Chemical change

1
Chemical change

• Chemical Change- Change in composition of matter
• Process involving chemical changes called
  Chemical Reactions
• Chemical equations- Representation of chemical
  reactions in terms of reactants and products
• Reactants- substances that undergo chemical
  change in a reaction
• Products- substances produced as result of
  reaction
• Chemical equation Reactants ? Products
• Reactant 1 Reactant 2 ? Product 1 Product 2

2
Chemical Equation

• hydrogen oxygen ? water
• 2H2(g) O2(g) ? 2H2O(l)
• Balanced chemical equation-
• atoms of each element in reactants atoms of
  each element in products
• Law of conservation of matter- Atoms are neither
  created not destroyed in chemical reactions
• Coefficient on left - of molecules
• State or form- symbol in parentheses on right
• (l), (g), (aq), (s)

3
Balancing Chemical Equation

• N2(g) H2(g) ? NH3(g)
• Example 5.2
• Learning check 5.2
• Methane- natural gas (fuel) burns in air
• CH4(g) O2 (g) ? CO2 (g) H2O(g)

4
Classification of Chemical Reactions
5
Redox Reactions
6
Oxidation Number

• Oxidation numbers (also called oxidation states)
  are positive or negative numbers assigned to
  elements in chemical formulas according to a set
  of rules.
• Abbreviated O.N..
• Rule 1
• The O.N. of any uncombined element is 0.
  Examples are Fe (0), Cl2 (0) and Ca(0).
• Rule 2
• The O.N. of a simple ion is equal to the charge
  on the ion. Examples are Mg2(2), O2-(-2) and
  Cl-(-1).

7
Oxidation Number

• Rule 3
• The O.N. of group IA and IIA elements when they
  are in compounds is always 1 and 2
  respectively. Examples are Na2S (Na 1), and
  Ca(NO3)2 (Ca 2).
• Rule 4
• The O.N. of hydrogen is always 1. Examples are
  HBr (H 1) and H2SO4(H 1).
• Rule 5
• The O.N. of oxygen is -2 except in peroxides
  where it is -1. Examples are MgO (O -2), HBrO3
  (O -2) and H2O2 (O -1).

8
Oxidation Number

• Rule 6
• The algebraic sum of the oxidation numbers of all
  atoms in a complete compound equals zero.
• MgSO4 (O.N. of Mg 2, O.N. of S 6, O.N. of O
  -2)
• Total positive oxidation number is 8.
• Total negative O.N. of -8.
• Rule 7
• The algebraic sum of the O.N. of all the atoms in
  a polyatomic ion is equal to the charge on the
  ion.
• HCO3- (O.N. of H is 1, O.N. of C is 4, O.N. of
  O is -2)
• The total positive O.N. is the sum of 1 and 4
  or 5.
• The total negative O.N. is 3 x (-2) or -6.
• (5) (-6) or -1 which is the charge on the
  polyatomic ion.

9
Oxidation Number

• Example 5.3
• Learning Check 5.3

10
Oxidation-Reduction

• An oxidation reaction is always accompanied by
  reduction
• Oxidation Loss of e-s ? O.N.
• Reduction Gain of e-s ? O.N.
• S O2 ? SO2
• ReactantsO.N. of elemental S 0O.N. of
  elemental O 0
• Products
• O.N. of S 4 (oxidation)
• O.N. of O -2 (reduction)

11
Redox Reactions

• Reducing agent- substance that contains an
  element that is oxidized during a reaction.
• Oxidizing agent- substance that contains an
  element that is reduced during a reaction.
• Learning Check 5.4

12
Decomposition Reactions

• One substance is broken down into two or more
  simpler substances
• A B C
• Redox decomposition reaction
• 2HI(g) H2 (g) I2 (g)
• Non-redox decomposition reaction
• H2CO3(aq) CO2(g) H2O(l)

13
Combination Reactions

• Two or more substances react to form a single
  substance
• A B C
• Redox combination reaction
• S(s) O2 (g) SO2 (g)
• Non-redox combination reaction
• N2O5(g) H2O(l) 2HNO3(aq)

