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Chemical Formula, Chemical Formula, Equations Equations

andandVarious Chemical ReactionsVarious Chemical Reactions

Week -10

Lecture 19 & 20

Chemical FormulasChemical Formulas

• We know a Chemical SymbolChemical Symbol identifies an element.

• A FormulaFormula is a combination of symbols that identifies a compound, an ion, or a molecule of an element.Ex:formula of compounds = HClHCl, (NH(NH44))33POPO4 4 , HH22OOformula of ions = OHOH-- , (NH(NH44))

++

formula of molecule = HH22, NN22, OO22, FF22, ClCl22, BrBr22, II22, PP44, SS88, OO33

• A Formula also indicates the relative quantities of elements contained in the compound or ion and implies some kind of chemical bonding between the atoms

FormulaFormula ––Subscripts Subscripts vs. vs. CoefficientsCoefficients

• The subscripts tell you how many atoms of a particular element are in a compound.

• The coefficient tells you about the quantity, or number, of molecules of the compound.

Give the example of (NH4)3PO4 and some others. Ref: Read page-128 to 132 of Goldberg – fundamental of Chemistry

Chemical Reaction

A process in which at least one new substance is produced as a result of chemical change.

– Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken

– Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

– Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

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– Chemical equations show the conversion of reactantsreactants (the molecules shown on the left of the arrow) into productsproducts (the molecules shown on the right of the arrow).

• + sign separates molecules on the same side

• The arrow is read as “yields”

• Example

C + O2 CO2

• This reads “carbon plus oxygen react to yield carbon dioxide”

Chemical Reaction Chemical Reaction EquationEquation Reading A Chemical Equation

4 NH3 + 5 O2 4 NO + 6 H2O

Four molecules of NH3 react with five molecules O2 to produce four molecules NO and six molecules of H2O

or

Four moles NH3 react with 5 moles O2 to

produce four moles NO and six moles H2O

• The charcoal used in a grill is basically carbon.

• The carbon reacts with oxygen to yield carbon dioxide.

• The chemical equation for this reaction

C + O2 CO2

• Contains the same information as the English sentence but has quantitative meaning as well.

Writing a Chemical Equation

Chemical symbols give a “before-and-after” picture of a chemical reaction

Reactants Products

MgO + C CO + Mg

magnesium oxide carbon-monoxide

reacts with carbon and magnesium

to form

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• Solid (s)

•Liquid (l)

•Gas (g)

•Aqueous solution (aq)

•Catalyst H2SO4

•Escaping gas ()

•Precipitate solid ( )

•Change of temperature ()

Ex:

4 Al(s) + 3 O4 Al(s) + 3 O22(g) (g) ------> 2 Al> 2 Al22OO33(s)(s)

Symbols Used in EquationsSymbols Used in EquationsA Balanced Chemical

Equation

Same numbers of each type of atom on each side of the equation

Al + S Al2S3 Not Balanced

2Al + 3S Al2S3 Balanced

Because of the principle of the Because of the principle of the

conservation of matterconservation of matter, ,

an an equation must be equation must be balanced.balanced.

It must have the same It must have the same number of atoms of the number of atoms of the

same kind on both sides.same kind on both sides. Lavoisier, 1788Lavoisier, 1788

Chemical EquationsChemical Equations

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Matter Is Conserved

H2 + Cl2 2 HCl

+ +

Total atoms = Total atoms

2 H, 2 Cl 2H, 2 Cl

Total Mass = Total Mass

2(1.0) + 2(35.5) 2(36.5)

73.0 g = 73.0 g

Balance Equations with Coefficients

Coefficients in front of formulas balance

each type of atom

4NH3 + 5O2 4NO + 6H2O

4 N = 4 N

12 H = 12 H

10 O = 10 O

Chemical EquationsChemical EquationsChemical EquationsChemical Equations

4 Al(s) + 3 O4 Al(s) + 3 O22(g) (g) ------> > 2 Al2 Al22OO33(s)(s)

This equation meansThis equation means

4 Al atoms + 3 O4 Al atoms + 3 O22 molecules molecules ------producesproduces------>>

2 molecules of Al2 molecules of Al22OO33

AND/ORAND/OR

4 moles of Al + 3 moles of 4 moles of Al + 3 moles of OO2 2 ------producesproduces------>>

2 moles of Al2 moles of Al22OO33

–When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but

you may notnot change the subscripts.

•Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Balancing EquationsBalancing Equations

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There are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if:

– The numbers of atoms on both sides of the equation are now balanced.

– The coefficients are in the lowest possible whole number ratios. (reduced)

Steps to Balancing EquationsSteps to Balancing Equations Some Suggestions to Help YouSome Suggestions to Help You

Some of Helpful Hints for balancing equations:

• Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

Read from Page 207 to Page 213 of Goldberg- Fundamentals of Chemistry

Ex: Steps in Balancing An Equation

Fe3O4 + H2 Fe + H2O

Fe: Fe3O4 + H2 3 Fe + H2O

O: Fe3O4 + H2 3 Fe + 4 H2O

H: Fe3O4 + 4 H2 3 Fe + 4 H2O

Learning Check E5

Balance each equation. The coefficients for each equation are read from left to right

A. Mg + N2 Mg3N2

1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1

B. Al + Cl2 AlCl3

1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2

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Learning Check E5

C. Fe2O3 + C Fe + CO2

1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3

D. Al + FeO Fe + Al2O3

1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1

E. Al + H2SO4 Al2(SO4)3 + H2

1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3

Solution E5

A. 3 Mg + N2 Mg3N2

B. 2 Al + 3 Cl2 2 AlCl3

C. 2 Fe2O3 + 3 C 4 Fe + 3 CO2

D. 2 Al + 3 FeO 3 Fe + Al2O3

E. 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2

Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

___ H___ H22(g) + ___ O(g) + ___ O22(g) (g) ------> ___ H> ___ H22O(l)O(l)

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) (l) ------> ___ Al> ___ Al22BrBr66(s)(s)

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

____C____C33HH88(g) + _____ O(g) + _____ O22(g) (g) -------->>

_____CO_____CO22(g) + _____ H(g) + _____ H22O(g)O(g)

____B____B44HH1010(g) + _____ (g) + _____ OO22(g) (g) -------->>

___ B___ B22OO33(g) + (g) + _____H_____H22O(gO(g))

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Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

Sodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphatesodium oxide + iron (III) phosphate

NaNa33POPO44 + Fe+ Fe22OO33 -------->>

NaNa22O + FePOO + FePO44