Chemical Reactions: An Introduction Chapter 6

Chemical Equations Chemical change involves a reorganization of the atoms in one or more substances.

4 Evidences a Chemical Reaction Has Occurred 1.A color change is apparent. 2. A precipitate (solid) forms.

4 Evidences a Chemical Reaction Has Occurred 3. A gas (bubbles) is formed. 4. Evolution/absorption of energy --heat, light, electrical energy. --heat, light, electrical energy.

Reaction of Zinc and Iodine Product: Zn I 2 Reactants: Zn + I 2

Chemical Equation A representation of a chemical reaction: word equation -- qualitative significance only ethanol + oxygen yields carbon dioxide + water vapor formula equation -- qualitative & quantitative C 2 H 5 OH (l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g) reactants products reactants products

Chemical Equations Chemical equations give two important pieces of information: 1. The identities of the reactants and products -- qualitative information. 2. The relative number of each --quantitative information.

Physical States solid (s)solid (s) liquid (l)liquid (l) gas (g)gas (g) aqueous (aq)aqueous (aq)

Important Equation Symbols yields >yields > cat. cat. catalyst >catalyst > H 2 SO 4 H 2 SO 4 catalyst >catalyst >  heat > light light > elect. electricity >

Chemical Equation Quantitative Significance C 2 H 5 OH (l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g) The equation is balanced. 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water

Chemical Equations Quantitative Significance 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules ---give---> 2 molecules of Al 2 O 3 2 molecules of Al 2 O 3 4 moles of Al + 3 moles of O 2 ---give---> 2 moles of Al 2 O 3 2 moles of Al 2 O 3

Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. The total mass for the reaction cannot change. Chemical Equations Lavoisier, 1788

Balancing Equation Prerequisites Student must have memorized: 44 chemical symbols44 chemical symbols Table 5.2 on page 133 in textTable 5.2 on page 133 in text Table 5.3 on page 137Table 5.3 on page 137 Table 5.4 on page 142Table 5.4 on page 142 Table 5.5 on page 146Table 5.5 on page 146 Table 5.6 on page 146Table 5.6 on page 146 Type I, II, III, and acid nomenclatureType I, II, III, and acid nomenclature Count HOFBrINClCount HOFBrINCl

Key Words Reactants are indicated by: reacts with….. combines with….. oxidizes…… burns in…… ….are required ….are needed ….decomposes Products are indicated by: ….is formed ….is produced ….is given off ….is precipitated yields…. ….is prepared …..is synthesized

Coefficients, Subscripts, & Superscripts Coefficient Superscript Subscript Subscript

Four Steps in Balancing Equations 1. Get the facts down. 2. Check for diatomic molecules (subscripts). 3. Balance charges on compounds containing a metal, ammonium compounds, and acids (subscripts). 4. Balance the number of atoms (coefficients). a. Balance most complicated molecule first. b. Balance other elements. c. Balance hydrogen next to last. d. Balance oxygen last.

Balancing Equations Caution The identities (formulas) of the compounds must never be changed in balancing a chemical equation! Only coefficients can be used to balance the equation- subscripts will not change!

Combustion of Methane methane gas burns to produce carbon dioxide gas and liquid water whenever something burns it combines with O 2 (g)whenever something burns it combines with O 2 (g) CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(l) H H C H H OO + O O C + O HH 1 C + 4 H + 2 O1 C + 2 O + 2 H + O 1 C + 2 H + 3 O

Combustion of Methane Balanced The balanced reaction must obey the Law of Conservation of Mass: CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(l) H H C H H OO + O O C + O HH OO + O HH + 1 C + 4 H + 4 O

Balancing Equations ___ Al (s) + ___ Br 2(l) ---> ___ Al 2 Br 6(s)

Chemical Reactions What evidence did you observe that a chemical reaction occurred? 1. Evolution of heat/light 2. Color change

Balancing Equations 2 Al (s) + 3 Br 2(l) ---> Al 2 Br 6(s)

Balancing Equations ___C 3 H 8(g) + ___ O 2(g) ----> ____CO 2(g) + ____ H 2 O (g) ___B 4 H 10(g) + ___ O 2(g) ----> ____ B 2 O 3(g) + ____ H 2 O (g)

Balancing Equations C 3 H 8(g) + 5 O 2(g) ----> 3CO 2(g) + 4 H 2 O (g) 2 B 4 H 10(g) + 11 O 2(g) ----> 4 B 2 O 3(g) + 10 H 2 O (g)

Final Equation Balancing Check SiO 2(s) + 4 HF (aq) -----> SiF 4(g) + 2 H 2 O (l) Make a grid to do a final check to be sure the Law of Conservation of Matter has been obeyed: Reactants Products 1 Si1 Si 1 Si1 Si 2 O2 O 2 O2 O 4 H4 H 4 H4 H 4 F4 F 4 F4 F

Figure 6.5(a): The reaction of potassium with water

Figure 6.5(b)&(c): The reaction of potassium with water What evidence is there for a chemical reaction?

Reaction of Potassium and Water Write and balance the word and formula equation for potassium and water. potassium (s) + water (l)  potassium hydroxide (aq) + hydrogen (g) 2K (s) + 2HOH (l)  2KOH (aq) + H 2(g)