Balancing Chemical Equations, Reaction Kinetics, & Heats of

ReactionsHomer Helps with Bart’s Chemistry

Homework

Play This as a Slide Show to see the Balancing in Action!

To Obey the LAW of Conservation of Matter!!!◦ We must have the same #atoms of each type at

the start & end of a chemical reaction◦ # Reactant atoms = # Product atoms◦ Otherwise we have created or destroyed Matter!◦ Ex: Na + CaF2 NaF + Ca

Na=1 Na=1Ca=1 Ca=1F=2 F=1

We broke the LAW Matter was DESTROYED!!

Why Do we Need to Balance Equations?

Subscripts◦ These are the numbers below the symbol◦ They tell how many atoms are bonded together in one

molecule◦ For example C2H6 has 2C atoms bonded to 6H atoms◦ The subscripts are determined by the valence electrons

◦WE CAN’T CHANGE THE SUBSCRIPT!!◦ We find the subscripts by using:

Bohr Model Lewis Dots Criss-cross /oxidation #

Balancing TipsHH

C C H

HH

H

Coefficients◦ These are the numbers in front of the symbol◦ They tell how many atoms or molecules we have◦ It multiplies the number of atoms or molecules

(including the subscript)◦ Ex: 3Na = 3Na atoms

2 C2H6 = 2x2 = 4C atoms

=2x6 = 12H atoms◦IT IS THE ONLY NUMBER THAT WE CAN CHANGE!!

Balancing tips

Polyatomic Ions◦ Are many atoms bonded together to form an ion◦ Examples

NO3 = nitrate; ion charge = -1 SO4 = sulphate; ion charge = -2 NH4 = ammonium ion; ion charge = +1 CO3 = carbonate; ion charge = -2 PO4 = sulphate; ion charge = -3

◦Count how many polyatomic ions you have when balancing; not the individual atoms Ex: 2HNO3

H=2 and NO3 =2 Not H=2 & N=2 and O=6

Balancing tips

Na + MgF2 NaF + MgNa=1 Na=1Mg=1 Mg=1F=2 F=1

Reaction #1

2

2

22 2

D’Oh!

H’mm

Yum

Cl2 + NaBr NaCl + Br2

Na=1 Na=1Br=1 Br=2Cl=2 Cl =1

Reaction #2

2

2

22 2

D’Oh!

H’mm

2Now I get it!

1. Na + MgF2 NaF + Mg

2. Mg + HCl MgCl2 + H2

3. Cl2 + KI KCl + I24. NaCl Na + Cl25. Na + O2 Na2O

6. Na + HCl H2 + NaCl

7. K + Cl2 KCl

8. N2 + H2 NH3

9. CH4 + O2 CO2 +H2O

10. Al + Fe2O3 Al2O3 +Fe

Practice Problems

◦ Called Reaction Kinetics◦ Reactions happen when the reactant atoms collide with enough

energy to form new product molecules Factors that influence the rates of chemical reactions:

Concentration More particles present in a given volume increases the probability of

collisions Temperature

Increases the vibrational movement of the atoms which increases the probability of collisions

Also increases the energy of the collisions Surface Area

Increases the number of atoms exposed to collisions Ex: grinding up a reactant into a powder

Catalysts Increase reaction rates, but are not consumed in the reaction Collect the reactant atoms and bring them close together so they can react

Rates of Chemical Reactions

Reactions either absorb (endothermic) or release heat (exothermic)

This has to do with the bond energy◦ It takes energy to break bonds◦ Energy is released when bonds form because an

atom has lower energy when it gains an electron

Exothermic & Endothermic Reactions

Exothermic reactions release energy Exo means “out” & thermic means heat

(“Heat Out”) The products have more energy than the

reactants When the products form, they release more

energy than it took to break the reactant bonds

HEAT IS RELEASED

Endo means “in” & thermic means heat (“Heat In”)

The products have less energy than the reactants

When the products form, they release less energy than it took to break the reactant bonds

HEAT IS ABSORBED

Endothermic reactions absorb energy