Chemical Reactions. Chapter 3. Chemical Reactions. Chemical Reactions. Chemical reactions are symbolically represented by a chemical equation.

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1 Chemical reactions are symbolically represented by a chemical equation. Chapter 3 Chemical equations represent the reaction using symbols for the elements or compounds in the reaction. Chemical equations have the form of reactants going to products. Reactants Products Check out examples of a chemical reaction. Chemical equations represent the reaction using symbols for the elements or compounds in the reaction. Reactants Products Fe(s) + S 8 (s) FeS(s) Balance Fe(s) + S 8 (s) FeS(s) Chemical equations represent the reaction using symbols for the elements or compounds in the reaction. Reactants Products Fe(s) + S 8 (s) FeS(s) Balance Coefficients are changed when balancing a chemical equation NOT subscripts! The substances in a chemical equation are represented with their formulas (NOT symbols) and their phases. 1

2 Chemical equations represent the reaction using symbols for the elements or compounds in the reaction. Reactants Products Fe(s) + S 8 (s) FeS(s) Balance Chemical reactions are characterized by heat and light explosion color change evolution of a gas formation of a solid (precipitate) Chemical equations represent the reaction using symbols for the elements or compounds in the reaction. Reactants Products Fe(s) + S 8 (s) FeS(s) Balance This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. 2H 2 (g) + O 2 (g) 2Al(s) + 3Br 2 (l) S 8 (s) + 8O 2 (g) N 2 (g) + 2O 2 (g) This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. 2H 2 (g) + O 2 (g) 2H 2 O(l) 2Al(s) + 3Br 2 (l) S 8 (s) + 8O 2 (g) N 2 (g) + 2O 2 (g) 2

3 This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. 2H 2 (g) + O 2 (g) 2H 2 O(l) 2Al(s) + 3Br 2 (l) 2AlBr 3 (s) S 8 (s) + 8O 2 (g) N 2 (g) + 2O 2 (g) This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. 2H 2 (g) + O 2 (g) 2H 2 O(l) 2Al(s) + 3Br 2 (l) 2AlBr 3 (s) S 8 (s) + 8O 2 (g) 8SO 2 (s) N 2 (g) + 2O 2 (g) This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. 2H 2 (g) + O 2 (g) 2H 2 O(l) 2Al(s) + 3Br 2 (l) 2AlBr 3 (s) S 8 (s) + 8O 2 (g) 8SO 2 (s) N 2 (g) + 2O 2 (g) 2NO 2 (g) This reaction is an example of a synthesis reaction: elements in their standard state forming a single product. 2H 2 (g) + O 2 (g) 2H 2 O(l) 2Al(s) + 3Br 2 (l) 2AlBr 3 (s) S 8 (s) + 8O 2 (g) 8SO 2 (s) N 2 (g) + 2O 2 (g) 2NO 2 (g) Reactant phases are always the phase at 25 C. When the product is an ionic compound the phase is always solid. When the product is a covalent compound you must know the formula and the phase. 3

4 Another reaction that was observed in the video CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (g) This is an example of a combustion reaction. A hydrocarbon reacting with oxygen to form carbon dioxide and water. Another reaction that was observed in the video CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (g) This is an example of a combustion reaction. A hydrocarbon reacting with oxygen to form carbon dioxide and water. Balance: Balance: CH 4 (g) + O 2 (g) 1 CO 2 (g) + H 2 O (g) Another reaction that was observed in the video CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (g) This is an example of a combustion reaction. A hydrocarbon reacting with oxygen to form carbon dioxide and water. Another reaction that was observed in the video CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (g) This is an example of a combustion reaction. A hydrocarbon reacting with oxygen to form carbon dioxide and water. Balance: CH 4 (g) + O 2 (g) 1 CO 2 (g) + H 2 O (g) Balance: CH 4 (g) + O 2 (g) 1 CO 2 (g) + H 2 O (g) CH 4 (g) + O 2 (g) 1 CO 2 (g) + 2 H 2 O (g) CH 4 (g) + O 2 (g) 1 CO 2 (g) + 2 H 2 O (g) CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O (g) 4

