Chemical Reactions

Chapter 10

Representing Chemical Changes 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 Pg. 277 list types of evidence that

a chemical reaction has occurred Temperature change Energy released in form of heat

and light Color change Odor Gas bubbles Appearance of a solid ( precipitate)

A correct chemical equation shows:

what changes take place the relative amounts of the

elements compounds that take part in the

changes

Representing Chemical Changes

Reactants are the starting substances in a chemical reaction.

Products are the substances formed by the chemical reaction.

Representing Chemical ChangesReactants yield Products

C2H2(g) + O2(g)

reactants

CO2 (g) + H2O(l)

productsyield

C2H2(g) + O2(g) CO2 (g) + H2O(l) The letters in parenthesis represent the physical

states. g = gases l = liquids s= crystalline solid (also represented “cr”). aq = substance dissolved in water

Representing reactions

Word equations Skeleton equations

-Formulas and states are represented, but not amounts – equation not balanced

Left pg. entry practice problems1. Hydrogen bromide gas is

produced from the reaction between hydrogen gas and bromine gas

2. Write a skeleton equation. Pg. 279 practice problems

Balancing Equations Reactants are written on the left

side of the chemical equations. Products are written on the right.

Balancing Equations 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

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

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.

CO2 + H2O C6H12O6 + O2

C6H12O6CO2

Balancing Equations

Multiply the carbon dioxide by six to equal the six carbons in glucose.

CO2 + H2O C6H12O6 + O2

C6H12O6CO2

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.

CO2 + H2O C6H12O6 + O26 6

Balancing Equations

Next, count the total oxygen atoms on the reactant side and compare that to the product side.

CO2 + H2O C6H12O6 + O26 6

(6x2) + (6x1)

=18

6

18 - 6= 12 2 =

Balancing Equations

CO2 + H2O C6H12O6 + O26 6 6

Balancing Equations

Finally, Check all atoms, comparing both sides.

6CO2 + 6H2O C6H12O6 + 6O2

6 C = 6 C 12 H = 12 H 18 O = 18 O

End of Part IPractice problems

pg.282 #’s 4-6

4. In water, iron(III) chloride reacts with sodium hydroxide, producing solid iron(III) hydroxide and sodium chloride.

Types of Chemical Reactions

Single Replacement (Displacement) Double Replacement (Displacement) Decomposition Synthesis Combustion

Single Displacement

element + compound compound + element

Cl2 + 2KBr

Re

2KCl + Br2

Single Displacement

Cl2 + 2KBr 2KCl + Br2

Cl-

replaces

The anion(in this case)

Anions swap places

Double Displacement

compound AB + compound CD compound AD + compound CB

PbCl2 + Li2SO4

Re

PbSO4 + 2LiCl

Decomposition

compound two or more elements or compounds

Pb(OH)2 PbO + H2O

See Pg 260

Synthesis

element/compound + element/compound compound

2H2(g) + O2(g) 2H2O(g)

Combustion

hydrocarbon + oxygen carbon dioxide + water

CH4 + 2O2 CO2 + 2H2O