Types of Chemical Reactions

Combination/Synthesis Reaction General Equation: R + S  RS Reactants: Generally two elements or two compounds. Probable Products: A single compound Example: 2Mg (s) + O 2(g)  2MgO (s )

Sample Exercise 3.3 Writing Balanced Equations for Combination Reactions Solution (a) The symbol for lithium is Li. With the exception of mercury, all metals are solids at room temperature. Fluorine occurs as a diatomic molecule (see Figure 2.19). Thus, the reactants are Li(s) and F 2 (g). The product will be composed of a metal and a nonmetal, so we expect it to be an ionic solid. Lithium ions have a 1+ charge, Li +, whereas fluoride ions have a 1– charge, F –.Thus, the chemical formula for the product is LiF. The balanced chemical equation is 2 Li(s) + F 2 (g) → 2 LiF(s) Write balanced equations for the following reactions: (a) The combination reaction that occurs when lithium metal and fluorine gas react. Write balanced chemical equations for the following reactions: (a) Solid mercury(II) sulfide decomposes into its component elements when heated. Practice Exercise Answer: (a) HgS(s) → Hg(l) + S(s)

Decomposition General Equation: RS  R + S Reactants: Generally a single binary or ternary compound Probable Products: Two or more elements Example: 2H 2 O (l)  2H 2(g) + O 2(g)

Sample Exercise 3.3 Writing Balanced Equations for Decomposition Reactions Solution (b) The chemical formula for barium carbonate is BaCO 3. As noted in the text, many metal carbonates decompose to form metal oxides and carbon dioxide when heated. In Equation 3.7, for example, CaCO 3 decomposes to form CaO and CO 2. Thus, we would expect that BaCO 3 decomposes to form BaO and CO 2. Barium and calcium are both in group 2A in the periodic table, which further suggests they would react in the same way: BaCO 3 (s) → BaO(s) + CO 2 (g) Write balanced equations for the following reactions: (b) The decomposition reaction that occurs when solid barium carbonate is heated. (Two products form: a solid and a gas.) Write balanced chemical equations for the following reactions: (b) The surface of aluminum metal undergoes a combination reaction with oxygen in the air. Practice Exercise Answer: (b) 4 Al(s) + 3 O2(g) → 2 Al2O3(s)

Combustion General Equation: fuel + O 2  CO 2 +H 2 O Reactants: A compound of C, H, (O) and oxygen Probable Products: Carbon dioxide and water Example: 2C 3 H 8 O (g) + 9O 2(g)  6CO 2(g) + 8H 2 O (l)

Sample Exercise 3.4 Writing Equations for Combustion Reactions Write the balanced equation for the reaction that occurs when methanol, CH 3 OH(l), is burned in air. Solution When any compound containing C, H, and O is combusted, it reacts with the O 2 (g) in air to produce CO 2 (g) and H 2 O(g). Thus, the unbalanced equation is CH 3 OH(l) + O 2 (g) → CO 2 (g) + H 2 O(g) In this equation the C atoms are balanced with one carbon on each side of the arrow. Because CH 3 OH has four H atoms, we place the coefficient 2 in front of H 2 O to balance the H atoms: CH 3 OH(l) + O 2 (g) → CO 2 (g) + 2 H 2 O(g) Adding the coefficient balances H but gives four O atoms in the products. Because there are only three O atoms in the reactants (one in CH 3 OH and two in O 2 ), we are not finished yet. We can place the fractional coefficient 2/3 in front of O 2 to give a total of four O atoms in the reactants (there are 2/3  2 = 3 O atoms in 3/2 O 2 ): Although the equation is now balanced, it is not in its most conventional form because it contains a fractional coefficient. If we multiply each side of the equation by 2, we will remove the fraction and achieve the following balanced equation: 2 CH 3 OH(l) + 3 O 2 (g) → 2 CO 2 (g) + 4 H 2 O(g)

Sample Exercise 3.4 Writing Equations for Combustion Reactions Write the balanced equation for the reaction that occurs when ethanol, C 2 H 5 OH(l), is burned in air. Practice Exercise Answer: C 2 H 5 OH (l) + 3 O 2(g) → 2 CO 2(g) + 3 H 2 O (g)

Single Displacement General Equation: R + TS  RS + T Reactants: An element and a compound. For displacement to occur, the element that is displaced must be less active than the element that is doing the displacing. Probable Products: A different element and a new compound Example: Zn (s) + 2HCl (l)  H 2(g) + ZnCl 2(l) Practice problems pg 218 #17

Double Displacement General Equation: RU + TS  RS + UT Reactants: Two ionic compounds Probable Products: Two new ionic compounds Example: Na 2 CO 3 + CuSO 4  Na 2 SO 4 + CuCO 3 Practice problems pg 220 #18-19