5 A balanced equation has equal numbers of elements on both side of the equation. Balancing involves introducing coefficients which proceed the formula in the equations. NEVER change subscripts! Trail and error method that will produce faster results with practice. Experience produce rules ; 1) Always balance the elemental form of a substance last in an equation. Other combustion equations: CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O (g) CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O (g) Other hydrocarbons that are combusted: Methane CH 4 Ethane C 2 H 6 Propane C 3 H 8 Butane C 4 H 10 Pentane C 5 H 12 Hexane C 6 H 14 Heptane C 7 H 16 Octane C 8 H 18 Nonane C 9 H 20 Decane C 10 H 22 CH 4 (g) + 2 O 2 (g) 1 CO 2 (g) + 2 H 2 O (g) C 5 H 12 (g) + O 2 (g) CO 2 (g) + H 2 O (g) Other hydrocarbons that are combusted: Methane CH 4 Ethane C 2 H 6 Propane C 3 H 8 Butane C 4 H 10 Pentane C 5 H 12 Hexane C 6 H 14 Heptane C 7 H 16 Octane C 8 H 18 Nonane C 9 H 20 Decane C 10 H 22 General formula for a hydrocarbon C n H 2n+2 Balance CO 2 (g) and H 2 O (g) first: C 5 H 12 (g) + O 2 (g) 5 CO 2 (g) + 6 H 2 O (g) Balance O 2 (g) last: C 5 H 12 (g) + 8 O 2 (g) 5 CO 2 (g) + 6 H 2 O (g) 5

6 C 4 H 9 SH(g) + O 2 (g)? C 4 H 9 SH(g) + O 2 (g) CO 2 (g) + H 2 O(g) C 4 H 9 SH(g) + O 2 (g) CO 2 (g) + H 2 O(g) + SO 2 (g) C 4 H 9 SH(g) + O 2 (g) CO 2 (g) + H 2 O(g) + SO 2 (g) Balance CO 2 (g), H 2 O(g) and SO 2 (g) 6

7 C 4 H 9 SH(g) + O 2 (g) CO 2 (g) + H 2 O(g) + SO 2 (g) Balance CO 2 (g), H 2 O(g) and SO 2 (g) C 4 H 9 SH(g) + O 2 (g) CO 2 (g) + H 2 O(g) + SO 2 (g) Balance CO 2 (g), H 2 O(g) and SO 2 (g) C 4 H 9 SH(g) + O 2 (g) 4 CO 2 (g) + 5 H 2 O(g) + 1 SO 2 (g) C 4 H 9 SH(g) + O 2 (g) 4 CO 2 (g) + 5 H 2 O(g) + 1 SO 2 (g) Balance O 2 (g) last: C 4 H 9 SH(g) + 15/2 O 2 (g) 4 CO 2 (g) + 5 H 2 O(g) + 1 SO 2 (g) C 4 H 9 SH(g) + O 2 (g) CO 2 (g) + H 2 O(g) + SO 2 (g) Balance CO 2 (g), H 2 O(g) and SO 2 (g) C 4 H 9 SH(g) + O 2 (g) 4 CO 2 (g) + 5 H 2 O(g) + 1 SO 2 (g) Balance O 2 (g) last: Recall that the unit for atomic mass for each element in the periodic table is u (amu). The relationship between mass in units of u and grams is; 1 u = x grams C 4 H 9 SH(g) + 15/2 O 2 (g) 4 CO 2 (g) + 5 H 2 O(g) + 1 SO 2 (g) Multiply through by 2: 2 C 4 H 9 SH(g) + 15 O 2 (g) 8 CO 2 (g) + 10 H 2 O(g) + 2 SO 2 (g) 7

8 Recall that the unit for atomic mass for each element in the periodic table is u (amu). The relationship between mass in units of u and grams is; 1 u = x grams So the mass, in grams, of an atom of sodium (at. Mass = u) is, u ( x grams/1 u) = 3.82 x grams So the mass, in grams, of an atom of sodium (at. Mass = u) is, u ( x grams/1 u) = 3.82 x grams So the mass, in grams, of 100 atoms of sodium (at. Mass = u) is, 100 atoms (22.99 u/1 atom) ( x grams/1 u) = 3.82 x grams or 1000 atoms 1000 atoms (22.99 u/1 atom) ( x grams/1 u) = 3.82 x grams What is the mass of 1 mol of sodium atoms? A mol is defined as the number of 12 C atoms in 12 grams of 12 C. This number is x atoms. A mol of any substance is defined as the amount of the substance that contains the same number of units as are in 12 g of 12 C or x units of that substance. How do we read this equation? So if a mol of sodium atoms contains x sodium atoms than, x atoms (22.99 u/1 atom) ( x grams/1 u) = grams How INTERESTING! 8

