of 23
8/14/2019 Sec 8-1 Describing Chemical Reactions
1/23
Describing ChemicalReactions
8-1
8/14/2019 Sec 8-1 Describing Chemical Reactions
2/23
What is a chemical reaction
A chemical reaction is the process bywhich one or more substances arechanged into one or more different
substances. Original substances = reactants
Resulting substances =products
8/14/2019 Sec 8-1 Describing Chemical Reactions
3/23
Chemical Equations
Are used to represent, with symbols, andformulas, the identities and relativeamounts of reactants and products in a
chemical reaction.
(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)
8/14/2019 Sec 8-1 Describing Chemical Reactions
4/23
Indications of a chemical reaction
Evolution of heat and light
8/14/2019 Sec 8-1 Describing Chemical Reactions
5/23
Indications of a chemical reaction
Production of a gas
8/14/2019 Sec 8-1 Describing Chemical Reactions
6/23
Indications of a chemical reaction
Formation of a precipitate (ppt)
A solid that is produced as a result of achemical reaction in solution and that
separates from the solution
8/14/2019 Sec 8-1 Describing Chemical Reactions
7/23
Characteristics of Chemical Equations
The equation must represent known facts
All reactant and products must be identified
The equation must contain the correct
formulas for the reactants and products Diatomic Elements - When using H,N,O or any
halogens they are represented by theirmolecular formulas
H2, N2, O2,F2, Cl2, Br2, I2 2H2(g) + O2(g) 2H2O(l)
8/14/2019 Sec 8-1 Describing Chemical Reactions
8/23
Characteristics of Chemical Equations
The law of conservation of mass must besatisfied
The same number of atoms of each element
must appear on each side of the equation Coefficients are used, by placing them in front
of a formula specifies the relative number ofmoles of a substance
H2 + O2
H2O 2H2(g) + O2(g) 2H2O(l)
8/14/2019 Sec 8-1 Describing Chemical Reactions
9/23
Word Equations
Is an equation in which the reactants andproducts in a chemical reaction arerepresented by words
Have on qualitative (descriptive) meaning Do not give us quantities
methane + oxygen carbon dioxide + water
8/14/2019 Sec 8-1 Describing Chemical Reactions
10/23
Formula Equation
Represents the reactants and products ofa chemical reaction by using their symbolsor formulas
CH4(g) + O2(g) CO2(g) + H2O(g)
CH4(g) + O2(g) CO2(g) + 2H2O(g)
CH4(g) +2O2(g) CO2(g) + 2H2O(g)
8/14/2019 Sec 8-1 Describing Chemical Reactions
11/23
Additional Symbols used in chemical
equations
8/14/2019 Sec 8-1 Describing Chemical Reactions
12/23
Practice
Solid sodium oxide is added to water atroom tempertature and forms sodiumhydroxide (dissolved in water).
Sodium oxide + water sodium hydroxide
Na2O(s) + H2O(l) NaOH(aq)Na2O(s) + H2O(l) 2NaOH(aq)
8/14/2019 Sec 8-1 Describing Chemical Reactions
13/23
Practice
Hydrogen peroxide in an aqueous solutiondecomposes to produce oxygen andwater.
Hydrogen peroxide oxygen + water
H2O2(aq) O2(g) + H2O(l)
2H2O2(aq) O2(g) + 2H2O(l)
8/14/2019 Sec 8-1 Describing Chemical Reactions
14/23
Practice
Solid zinc metal reacts with aqueouscopper sulfate to produce solid coppermetal and aqueous zinc sulfate.
Zinc + copper sulfate copper + zinc sulfate
Zn(s) + CuSO4(aq) Cu(s) + ZnSO4(aq)
8/14/2019 Sec 8-1 Describing Chemical Reactions
15/23
Significance of a Chemical Reaction
The coefficients of a chemical reactionindicate relative, not absolute amounts ofreactants and products.
H2 + Cl2 2HCl
1moleH2 : 1mole Cl2: 2moles HCl.5 mol : .5 mol : 1 mol
10 mol : 10 mol : 20 mol
8/14/2019 Sec 8-1 Describing Chemical Reactions
16/23
Significance of a Chemical Reaction
The relative masses of the reactants andproducts of a chemical reaction can bedetermined from the reactions
coefficients.H2 + Cl2 2HCl
1mol H2 x 2.02g/mol = 2.02 g H2
1mol Cl2x 70.90g/mol = 70.90g Cl22mol HCl x 36.46g/mol = 72.92 g HCl
8/14/2019 Sec 8-1 Describing Chemical Reactions
17/23
Significance of a Chemical Reaction
8/14/2019 Sec 8-1 Describing Chemical Reactions
18/23
Significance of a Chemical Reaction
The reverse reaction for a chemicalequation has the same relative amounts ofsubstances as the forward reaction
8/14/2019 Sec 8-1 Describing Chemical Reactions
19/23
Balancing Chemical Equations
First and foremost make sure you have thecorrect formula equation.
Steps
Balance the different types of atoms one at a
time First balance the atoms of elements that are
combined and that appear only once on eachside of the equation
Balance polyatomic ions that appear on bothsides of the equation as single units
Balance H and O atoms after atoms of all otherelements have been balanced
8/14/2019 Sec 8-1 Describing Chemical Reactions
20/23
Practice
Zinc + hydrochloric acid
zinc chloride + hydrogen
Zn + HCl ZnCl2 + H2
Zn + 2HCl ZnCl2 + H2(1Zn)+(2H+2Cl) (1Zn+2Cl)+(2H)
8/14/2019 Sec 8-1 Describing Chemical Reactions
21/23
Practice
Al4C3 + H2O CH4 + Al(OH)3
Al4C3 + 12H2O
3CH4 + 4Al(OH)3
8/14/2019 Sec 8-1 Describing Chemical Reactions
22/23
Practice
Aluminum Sulfate and Calcium hydroxideare combined they react to producealuminum hydroxide and calcium sulfate.
Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4
Al2(SO4)3 + 3Ca(OH)22Al(OH)3 + 3CaSO4
8/14/2019 Sec 8-1 Describing Chemical Reactions
23/23
Homework
Pg 290
#s 10, 13-16