14
Single-replacement Reactions

• Single replacement (substitution) reactions- An
  element reacts with a compound and displaces
  another element from the compound. Always redox
  reactions
• A BX B AX
• Zn(s) CuSO4(aq) Cu(s) ZnSO4(aq)

15
Double-replacement Reactions

• Double replacement (metathesis) reactions- Two
  compounds react and exchange partners forming new
  compounds
• AX BY BX AY
• Ba(NO3)2(aq) Na2S(aq) BaS(s) 2NaNO3(aq)

16
Classification of Chemical Reactions
Redox ? ? O.N. Nonredox O.N. unchanged
Example 5.5 Learning check 5.5
17
Reactions and Energy

• Reactions involve changes in energy
• 2H2(g) O2(g) ? 2H2O(g)
• Energy absorbed or evolved
• Forms of energy- heat (most common), electricity
  (batteries), sound, light (LEDs, bulbs), motion..
• Energy expressed in joules or calories
• Exothermic reactions- release heat
• Energy of products is less than that of reactants
• Example- burning of fuel
• Endothermic reactions- absorb heat
• Energy of products is more than that of reactants
• Example- photosynthesis
• Chemistry around us 5.1

energy
18
Stoichiometry

• stoicheion (element) metron (measure)
  Measurement of (mass of) elements (in reactions)
• The study of mass relationships in chemical
  reactions
• Applying mole concept to balanced chemical
  reactions
• CH4(g) 2 O2 (g) ? CO2 (g) 2 H2O(g)
• 1 CH4 molecule 2 O2 molecules ? 1 CO2 molecule
  2 H2O molecules
• 2 CH4 molecules 4 O2 molecules ?
• 2 CO2 molecules 4 H2O molecules
• 10 CH4 molecules 20 O2 molecules ?
• 10 CO2 molecules 20 H2O molecules
• 6.02 1023 CH4 molecules 12.0 1023 O2
  molecules ?
• 6.02 1023 CO2 molecules 12.0 1023 H2O
  molecules
• 1 mol CH4 2 mol O2 ? 1 mol CO2 2 mol H2O

19
Stoichiometry

• CH4(g) 2 O2 (g) ? CO2 (g) 2 H2O(g)
• 1 mol CH4 2 mol O2 ? 1 mol CO2 2 mol H2O
• 1 mol CH4 ? 1 mol CO2
• 1 mol CH4 ? 2 mol H2O
• 1 mol CH4 ? 2 mol O2
• 1 mol CH4 16.04 g CH4 ? 1 mol CO2
• 1 mol CH4 16.04 g CH4 ? 2 mol H2O
• 1 mol CH4 16.04 g CH4 ? 2 mol O2
• 1 mol CH4 16.04 g CH4 ? 1 mol CO2 ? 44.01 g CO2
  
• 1 mol CH4 16.04 g CH4 ? 2 mol H2O ? 18.02 g H2O
• 1 mol CH4 16.04 g CH4 ? 2 mol O2 ? 32.00 g O2

20
Stoichiometry

• Factor-unit (dimensional analysis) method
• Step 1 known quantity (numerical and units)
• Step 2 known qty. unknown qty.
• Step 3 multiply known qty. by (1 or more)
  factors to cancel units of known qty. and
  generate units of unknown qty.
• Step 4 arithmetic to get numerical answer for
  unknown qty.
• Example 5.7
• Learning check 5.7

21
Limiting reactant

• Limiting reactant-
• Reactant present in least amount (based on
  reaction coefficient and mol. weight)
• Determines maximum amount of product that can be
  formed
• Technique
• Find out which reactant gives the least amount of
  products
• Calculate the maximum amount of SO2 that could be
  produced by reacting 55.2 g of O2 with 50.8 g of
  H2S.
• Learning check 5.8

22
Reaction yields

• Less products formed
• Side reactions (reactions not giving the desired
  products)
• Poor laboratory technique
• Reaction does not reach completion (unfavorable
  conditions)
• Percentage yield-
• Example 5.10, Learning check 5.9