9 How do we read this equation? One atom of magnesium reacts with 1 molecule of chlorine to product one formula unit of magnesium chloride. How do we read this equation? 10 atoms of magnesium reacts with 10 molecules of chlorine to product 10 formula units of magnesium chloride. NOTE: CAN NOT say molecule of magnesium chloride because it is an ionic compound and does not exist as molecules at the atomic level. It exists as spherical ions arranged in a pattern that maximizes attractions between oppositely charged ions and minimizes repulsions between like charges. NOTE: CAN NOT say molecule of magnesium chloride because it is an ionic compound and does not exist as molecules at the atomic level. It exists as spherical ions arranged in a pattern that maximizes attractions between oppositely charged ions and minimizes repulsions between like charges. How do we read this equation? 10,000 atoms of magnesium reacts with 10,000 molecules of chlorine to product 10,000 formula units of magnesium chloride. NOTE: CAN NOT say molecule of magnesium chloride because it is an ionic compound and does not exist as molecules at the atomic level. It exists as spherical ions arranged in a pattern that maximizes attractions between oppositely charged ions and minimizes repulsions between like charges. How do we read this equation? 6.02 x atoms of magnesium reacts with 6.02 x molecules of chlorine to product 6.02 x formula units of magnesium chloride. Remember 6.02 x = 1 mol NOTE: CAN NOT say molecule of magnesium chloride because it is an ionic compound and does not exist as molecules at the atomic level. It exists as spherical ions arranged in a pattern that maximizes attractions between oppositely charged ions and minimizes repulsions between like charges. 9

10 How do we read this equation? 1 mol of magnesium atoms reacts with 1 mol of chlorine molecules to product 1 mol of magnesium chloride formula units. Remember 6.02 x = 1 mol NOTE: CAN NOT say molecule of magnesium chloride because it is an ionic compound and does not exist as molecules at the atomic level. It exists as spherical ions arranged in a pattern that maximizes attractions between oppositely charged ions and minimizes repulsions between like charges. But how do we measure 1 atom of magnesium, 10 atoms, 10,000 atoms or 6.02 x magnesium atoms? Our earlier definition of a mol stated A mol of any substance is defined as the amount of the substance that contains the same number of units as are in 12 g of 12 C or x units of that substance. So a mol of 12 C atoms weighs 12 g. Interestingly the mass of 1 atom of 12 C is 12 u, and the mass of 1 mol of 12 C atom is 12 g! What is the mass, in atomic mass units, of one atom of magnesium? What is the mass, in atomic mass units, of one atom of magnesium? u What is the mass of one mol of magnesium atoms (6.022 x )? 10

11 What is the mass, in atomic mass units, of one atom of magnesium? What is the mass, in atomic mass units, of one molecule of chlorine? u What is the mass of one mol of magnesium atoms (6.022 x )? g x Mg atoms (24.31 u/1 Mg atom) ( x g/1 u) = g What is the mass, in atomic mass units, of one molecule of chlorine? u What is the mass of one mol of chlorine molecules (6.022 x )? What is the mass, in atomic mass units, of one molecule of chlorine? u What is the mass of one mol of chlorine molecules (6.022 x )? g x Mg atoms (70.90 u/1 Mg atom) ( x g/1 u) = g 11

12 What is the mass, in atomic mass units, of one formula unit of magnesium chloride? What is the mass, in atomic mass units, of one formula unit of magnesium chloride? u u = u What is the mass, in atomic mass units, of one formula unit of magnesium chloride? u u = u What is the mass of one mol of formula unit of magnesium chloride (6.022 x )? What is the mass, in atomic mass units, of one formula unit of magnesium chloride? u u = u What is the mass of one mol of formula unit of magnesium chloride (6.022 x )? g g = g x Mg atoms (95.21 u/1 Mg atom) ( x g/1 u) = g 12

13 My favorite problem! 13