Lesson on Chemical Reactions

Lesson on Chemical Reactions Norma Thompson

Materials:  A bottle of sour milk  A bottle of plain milk  Sheet of Cartridge Paper to make a chart showing reactions  Paper circles about 5 to 8 cm diameter labeled and of different colours representing elements and compounds  Magnetic tape - at back of circles to stick circles to board.  Markers to label  Chalkboard and chalk

Method  First indicate dilemma with milk going bad, to introduce the topic of Chemical Reaction.  Question students (Socratic method) about the difference between the bottle of sour milk and bottle of plain milk. (Curdling, sour taste, colour change).  Illustrate the four main types of chemical reactions using the labeled circles in equations.  End class by letting students demonstrate the four types of reactions, to show their understanding: i. Synthesis- they pair up like a marriage formed ii. Decomposition – the pairs break apart like a divorce iii.Single displacement – one pair and one single show taking away a person’s partner iv. Double displacement – two pairs show switching partners

Resources:

 The Ontario curriculum Grade 9 and 10 – Chemistry: Chemical Processes, pg 27

 Ritter et al, Science 10, pages 233 – 247, Nelson, (2001) Canada

 To visualize a synthesis reaction look at the following cartoon: usoe.k12.ut.us/curr/science/sciber00/8th/matter/sciber/chemtype.htm

 Chemical Equations and Reactions. Multiple-Choice Exercise: http://www.iq.poquoson.org/hsscience/chemistry/chemicalequationsandreactionsch08/ch emequationsandreactions.htm Explanation Chemical reaction occurred in sour milk: Curdling = formation of precipitate Taste change Colour change

General signs that a chemical reaction has occurred 1. New substance formed 2. Change in temperature 3. Change in taste 4. Change on colour 5. Change in odour 6. Gas evolved 7. Formation of a precipitate

There will always be a new substance formed, but the others do not all have to occur in the same reaction.

For a chemical reaction to occur, bond must break, then new bonds will form. e.g 2H2 + O2 2H2O If yellow circles are H atoms and red circles are O atoms, as bonds in molecules break

+ + H2

+ O2 Then atoms rearrange and new bonds are formed.

Types of Reactions Reactions can be classified into four types according to how the atoms rearrange themselves 1. Synthesis 2. Decomposition 3. Single displacement 4. Double displacement Synthesis (like a marriage) Generally: A + B AB (illustrate using circles) e.g 2H2 + O2 2H2O and Na + Cl NaCl

Decompostion (opposite of synthesis, like a divorce) (illustrate using circles) Generally: AB A + B e.g. 2H2O 2 H2 + O2 and CaCO3 CaO + CO2

Single displacement (like cheating on your partner) Generally: A + BC AC + B (illustrate using circles) e.g. Mg + CuSO4 MgSO4 + Cu and Zn + 2HCl ZnCl2 + H2 ( Note that two metals will not bond together so you could not say MgCu formed)

Double Displacement (like wife swapping) (illustrate using circles) Generally: AB + CD AD + CB e.g. 2KI + Pb(NO3)2 2KNO3 + PbI2 and 3NaOH + AlCl3 3NaCl + Al(OH)3