Chemical Reactions Chapter 10

Representing Chemical Changes n Chemical equations are used to represent chemical reactions (the process by which one or more substances are changed into one or more different substances).

Evidence of chemical reactions n Pg. 277 list types of evidence that a chemical reaction has occurred n Temperature change n Energy released in form of heat and light n Color change n Odor n Gas bubbles n Appearance of a solid ( precipitate)

A correct chemical equation shows: n what changes take place n the relative amounts of the elements n compounds that take part in the changes

Representing Chemical Changes n Reactants are the starting substances in a chemical reaction. n Products are the substances formed by the chemical reaction.

Representing Chemical Changes Reactants yield Products C 2 H 2 (g) + O 2 (g) reactants CO 2 (g) + H 2 O(l) products yield

C 2 H 2 (g) + O 2 (g)  CO 2 (g) + H 2 O (l) n The letters in parenthesis represent the physical states. n g = gases n l = liquids n s= crystalline solid (also represented “cr”). n aq = substance dissolved in water

Representing reactions n Word equations n Skeleton equations -Formulas and states are represented, but not amounts – equation not balanced

Left pg. entry practice problems 1. Hydrogen bromide gas is produced from the reaction between hydrogen gas and bromine gas 2. Write a skeleton equation. n Pg. 279 practice problems

Balancing Equations n Reactants are written on the left side of the chemical equations. n Products are written on the right.

Balancing Equations n Use the following steps to balance equations: 1. Determine the reactants and the products. 2. Assemble the parts of the chemical equation. The symbols and the formulas must be correct. 3. Balance the equation. Balanced equations have the same kind and number of atoms on each side. In a chemical reaction no mass is lost or gained. Law of conservation of mass

Balancing Equations n Balance the equation(continued) The same amount of matter is present before and after the reactions. Coefficients, not subscripts, may be changed to balance an equation. Never change a subscript to balance an equation.

Balancing Equations n Balance the equation(continued) The same amount of matter is present before and after the reactions. Coefficients, not subscripts, may be changed to balance an equation. Never change a subscript to balance an equation. EVER! Don’t do it !!!

Balancing Equations Notice one C in carbon dioxide and six carbons in glucose. CO 2 + H 2 O  C 6 H 12 O 6 + O 2 C 6 H 12 O 6 CO 2

Balancing Equations Multiply the carbon dioxide by six to equal the six carbons in glucose. CO 2 + H 2 O  C 6 H 12 O 6 + O 2 C 6 H 12 O 6 CO 2 6

Balancing Equations Next look at the water molecule. Notice that there are 12 H in glucose. Therefore you’ll need six waters to balance. CO 2 + H 2 O  C 6 H 12 O 6 + O 2 6 6

Balancing Equations Next, count the total oxygen atoms on the reactant side and compare that to the product side. CO 2 + H 2 O  C 6 H 12 O 6 + O (6x2) + (6x1) = = 12  2 =

Balancing Equations CO 2 + H 2 O  C 6 H 12 O 6 + O

Balancing Equations n Finally, Check all atoms, comparing both sides. n 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 n 6 C = 6 C F 12 H = 12 H F 18 O =18 O

End of Part I Practice problems pg.282 #’s In water, iron(III) chloride reacts with sodium hydroxide, producing solid iron(III) hydroxide and sodium chloride.

Types of Chemical Reactions n Single Replacement (Displacement) n Double Replacement (Displacement) n Decomposition n Synthesis n Combustion

Single Displacement n element + compound + element n Cl 2 + 2KBr Re 2KCl + Br 2

Single Displacement n Cl 2 + 2KBr 2KCl + Br 2 Cl - replaces The anion(in this case)

Anions swap places Double Displacement compound AB + compound CD  compound AD + compound CB PbCl 2 + Li 2 SO 4  Re PbSO 4 + 2LiCl

Decomposition n compound two or more elements or compounds n Pb(OH) 2 PbO + H 2 O See Pg 260

Synthesis n element/compound + element/compound compound n 2H 2 (g) + O 2 (g)2H 2 O(g)

Combustion n hydrocarbon + oxygencarbon dioxide + water n CH 4 + 2O 2 CO 2 + 2H 2 O