sec 8-1 describing chemical reactions

8/14/2019 Sec 8-1 Describing Chemical Reactions

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Describing ChemicalReactions

8-1


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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


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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)


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Indications of a chemical reaction

Evolution of heat and light


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Production of a gas


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Formation of a precipitate (ppt)

A solid that is produced as a result of achemical reaction in solution and that

separates from the solution


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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)


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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)


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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


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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)


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Additional Symbols used in chemical

equations


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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)


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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)


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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)


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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


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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


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The reverse reaction for a chemicalequation has the same relative amounts ofsubstances as the forward reaction


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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


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Practice

Zinc + hydrochloric acid

zinc chloride + hydrogen

Zn + HCl ZnCl2 + H2

Zn + 2HCl ZnCl2 + H2(1Zn)+(2H+2Cl) (1Zn+2Cl)+(2H)


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Practice

Al4C3 + H2O CH4 + Al(OH)3

Al4C3 + 12H2O

3CH4 + 4Al(OH)3


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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


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Homework

Pg 290

#s 10, 13-16

sec 8-1 describing chemical reactions

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