Lecture 47 – Lecture 48 Classifying Chemical Reactions Ozgur Unal NIS – CHEMISTRY Lecture 47 – Lecture 48 Classifying Chemical Reactions Ozgur Unal

Classifying Chemical Reactions Have you ever been to a library? How are the books arranged in a big library? How does this arrangement help you find a book you are looking for?

Classifying Chemical Reactions Similar to the books in a library, chemical reactions are categorized into different groups to help identify them easily. There are four categories of chemical reactions: Synthesis reactions Combustion reactions Decomposition reactions Replacement reactions These categories help us recognize patterns and identify the products of many chemical reactions.

Synthesis Reactions Synthesis reactions are chemical reactions in which two or more substances (A and B) react to produce a single product (AB). A + B  AB When two elements react, the reaction is always a synthesis reaction: 2Na (s) + Cl2 (g)  2 NaCl (s) Two compounds can also combine to form one compound in a synthesis reaction: CaO (s) + H2O (l)  Ca(OH)2 (s) Another type of synthesis reaction incolves a reaction between a compound and an element: 2SO2 (g) + O2 (g)  2SO3 (g)

CH4 (g) + 4O2 (g)  CO2 (g) + 2H2O (l) Combustion Reactions In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light. Examples: C (s) + O2 (g)  CO2 (g) 2H2 (g) + O2 (g)  2H2O (l) Some combustion reactions are also synthesis reactions However, not all combustion reactions are synthesis reactions. Example: CH4 (g) + 4O2 (g)  CO2 (g) + 2H2O (l)

London Smog Disaster – 1952

Decomposition Reactions Some chemical reactions are the opposite of synthesis reactions  decomposition reactions A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds  AB  A + B Example: NH4NO3 (s)  N2O (g) + 2H2O (l)

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Replacement Reactions Have you ever had an allergic reaction to metal jewelries?

Replacement Reactions In addition to synthesis, combustion and decomposition reactions, there are replacement reactions (also called displacement reactions) which involve the replacement of an element of a compound. Two types of replacement reactions: single replacement reactions and double replacement reactions. Single-replacement reactions: A reaction in which the atoms of one element replaces the atoms of another element in a compound is called a single replacement reaction: A + BX  AX + B Example: 2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g) In this reaction lithium atom replaces hydrogen atom

Single Replacement Reactions In some single replacement reactions, metals may replace hydrogen atoms. There are other types of single replacement reactions in which metals replace other metals. Example: Cu (s) + 2AgNO3 (aq)  2Ag (s) + Cu(NO3)2 (aq) A metal will not always replace another metal  metals differ in reactivities. Reactivity is the ability to react with another substance Check out Figure 9.13 A specific metal can replace any metal listed below it that is in a compound.

Single Replacement Reactions Example: The following reaction can happen: K (s) + NaOH (aq)  Na (s) + KOH (aq) But, the reaction below cannot happen: Ag (s) + AlCl3 (aq)  NR NR indicates that no reaction takes place. Similarly, nonmetals can replace other nonmetals in a compound, because nonmetals differ in reactivity  Figure 9.13 Fluorine has the highest reactivity F2 (g) + HCl (aq)  Cl2 (g) + HF (aq) The reactivity of an atom is closely related to its electronegativity. Check out Problem Solving Lab!

Single Replacement Reactions Example: Bromine does not replace fluorine in water containing dissolved sodium fluoride. Br2 (g) + 2NaF (aq)  NR Example: Predict the products that will result when these reactants combine, and write a balanced chemical equation for each reaction. Fe (s) + CuSO4 (aq)  Br2 (s) + MgCl2(aq)  Mg (s) + AlCl3 (aq) 

Double Replacement Reactions Another type of replacement reaction is double replacement reaction, which involves an exchange of ions between two compounds. AX + BY  AY + BX In the generic equation, X replaces Y, and Y replaces X  double replacement. Examples: Ca(OH)2 (aq) + HCl (aq)  CaCl2 (aq) + 2H2O (l) 2NaOH (aq) + CuCl2 (aq)  2NaCl (aq) + Cu(OH)2 (s) In the second reaction, a solid is produced  Check out Figure 9.15 A solid produced during a chemical reaction in a solution is called a precipitate. Check out Table 9.3 for guidelines!

Double Replacement Reactions The products of a double replacement reaction can be gas, water or a solid. Example: KCN (aq) + HBr (aq)  KBr (aq) + HCN (g) Example: Aqueous barium chloride and aqueous potassium carbonate react to produce solid barium carbonate and aqueous potassium chloride. Write the balanced chemical equation for this double replacement reaction. BaCl2 (aq) + K2CO3 (aq)  BaCO3 (s) + 2KCl (aq)