CHEMICAL REACTIONSCHEMICAL REACTIONS

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

– Chemical reactions occur when bonds between atoms are formed or broken

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

– Chemical reactions are written in symbolic form

– How can you tell a chemical reaction is happening?

IntroductionIntroduction

– Color, heat/light change, odor, bubbles

Energy is in the bonds!Energy is in the bonds!• Exothermic rxns –

bonds broken and energy is released

• Endothermic rxns – bonds made and energy is stored

– reactantsreactants (molecules on left of arrow)– productsproducts (molecules on right of arrow).

– “+” sign separates molecules on same side

– “” is read as “yields”

– Ex: carbon plus oxygen react to yield carbon dioxide

C + O2 CO2

Parts of a Reaction Parts of a Reaction EquationEquation

Other symbolsOther symbols•CoefficientsCoefficients - The numbers in - The numbers in

frontfront– Show relative amounts of reactants & Show relative amounts of reactants &

productsproducts

– Shows the “recipe”Shows the “recipe”

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

• Letters (s), (g), and (l) are the Letters (s), (g), and (l) are the physical states of compounds.physical states of compounds.

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 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

– 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

Subscripts vs. Subscripts vs. CoefficientsCoefficients

• The The subscriptsubscript tell you how many atoms of a tell you how many atoms of a particular element are in a compound. particular element are in a compound. – CO carbon monoxide VS. COCO carbon monoxide VS. CO22 carbon dioxide carbon dioxide

• The The coefficientcoefficient tells you about the quantity, or tells you about the quantity, or number, of molecules of the compound.number, of molecules of the compound.– 33 COCO22 = 3 molecules of carbon dioxide = 3 molecules of carbon dioxide

1. DO NOT CHANGE THE FORMULAS!2. Always start on the left.3. Compare the number of atoms for each element on

either side of the reaction4. Add or change COEFFICIENTS to balance.5. Every time you make a change, go back to the

beginning.6. If polyatomic ions are present on both sides, treat

them as one unit.7. 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

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

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

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

This equation is not balanced!

What coefficients need to be added to balance this equation?

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) ---> ___ Al(l) ---> ___ Al22BrBr66(s)(s)2 3

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

___ B___ B22OO33(g) + _____ H(g) + _____ H22O(g)O(g)

3 4

5

2 5

5.52 11

4 10

Balancing EquationsBalancing EquationsBalancing EquationsBalancing EquationsSodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide

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

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

NaNa22O + FePOO + FePO44 3

2

2

Now Try These:5

2 33.52 7 4 6

2 222 63

C2H6 + O2 CO2 + H2O

a) P4 + O2 P4O10

b) Li + H2O H2 + LiOHc) Bi(NO3)3 + K2S Bi2S3 + KNO3

d) C2H6 + O2 CO2 + H2O

Mg + O2 MgO 2Mg + O2 2MgO Mg + ½O2 MgO Mg2+ O2 2MgO

4Mg + 2 O2 4MgO

Which Is Balanced?

a) Mg + 2HCl MgCl2 + H2

b) 3Ca + N2 Ca3N2

c) NH4NO3 N2O + 2H2O

d) 2BiCl3 + 3H2S Bi2S3 + 6HCl

e) 2C4H10 + 13O2 8CO2 + 10H2O

f) 6O2 + C6H12O6 6CO2 + 6H2O

g) 3NO2 + H2O 2HNO3 + NO

h) Cr2(SO4)3+ 6NaOH 2Cr(OH)3+ 3Na2SO4

i) Al4C3 + 12H2O 3CH4 + 4Al(OH)3

Balance these skeleton equations:

a) 2KNO3 2KNO2 + O2

b) 2Pb(NO3)2 2PbO + 4NO2 + O2

c) P4 + 6I2 4PI3

d) 3MgO + 2H3PO4 Mg3(PO4)2 + 3H2O

e) Br2 + 2KI I2 + 2KBr

f) Ca(OH)2 + 2HNO3 Ca(NO3)2 + 2H2O

g) Bi2O3 + 3H2 2Bi + 3H2O

h) 3Fe + 2O2 Fe3O4

i) 2CaO + 5C 2CaC2 + CO2

Here are some more to balance:

a) 2Li + 2H2O H2 + 2LiOH

b) P4 + 5O2 P4O10

c) 2C2H6 + 7O2 4CO2 + 6H2O

d) CS2 + 3O2 CO2 + 2SO2

e) 2AsCl3 + 3H2S As2S3 + 6HCl

f) 3AgNO3 + FeCl3 3AgCl + Fe(NO3)3

g) 2KClO3 2KCl + 3O2

h) 2SO2 + O2 2SO3