Types of chemical reactions

1. Types of chemical reactions • Imagine that you are baking some cookies at home. • You follow a recipe • In the same way chemists know that elements and compounds undergo particular type of chemical reactions

2. How do you classify elements? • Chemical famillies • Members react in a similar way How do we classify compounds? Ionic or molecular - Have definite patterns of chemical properties. • Chemistsusethesepatterns to classifygroups of chemicalchanges • 4 categories • Synthesis • Decomposition • Single displacement • Double displacement

3. Why is it useful to have any knowledge of of the types of reactions? We can better understand experimental observations. We can predict products of unknown reactions

4. Look at thereactantsrepresentedbelow, whichreactioninvolves elements as reactants? Whichreactioninvolves compounds as reactants? Synthesis reaction Decomposition reaction A synthesis reaction involves the combination smaller molecules A decomposition reaction involves the breaking apart of larger molecules

5. Recognizingthe types of reactants is key to identifyingthereaction type Lets uslook at two of thereactiontypes:synthesis and decomposition

6. Synthesis Reactions • Involve the combination of smaller atoms and /or molecules into larger molecules. • They are also called combination reactions • General formula • A + BAB • If you see two elements as reactants, you know the reaction has to be a sysnthesis reaction

7. For example, hydrogen and oxygen gases react to produce water hydrogen + oxygen water H2 + O2 H2O 2H2 + O22H2O • Most of these elements are represented as single atoms, but some occur naturally as diatomic molecules.

8. Synthesis reactions can also involve combinations of small molecules. • For example, when ammonia and hydrogen chloride vapours combine, they form a white smoke as solid particles of ammonium chloride are formed. Hydrogen chloride + ammonia ammonium chloride HCl + NH3 NH4Cl

9. Similarly, combination of water and carbon dioxide molecules Synthesis reaction CO2 + H2O H2CO3(aq) Typical example , rainwater

10. Decomposition Reactions • It involve the splitting of a large molecule into elements or smaller molecules. • General formula AB A + B • Example, electrolysis of water uses electricity to split water molecules into their elements water hydrogen + oxygen H2O H2 + O2 2H2O 2H2 + O2

11. Sometimes it involves large amounts of energy. • Nitrogen triiodide decomposes rapidly to form the elements nitrogen and iodine nitrogen triiodide nitrogen + iodine • NI3 N2 + I2 • 2NI3 N2 + 3I2 • Can also involve the production of two small molecules from a large molecule • When ammonium nitrate is heated to above 250oC, it decomposes explosively to form nitrous oxide and water molecules • NH4NO3 N2O + H2O • NH4NO3 N2O + 2H2O

12. Single Displacement Reactions • Are chemical changes that involve an element and a compound as reactants. • One element displaces or replaces another element from the compound. • Example, when magnesium ribbon is placed in a solution of silver nitrate Mg + AgNo3 Ag + Mg(NO3)2 Mg + 2AgNO32Ag + Mg(NO3)2

13. Example, when the element bromine is added to a solution of calcium iodide • the bromine displaces (or takes the place of) the iodide ion • bromine + calcium iodide iodine + calcium bromide • Br2 + CaI2 I2 + CaBr2