1. Chapter 9 How Chemicals React
Lecture Presentation
Chapter 9 How Chemicals React
Bradley Sieve
Northern Kentucky University
Highland Heights, KY

2. 9.1 Chemical Reactions Are Represented by Chemical Reactions
Chemical reactions describe how atoms are rearranged
Reacting substances are called reactants
New substances are called products
Reactants  Products

3. 9.1 Chemical Reactions Are Represented by Chemical Reactions
Phases are shown by abbreviations
(s) for solid
(l) for liquid
(g) for gas
When compounds are dissolved in water, it is said to be aqueous (aq)

4. 9.1 Chemical Reactions Are Represented by Chemical Reactions
Coefficients
Denote the number of each type of atom or molecule present
Are shown before each type of atom or molecule

5. 9.1 Chemical Reactions Are Represented by Chemical Reactions
Balancing of an equation
The same number of each type must go into and come out of a reaction
Demanded by the law of conservation of mass
In balancing, only the coefficients can be changed
Coefficient of 1 is omitted

6. 9.1 Chemical Reactions Are Represented by Chemical Reactions
4 H and 2 O atoms are present on both sides of the arrow

7. Concept Check
How many oxygen atoms are indicated by the following balanced equation?
3 O2(g) 2 O3(g)

8. Concept Check
There are six oxygen atoms. Before the reaction, these six oxygen atoms are found in three O2 molecules. After the reaction, these same six atoms are found in two O3 molecules.
3 O2(g)  2 O3(g)

9. 9.1 Chemical Reactions Are Represented by Chemical Reactions
Remember the law of mass conservation, which tells us that atoms are neither created nor destroyed in a chemical reaction—they are simply rearranged.

10. 9.1 Chemical Reactions Are Represented by Chemical Reactions
Balancing equations quick guide
1. Focus on one element at a time
2. Move on to next element and balance
3. Continue through all the elements
4. Repeat 1–3 until all elements are balanced

11. 9.2 Counting Atoms and Molecules by Mass
Because different atoms and molecules have different masses, there are different numbers in a given mass

12. 9.2 Counting Atoms and Molecules by Mass
Atomic masses are given on the periodic table
One carbon atom is roughly 12 amu
Molecular mass is the sum of the atomic masses
O2 is about 32 amu (~16 amu from each O)

13. 9.2 Counting Atoms and Molecules by Mass

14. 9.3 Converting between Grams and Moles
Mole is an Avogadro’s number of item
6.02 × 1023 atoms or molecules
One mole of an atom is equal to the amount of mass in grams
1 mole of sodium is grams
1 mole of lead is grams

15. 9.3 Converting between Grams and Moles
One mole of a molecule is equal to the molecular mass in grams
1 mole of O2 is grams
1 mole of CO2 is grams

16. Concept Check
How many atoms are in a gram sample of lithium, Li (atomic mass = amu)?

17. Concept Check
Because this number of grams of lithium is numerically equal to the atomic mass, × 1023 atoms are in the sample, which is 1 mole of lithium atoms.

18. 9.3 Converting between Grams and Moles
Molar mass is defined as the mass of 1 mole of the substance
Unit is grams per mole
Also defines the mass of 6.02 × 1023 of the item in the mole

19. 9.3 Converting between Grams and Moles

20. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
Reactions may either absorb or release energy
If energy is released, the reaction is exothermic
If energy is absorbed, the reaction is endothermic

21. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
The change in energy is due to the formation or breaking of chemical bonds
The energy to break or form a bond is called bonding energy
Positive energy is the energy absorbed as a bond breaks
Negative energy is the energy released as the bond forms

22. 9.4 Chemical Reactions Can Be Exothermic or Endothermic

23. Concept Check
Do all covalent single bonds have the same bond energy?

24. Concept Check
No. Bond energy depends on the types of atoms bonding. The H–H single bond, for example, has a bond energy of 436 kilojoules per mole, but the H–O single bond has a bond energy of 464 kilojoules per mole. Not all covalent single bonds have the same bond energy.

25. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
Exothermic reactions involve a net release of energy
So less energy is contained in the product’s bonding
Amount of energy released can be calculated from the individual bond differences
Net energy of reaction =
energy absorbed + energy released

26. Concept Check
Where does the net energy released in an exothermic reaction go?

27. Concept Check
This energy goes into increasing the speeds of reactant atoms and molecules and often into electromagnetic radiation such as light.

28. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
Endothermic reactions involve a net absorption of energy
More energy is contained in the product’s bonding
Amount of energy released can be calculated from the individual bond differences
Net energy of reaction =
energy absorbed + energy released

29. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
Exothermic Reactions

30. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
Endothermic Reactions
[insert fig. 9.9]

31. Concept Check
Should the following reaction be endothermic or exothermic?
O2NNO2  O2N + NO2

32. Concept Check
No calculations are necessary. This reaction is endothermic because it involves only the breaking of a chemical bond.

33. 9.4 Chemical Reactions Can Be Exothermic or Endothermic
Energy is conserved in a chemical reaction
Thermodynamics is the area of science concerned with energy’s roles
First law of thermodynamics states that energy does not appear or disappear

34. 9.5 Chemical Reactions Are Driven by the Spreading of Energy
Energy tends to disperse
One of the main driving forces of chemical and physical changes
Reactions that concentrate energy do not tend to occur
Described by the second law of thermodynamics

35. 9.5 Chemical Reactions Are Driven by the Spreading of Energy
Entropy
Measure of the spreading of energy
Increase in entropy corresponds to a spreading of energy

36. 9.5 Chemical Reactions Are Driven by the Spreading of Energy
The sun is truly a “hothouse”— dispersing enormous amounts of energy from exothermic nuclear reactions. A tiny fraction of the energy is used to drive photosynthesis, which is vital for plants and plant-eating creatures like us.

37. Concept Check
Sugar crystals form naturally within a supersaturated solution of sugar water. Does the formation of these crystals result in an increase or a decrease in entropy?

38. Concept Check
The formation of these sugar crystals results in an increase in entropy. Your clue to an increase in entropy here is that the crystals form “on their own,” a spontaneous process and thus one that must result in an entropy increase.

39. 9.5 Chemical Reactions Are Driven by the Spreading of Energy
Laws of Thermodynamics
You can’t win because you can’t get any more energy out of a system than you put into it. You can’t break even because no matter what you do, some of your energy will be dispersed as ambient heat.

40. 9.6 Chemical Reactions Can Be Slow or Fast
The speed of a reaction depends upon
Concentration of reactants
Temperature of the reactants
Speed of a reaction is called the reaction rate

41. 9.6 Chemical Reactions Can Be Slow or Fast
Reactants must come in contact to react
The easier this occurs, the higher the rate
The more interactions or the higher the energy, the faster the overall rate

42. 9.6 Chemical Reactions Can Be Slow or Fast
Reactants must collide in the correct orientation to react

43. Concept Check
An internal-combustion engine works by drawing a mixture of air and gasoline vapors into a chamber. The action of a piston then compresses these gases into a smaller volume prior to ignition by the spark of a spark plug. What is the advantage of squeezing the vapors into a smaller volume?

44. Concept Check
Squeezing the vapors into a smaller volume effectively increases their concentration and hence the number of collisions between molecules. This, in turn, promotes the chemical reaction.

45. 9.6 Chemical Reactions Can Be Slow or Fast
Activation Energy (Ea)
Energy required for the initial breaking of bonds
Can be viewed as an energy barrier the reaction must overcome
The faster moving a molecule, the more energy present

46. 9.7 Catalysts Speed Up the Destruction of Stratospheric Ozone
Catalysts speed up a reaction
Not consumed in the process of the reaction
Lowers the overall activation energy needed

47. Chlorofluorocarbons (CFCs)
Chlorine acts as a catalyst to destroy ozone in the stratosphere
Cl + O3  ClO + O2
ClO + O3  Cl + 2O2
Depletion of stratospheric ozone leads to the “ozone hole”

50. Catalysts are mostly beneficial
Catalytic converters
Convert dangerous pollutants from exhaust to less toxic substances

51. Concept Check
How is a catalyst different from a chemical reactant?

52. Concept Check
A catalyst is not used up during a chemical reaction. Instead, it is released as its original self. The catalyst can then serve to catalyze more reactions. A chemical reactant, by contrast, is consumed during a reaction as it is transformed into a chemical product.