1. Introduction to Chemistry Chapter 21
Chemical Reaction
Introduction to Chemistry
Chapter 21

2. Chemical Changes
Section 21-1

3. Chemical Reactions Atoms re-arrange to form new compounds.
Seating chart analogy
How do you know a reaction has occurred?
Gas is produced.
Change in temperature.
Precipitate formed.
Light given off.
Reactants react to form products.
Reactants Products.

4. Conservation of mass Law of Conservation of mass.
Antoine Lavoisier
Mass of the products will equal the mass of the reactants
Discovered by heating
Mercury (II) oxide to
produce oxygen gas and mercury.
Mercury (II) oxide  oxygen plus mercury.
10 g = g g

5. Letters, Words & Sentences
The atomic symbols are like the chemist alphabet.
Those “letters” are combined to form words - the chemical formulas.
Those words are combined into sentences called chemical equations.
Like a sentence, they describe what happened.

6. Equation Symbols Symbol Meaning  Produces, yields, or forms. +
Plus or and
(s)
solid
(l)
Liquid as in melted
(g)
gas
(aq)
Aqueous, dissolved in water

7. Coefficients
Since mass is conserved, you sometimes need more than one molecule of a compound.
To show the number needed, coefficients are placed in front of the compound.
2NaCl would mean two sodium chloride molecules are involved in the reaction.

8. A reaction . . . Lead (II) nitrate is added to potassium iodide.
One molecule of lead (II) nitrate dissolved in water is added to two molecules of potassium iodide dissolved in water to produce solid lead (II) iodide and potassium nitrate dissolved in water.
Pb(NO3)2 (aq) + 2KI (aq) PbI2 (s) + 2KNO3 (aq)
Lead Nitrate and potassium Iodide

9. Reactions: Practice What is the solid that is formed?
NiCl2 (aq) + 2NaOH (aq) Ni(OH)2 (s) + 2NaCl (aq)
What is the solid that is formed?
What product is dissolved in the solution?
How many of each element are on the reactant side?
How many of each element are on the product side?

10. Practice Answers NiCl2 (aq) + 2NaOH (aq) Ni(OH)2 (s) + 2NaCl (aq)
What is the solid that is formed?
Nickel (II) hydroxide.
What product is dissolved in the solution?
Sodium chloride
How many of each element are on the reactant side?
Ni: 1, Cl: 2, Na:2, O:2, H:2 (or OH:2)
How many of each element are on the product side?
Ni:1, Cl:2, Na:2, O:2, H:2 (or OH:2)

11. Chemical Equations
Section 21-2

12. Balancing reactions Why balance? HgO  Hg + O2 2HgO  2Hg + O2
Matter can not be created or destroyed!
HgO  Hg + O2
Where did the extra oxygen come from?
2HgO  2Hg + O2
Now there are 2 mercury atoms and 2 oxygen atoms on both sides.
This is a balanced reaction.
So, why is the oxygen O2 and not the mercury?

13. Great Question! The Oxygen is diatomic – two atoms.
Hydrogen, Nitrogen, Oxygen, Fluorine, chlorine, and Bromine will pair up as gases.
Hydrogen and the “upside down L” or “7”.
Whenever you see these as gases the formula is X2.
Cl2 (g), O2 (g), F2 (g), H2 (g)

14. Balancing Rules Write the reaction with all the proper formulas.
Once you figure out the formula, do not touch the subscripts.
Balancing is done with coefficients (numbers in front).
Count the number of atoms of each element on both sides.
Change one coefficient, recount, and check again.
Continue changing coefficients and rechecking until it is balanced.
Coefficients must be in the lowest ratio.

15. Balancing Practice
Lithium metal is added to water to produce lithium hydroxide and hydrogen gas.
Copper (II) nitrate is added to sodium hydroxide to produce copper (II) hydroxide and sodium nitrate.
Do It!

16. Balancing: Tricks of the Trade
Only change one coefficient at a time, and then recheck.
If a polyatomic ion appears on both sides keep together.
If an atom appears in more than one compound balance it last.
If you have oxygen gas (O2) and need an odd number, double everything to need an even number of oxygen.

17. Classifying Chemical Reactions
Section 21-3

18. Synthesis Two or more substances combine to form one new substance.
More reactants than products.
A + B  AB
2H2 + O2  2H2O

19. Decomposition One compound breaks apart in to many.
Opposite of synthesis reaction.
More products than reactants.
AB  A + B
H2O2  H2 + O2

20. Single Displacement Between a compound and an element.
The element (usually a metal) replaces another in a compound.
A + BC  B + AC
Cu + 2AgNO3  Cu(NO3)2 + Ag

21. Activity Series Metals can be ranked by how reactive they are.
List is called activity series.
A metal on top of the series will replace a metal lower down.
Metals on the bottom of the list are often found uncombined in nature.
Gold, Silver and copper are on the bottom.

22. Making Predictions
Series can be used to predict if a reaction will occur.
Mg + Pb(NO3)2 ?
Cu + FeCl3  ?
Cu + AgNO3  ?

23. Double Displacement Between two compounds.
Positive ions exchange places.
AB + CD  AD + CB
AgNO3 + NaCl  AgCl + NaNO3

24. Combustion
Combining a substance with oxygen to produce carbon dioxide and water.
Combustion of Methane:
CH4 + O2  CO2 + H2O
CH4 + 2O2  CO2 + 2H2O

25. Oxidation and Reduction
In a reaction, there is a transfer of electrons from one compound to another.
Reduction is the gain of electrons.
Charge is reduced (becomes more negative)
Oxidation is the loss of electrons.
OIL RIG
Oxidation Is Loss, Reduction Is Gain
Oxidation and reduction occur together
Redox reactions

26. Chemical Reactions and Energy
Section 21-4

27. Activation Energy – The Hill
A reaction is like a rock at the bottom of a hill.
Push it to the top and it will roll down the other side on its own.
Pushing it to the “top” of the hill is the activation energy.
Some hills are larger than others.
Bond breaking requires energy.
Bond making releases energy.
Ethanol cannon

28. Energy Out
Bond making release more energy than the bond breaking requires.
Making > Breaking.
Called exergonic reactions
Excess energy is released as heat or light.
If heat is released, called exothermic reaction.
Exo = Out
Therm = Heat
Feels hot to the touch.

29. Energy In
Bond breaking requires more energy than bond making provides.
Breaking > Making
Called endergonic reactions
Additional energy must be provided to the reaction.
Light (photosynthesis), Electricity (hydrolysis of water)
If the reaction absorbs heat, it is called an endothermic reaction.
Endo = In
Therm = Heat
Feels cold to the touch.
Barium hydroxide and ammonium chloride rxn.

30. Energy Diagram - Exothermic

31. Energy Diagram - Endothermic

32. Reaction Rates To react, molecules must collide with enough energy.
Concentration
If the molecules are tighter packed, greater chance of collision.
Surface Area
Reactions occur at surfaces – more surface, more reactions.
Temperature
The lower the temperature, the slower the molecules – less energy.

33. Catalysts and Inhibitors
Catalysts speed up a reaction.
Enzymes in body
Inhibitors slow down the reaction.
BHT and BHA prevent spoilage.
Nitroglycerine and dynamite.
A catalyst / inhibitor does not react.
Demos: Hydrogen peroxide and MnO /