Chemical Equations

1
Chemical Equations

• Law of Conservation of Mass
• Matter can not be created or destroyed
• Balanced chemical equations reflect this

2
Balancing Chemical Equations
Reactants
Products
Not balanced
MgCl2 (aq) AgNO3 (aq) ? AgCl (s)
Mg(NO3)2 (aq)
Magnesium chloride
Silver(I) chloride
Magnesium nitrate
Silver(I) nitrate

• Numbers and identities of atoms must be the same
  on both sides of a reaction (rxn) arrow
• Total charge must be the same on both sides of
  the rxn arrow

3
Balanced Equations and Coefficients
MgCl2 (aq) AgNO3 (aq) ? AgCl (s)
Mg(NO3)2 (aq)

• Coefficients go in front of chemical and indicate
  how many
• Coefficients should be the smallest possible
  whole number

4
Reaction Amounts
A B ? C

• How much A do we mix with B to get a certain
  amount of C?
• Depends on relationship between A, B, and C
• Coefficients tell us relationship
• parts 1 part A to 1 part B makes 1 part C

5
Parts in Chemistry equals Moles

• Avogadros number and Moles
• 6.022 ? 1023 of anything equals 1 mole
• In Chemistry 6.022 ? 1023 atoms or molecules
  equal 1 mole
• 2 parts red to 2 parts green to 1 part yellow
  equals 1 Rah
• (microscopic world)
• 2 moles red to 2 moles green to 1 mole yellow
  equals
• 1 mole Rah
• (macroscopic world)

1 yellow 2 red 2 Green ? 1 rah C 2 Cl
2 H ? CH2Cl2
6
Moles and Grams

• Avogadros number
• 6.022 ? 1023 atoms or molecules equal 1 mole
• 1 mole of atoms equals the atomic weight in
  grams
• ie. 1 mole of O atoms (6.022 ? 1023 atoms)
  16.00 g
• 1 atom of O 16.00 amu, how many atoms weigh
  16.00 g?
• 16.00 g O ? 6.022 ? 1023 amu ? 1 atom O 6.022
  ? 1023 atoms O
• 1 g 16.00 amu

7
Moles A New Conversion Factor

• 1 mol 6.022 ? 1023 parts
• 1 mol atoms atomic weight in grams
• 1 mol of molecules molar mass in grams
• Molar Mass, Molecular Weight, Formula Weight
• Sum of all atomic weights of the atoms in a
  formula
• Can be determined using a mass spectrometer

8
Converting Between Grams, Molecules, and Moles

• If you have 20.0 g of CO2, how many moles of CO2
  do you have? How many molecules of CO2 do you
  have?

9
Formulas, moles, and atoms
Moles talk to moles

• Subscripts indicate the relative molar ratio of
  elements
• ie. CO2 has 1 mol of C for every 1 mol of CO2
  and
• 2 mol of O for every 1 mol of CO2 and 2 mol of O
  for every 1 mol of C

10
Percent Composition (By Mass)
mass of x in substance total mass of substance
Mass Percent of x
? 100
Percent Composition in a compound

• Determine the molar mass of the compound (total
  mass of 1 mole of substance)
• Determine the mass of each element in 1 mole of
  compound (mass of x in substance)
• Use the above formula
• The mass percent of all elements in a compound
  should add up to 100

11
Working backwards from compositions to formulas

• Start with mass of elements (i.e. empirical
  data) and calculate a formula
• Mass often determined using combustion analysis

12
Stoichiometric Amounts of Products and Reactants

• Stoichiometric amounts represent the ideal or
  perfect amount of reactants and products
• Stoichiometric amounts come from the mole to mole
  ratios of reactants and products (coefficients in
  balanced chemical equation)

13
Mole Relationships and Chemical Equations
Pb(NO3)2 (aq) KI (aq) ?
KNO3 (aq) PbI2 (s)
2
2
2.54 g

• 1. Start with balanced chemical equation

2. Coefficients in front of compounds tell you
the mole relationships
1 mole of Pb(NO3)2 plus 2 moles of KI makes 2
moles of KNO3 and 1 mole of PbI2
14
Mole Relationships and Grams

• Determine the molar mass of compounds of interest
• 1 mole of a compound or atom is equal to its
  molar mass in grams (conversion factor)
• Calculate the moles of each starting material (if
  moles are given in the beginning start here)
• Convert to moles of compound of interest
• Convert to grams of compound of interest

15
Experimentally Determine Amount of Product

• Mix preweighed starting materials together
• Filter final solution into pre-weighed piece of
  filter paper
• Let filter paper and product dry
• Weigh product and filter paper
• Subtract the weight of the filter paper from the
  weight in step 3.
• Mass of Product also called experimental yield of
  product or actual yield of product.

16
Percent Yield

• Relates experimental findings to theoretical
  predictions
• Percent Yield Experimental yield ? 100
• Theoretical yield

17
Limiting Reagent
Pb(NO3)2 (aq) 2 KI (aq) ?
2 KNO3 (aq) PbI2 (s)
2.54 g
3.00 g

• How much PbI2 (s) should we get from mixing the
  amounts of reactants shown above?
• One reactant runs out before the other, ie. it
  limits the amount of product(s) that can be
  formed
• The other product is in excess, ie. some is left
  over at the end of the reaction