Types of chemical reactions

1. Types of chemicalreactions • Imagine thatyou are baking somecookies at home. • Youfollow a recipe • In the same waychemists know that elements and compounds undergoparticular type of chemicalreactions

2. How do youclassify elements? • Chemical famillies • Membersreact in a similarway How do weclassify compounds? Ionic or molecular - Have definitepatterns of chemicalproperties. • Chemistsusethesepatterns to classifygroups of chemicalchanges • 4 categories • Synthesis • Decomposition • Single displacement • Double displacement

3. Why is it useful to have anyknowledge of ofthe types of reactions? Wecanbetter understand experimentalobservations. Wecanpredictproducts of unknownreactions

4. Look at thereactantsrepresentedbelow, whichreactioninvolves elements as reactants? Whichreactioninvolves compounds as reactants? Synthesisreaction Decompositionreaction A synthesisreactioninvolvesthe combination smallermolecules A decompositionreactioninvolvesthe breaking apart of largermolecules

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

6. SynthesisReactions • Involvethe combination of smaller atoms and /or moleculesintolargermolecules. • They are alsocalledcombination reactions • General formula • A + BAB • Ifyouseetwo elements as reactants, you know thereaction has to be a sysnthesisreaction

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, butsomeoccurnaturally as diatomicmolecules.

8. Synthesisreactionscanalsoinvolve combinations of smallmolecules. • For example, whenammonia and hydrogen chloridevapourscombine, they form a whitesmoke 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