrate ( ${\mathrm{AgNO}}_3$ ) was added to test tube Z. B: A white precipitate formed in test tubes X and Y when barium chloride ( ${\mathrm{BaCl}}_2$ ) was added. C: The precipitate in test tube X dissolved in hydrochloric acid ( $\mathrm{HCl}$ ) and a gas was released. D: The precipitate in test tube Y was insoluble in hydrochloric acid.
   1. Use the above information to identify the solutions in each of the test tubes X, Y and Z.
   2. Write a chemical equation for the reaction that took place in test tube X before hydrochloric acid was added.
   (DoE Exemplar Paper 2 2007)