HomeChapter 4 The Effects of Chemical Reactions. • Introduction to Chemical Reactions. - Chemical reaction: a process in which one or more substances change into one or more new substances. - Clues that a chemical reaction has occurred : 1. Color change Example: two colorless aqueous solutions mix together to produce a bright yellow precipitate. 2. A precipitate (solid) is formed when mixing two solutions together. 3. Gas formation. Bubbles of gas (effervescence) are produced when mixing substances together (solid – liquid or aqueous – aqueous ….) 4. Heat is produced. - Chemical reactions are described by using word equations or chemical equations. - Chemical equations need to be balanced when written because it shows the correct proportions (amounts) of chemicals in a reaction. - A balanced chemical equation has equal number of atoms of each element in the reactants (left hand side) and the products (right hand side). - Exercise: Balance the following equations. a) KClO3→KCl + O2 b) Na2O + H2O NaOH c) Cu + AgNO3 Cu(NO3)2 + Ag d) C3H7OH + O2 CO2 + H2O • Synthesis and Decomposition Reactions. Synthesis: Two or more substances (elements and / or compounds) combine to form one larger compound. General pattern: A + B → C Examples: N2 + 3 H2 → 2 NH3 CaO + CO2 → CaCO3 2 P + 3 Cl2 → 2 PCl3 Decomposition: This is opposite to synthesis; that is, one large compound breaks down (decomposes) into 2 or more simpler substances. Example: 2 KClO3 → 2 KCl + 3 O2 General pattern: R → S + T Remark: Usually decomposition happens due to heat or electricity. - Predicting the product of decomposition or synthesis reactions. 2 AlCl3 (s) → 2 Al (s) + 3 Cl2 (g) Zn (s) + S (s) → ZnS (s) 2 Zn (s) + O2 (g) → 2 ZnO(s) - Single Displacement (Replacement) Reactions. Definition: A reaction in which an element displaces (replaces) another element in a compound, producing a new compound and a new element. General pattern: A + BC → AC + B Example: Mg (s) + CuSO4 (aq) → MgSO4 (aq) + Cu (s) Zn (s) + 2 AgNO3 (aq) → Zn(NO3)2 (aq) + 2 Ag (s) Fe (s) + MgCl2 (aq) → no reaction. Remark: The element that displaces the other element in a compound must be more reactive (active) than that element, otherwise no reaction takes place. In the general pattern above, A should be more reactive than B for the reaction to proceed. The following reactivity (activity) series lists the chemical strength (reactivity) of the metals in order from the more reactive to the less reactive. KPlease stop calling my amazing zebra in the long Nahungry class. sorry !! Ca Mg Al Zn Fe Sn Pb H Cu Ag Examples of single displacement reactions : 2 Al (s) + 3 CuSO4 (aq) → Al2(SO4)3 (aq) + 3 Cu (s) Sn (s) + Zn(NO3)2 (aq) → no reaction Exercise: Complete and balance the following equations. If there is no reaction occurring write no reaction. a) 2 Al (s) + 6 HCl (aq) → 2 AlCl3 (aq) + 3 H2 (g) b) Cu (s) + H2SO4 (aq) → no reaction c) 2 AlCl3 (aq) + 3 Ca (s) → 3 CaCl2 (aq) + 2 Al (s) d) Mg (s) + 2 HNO3 (aq) → Mg(NO3)2 (aq) + H2(g) - Reactivity of halogens decreases down the group. F2> Cl2> Br2> I2 The reactions taking place for the halogens or their compounds are in solution (aqueous) Examples: Cl2 (aq) + 2 KBr (aq) → 2 KCl (aq) + Br2 (l) Cl2 (aq) + NaF (aq) → no reaction. Exercise: F2 (aq) + 2 LiCl (aq) → 2 LiF (aq) + Cl2 (g) I2 (aq) + NaCl (aq) → no reaction • Double displacement reactions. - Definition: A reaction in which two compounds mix together and an exchange of ions (elements) occurs which results in the formation of 2 new compounds. - General pattern: AB + CD → AD + CB - Solubility: the amount of solute that dissolves in a given amount of solvent at a given temperature. - When we say a substance is soluble, it means it dissolves in water; whereas if it is insoluble it means it doesn’t dissolve in water. - The compound in a reaction that is soluble is in aqueous (aq) phase, whereas the compound which is insoluble is in the solid state (s). - The solid which is formed in a double displacement reaction is called the precipitate and it is insoluble. - Solubility rules (used in double displacement reactions). 