11Unit 14 - CHEMICAL Unit 14 - CHEMICAL REACTIONSREACTIONS

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

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

• Symbols represent elements, formulas describe Symbols represent elements, formulas describe compounds, chemical equations describe a chemical compounds, chemical equations describe a chemical reaction: 2 Hreaction: 2 H22 + O + O22 → 2 H→ 2 H22OO

• Chemical reactions occur Chemical reactions occur when bonds between the when bonds between the outermost parts of atoms outermost parts of atoms are are brokenbroken and new bonds and new bondsare are formedformed..

IntroductionIntroduction

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A chemical reaction is a process in A chemical reaction is a process in which a substance (or substances) which a substance (or substances) called called reactantsreactants are changed into are changed into one or more new substances which one or more new substances which are called are called productsproducts. .

44 Al Al (s)(s) + + 33 OO2 (g)2 (g) ---> ---> 22 AlAl22OO3 (s)3 (s)

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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).• A + 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”

Parts of a Reaction Parts of a Reaction EquationEquation

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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.

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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

balancedbalanced..It must have the same number It must have the same number

of atoms of the same kind on of atoms of the same kind on

both sides.both sides. Lavoisier, 1788Lavoisier, 1788

Chemical Chemical EquationsEquations

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A chemical reaction is written as aA chemical reaction is written as a chemical equationchemical equation using chemical using chemical formulas withformulas with symbolssymbols which represent which represent the chemical elements andthe chemical elements and numbersnumbers written as sub-indexswritten as sub-indexs, which indicate , which indicate the number of atoms of each element.the number of atoms of each element.

44 AAll (s)(s) + + 33 OO22 (g)(g) ---> ---> 22 AAll22OO33 (s)(s)

The The numbers in the frontnumbers in the front are called are called stoichiometric coefficientsstoichiometric coefficients..

The letters The letters (s)(s), , (g)(g) are the physical are the physical states of compounds.states of compounds.

99Symbols Used in Equations: Chemical Symbols Used in Equations: Chemical formulasformulas

The chemical formulas are represented by letters and numbers:

Na2CllO4

• The letters represent the symbols of the chemical elements present (as in the Periodic Table).

Na: sodium, Cll: Chloride, O:oxygen

• The numbers indicate the number of atoms of each element involved.

2 atoms Na, 1 atoms Cl, 4 atoms O

1010Symbols Used in Equations: Chemical Symbols Used in Equations: Chemical elementselements

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Subindexs indicate the number of atoms of each element involved. The absence of subindex means that there is 1 atom.

MgBr2 1 atom Mg, 2 atoms Br

Coefficients represent the number of units of that substance.

2 MgBr2 2 x (1 atom Mg, 2 atoms Br)=

2 atoms Mg, 4 atoms Br

Symbols Used in Equations: Symbols Used in Equations:

Subindexs and CoefficientsSubindexs and Coefficients

1212Symbols Used in Equations: Chemical Symbols Used in Equations: Chemical formulasformulas

If there is a parenthesis applies to everything inside it.

Fe2 ( S O4 ) 3

Fe: iron S:sulphur, O:oxygen

2 atoms Fe

3 x 1 = 3 atoms S

3 x 4 = 12 atoms O

1313PRACTISE: Count the number of atoms in a PRACTISE: Count the number of atoms in a chemical formulachemical formula

• H2

• KCll• Na2O

• Be(OH)2

• P2O5

• Li2SO4

• Ni(BrO3)2

• Ca3(PO4)2

• 2 Co2O3

• 3 C5H10

1414PRACTISE: Count the number of atoms in a PRACTISE: Count the number of atoms in a chemical formulachemical formula

• H2

• KCll• Na2O

• Be(OH)2

• P2O5

• Li2SO4

• Ni(BrO3)2

• Ca3(PO4)2

• 2 Co2O3

• 3 C5H10

• H2 : 2 atoms H

• KCll : 1 atom K, 1 atom Cl

• Na2O : 2 atoms Na, 1 atom O

• Be(OH)2 : 1 atom Be, 2 atoms O, 2 atoms H

• P2O5: 2 atoms P, 5 atoms O

• Li2SO4 : 2 atoms Li, 1 atom S, 4 atoms O

• Ni(BrO3)2 : 1 atom Ni, 2 atoms Br, 6 atoms O

• Ca3(PO4)2 : 3 atoms Ca, 2 atoms P, 8 atoms O

• 2 Co2O3 : 4 atoms Co, 6 atoms O

• 3 C5H10 : 15 atoms C, 30 atoms H

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• Solid (s)

• Liquid (ll)• Gas (s)

• Aqueous solution (aq)

• Escaping gas ()• Precipitating solid ()

Symbols Used in Symbols Used in Equations Equations

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

may notnot change the sub-indexs.

• Changing the sub-indexs changes the compound.

Balancing EquationsBalancing Equations

1717Subindexs vs. Subindexs vs. CoefficientsCoefficients

The sub-indexs The sub-indexs tell you how tell you how many atoms of many atoms of a particular a particular element are in a element are in a compound. The compound. The coefficient tells coefficient tells you about the you about the quantity, or quantity, or number, of number, of molecules of molecules of the compound.the compound.

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Chemical EquationsChemical Equations

4 Al(s) + 3 O4 Al(s) + 3 O22(g) (g)

---> 2 Al---> 2 Al22OO33(s)(s)

This equation meansThis equation means

4 Al atoms + 3 O4 Al atoms + 3 O22

molecules ---produces--->molecules ---produces--->

2 molecules of Al2 molecules of Al22OO33

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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

2020Some Suggestions to Help YouSome Suggestions to Help You

• 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)

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Balancing EquationsBalancing Equations

___ H___ H22(g) + ___ O(g) + ___ O22(g) ---> ___ H(g) ---> ___ H22O(O(ll))2 2

What Happened to the Other Oxygen Atom?????

This equation is not balanced!

Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

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Balancing Balancing Equations Equations

___ A___ All (s) + ___ Br(s) + ___ Br22((ll) ---> ___ A) ---> ___ All22BrBr66(s)(s)2 3

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PRACTISE: PRACTISE: Balancing Balancing EquationsEquations

____C____C33HH88(g) + _____ O(g) + _____ O22(g) (g) →→

____CO____CO22(g) + _____ H(g) + _____ H22O(g)O(g)

____B____B44HH1010(g) + _____ O(g) + _____ O22(g) (g) → →

___ ___ BB22OO33(g) + _____ H(g) + _____ H22O(g)O(g)

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PRACTISE: PRACTISE: Balancing Balancing EquationsEquations

CC33HH88(g) + (g) + 55 O O22(g) (g) → → 3 3 COCO22(g) + (g) + 4 4 HH22O(g)O(g)

22 B B44HH1010(g) + (g) + 1111 O O22(g) (g) →→ 44 B B22OO33(g) + (g) + 10 10 HH22O(g)O(g)

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Balancing EquationsBalancing Equations

Mg + HI Mg + HI → → MglMgl22 + H + H22

Ca(OH)Ca(OH)22 + HC + HCll→ → CaCCaCll22 + H + H22OO

NaNa33POPO44 + Fe + Fe22OO33 → → NaNa22O + FePOO + FePO44

CC44HH88 + O + O22 → → COCO22 + H + H22O O