1. All alkali metal ions and ammonium ion (NH4+) are soluble. 2. All nitrates (NO3-) are soluble. 3. All sulfates (SO4-2) are solubleexceptwith Ba+2 , Pb+2 , Ca+2 , Sr+2 , Ag+ . 4. All chlorides, bromides and iodides(Cl-, Br-, I-) aresolubleexcept with Ag+ , Pb+2 , Hg+, Cu+ 5. All OH- are insolubleexceptwith rule 1, and Ba+2 and Sr+2 . 6. All oxides (O2-), sulfides (S2-), sulfites (SO32-), carbonates (CO32-), phosphates (PO43-) are insoluble except with rule 1 Remark: If all compounds formed in a double displacement reaction are soluble (aqueous) then no reaction takes place. Exercise: State whether each of the following compounds is soluble or insoluble ? Na2SO4 : Fe(NO3)2: LiOH: ZnSO4: PbBr2: BaSO4: Mg(OH)2: PbO: NH4Cl: Na2S: Cu(OH)2: KF: Exercise: Complete and balance the following chemical equations: - KNO3 (aq) + NaCl (aq) → - LiCl (aq) + AgNO3 (aq) → - Zn (s) + FeSO4 (aq) → - NaOH (aq) + CuCl2 (aq) → - ZnCl2 (aq) + Na3PO4 (aq) → - Pb(NO3)2 (aq) + K2S (aq) → • Net ionic equation: a chemical equation which shows ONLY the ions that are involved in the formation of the precipitate (solid). Examples: Pb+2 (aq) + S-2 (aq) → PbS (s) Ag+ (aq) + Cl- (aq) → AgCl (s) Cu+2 (aq) + 2 OH- (aq) → Cu(OH)2 (s) • Full ionic equation: an equation which shows All the ions in the soluble (aqueous compounds) in both reactants and products. Example: - 2 NaOH (aq) + CuCl2 (aq) → 2 NaCl (aq) + Cu(OH)2 (s) 2 Na+ (aq) + 2 OH- (aq) + Cu+2 (aq) + 2 Cl- (aq) → 2 Na+ (aq) + 2 Cl- (aq) + Cu(OH)2 (s) - 3 ZnCl2 (aq) + 2 Na3PO4 (aq) → Zn3(PO4)2 (s) + 6 NaCl (aq) Full ionicequation: 3 Zn+2(aq) + 6 Cl-(aq) + 6 Na+ (aq) + 2 PO4-3 (aq) → Zn3(PO4)2 (s) + 6 Na+ (aq) + 6 Cl- (aq) Net ionic equation: 3 Zn+2 (aq) + 2 PO4-3 (aq) → Zn3(PO4)2 (s) Exercise: Complete and balance the following equation, then write full ionic and net ionic equations for the reaction. Pb(NO3)2 (aq) + 2 NaI (aq) → Full ionic equation: Net ionic equation: Spectator ions: the ions that are not involved in the formation of the precipitate (solid). Note that the spectator ions appear on both sides of the full ionic equation. For example, in the above reaction, Na+ (sodium ions) and NO3- (nitrate ions) are the spectator ions. Exercise: Complete and balance the following equation, then write the net ionic equation and identify the spectator ions. BaCl2 (aq) + K2SO4 (aq) → Net ionic equation: Ba+2 (aq) + SO4-2 (aq) → Spectator ions: - Combustion reaction is a special type of (synthesis) reaction in which the substance reacts with (burns in) oxygen. Examples: C(s) + O2(g) → CO2(g) • Production of gases (lab scale): 1. CO2 2. SO2 3. H2 4. H2S (hydrogen sulfide) 5. NH3 (ammonia) General pattern of the chemical reactions to produce the above gases: 1. Metal carbonate + acid → CO2 Example: Na2CO3 (aq) + 2 HCl (aq) → 2 NaCl(aq) + CO2(g) + H2O(l) 2. Metal sulfite + acid → SO2 K2SO3 (aq) + 2 HCl (aq) → 2 KCl(aq) + SO2(g) + H2O(l) 3. Metal + acid → H2 Remark: This is a single displacement reaction therefore the metal used in the reaction should be higher in the reactivity series than hydrogen. Zn (s) + 2 HCl (aq) → ZnCl2 (aq) + H2(g) 4. Metal sulfide + acid → H2S Na2S (aq) + 2 HCl (aq) → 2 NaCl (aq) + H2S (g) 5. Ammonium compound + base (alkaline solution) → NH3 NH4Cl (aq) + NaOH (aq) → NaCl (aq) + NH3 (g) + H2O (l) Exercise: Write the net ionic equations for each of the above 5 reactions. Answers 1. 2 H+ (aq) + CO3-2(aq) → CO2(g) + H2O (l) 2. 2 H+ (aq) + SO3-2(aq) → SO2(g) + H2O (l) 3. Zn(s) + 2 H+(aq) → Zn+2(aq) + H2(g) 4. 2H+ (aq) + S-2 (aq) → H2S (g)