1. Chemical Reactions

2. Chemical Reactions
Processes in which one or more substances are converted into new substances with different physical and chemical properties
Ex: rusted metal, burned toast

3. Chemical Reactions Parts of a chemical reaction:
Reactants – substances before reaction occurs
Products – substances produced by the reaction

4. Chemical Reactions Reason why reactions occur:
Increase in stability for products

5. Types of Reactions
Combination - two substances combine to form new substance
A + B  AB
Ca + Cl2  CaCl2

6. Types of Reactions Combustion – substance reacts with oxygen
A + O2  AO
CH4 + O2  CO2 + H2O

7. Types of Reactions
Decomposition – substance breaks into two or more substances
AB  A + B
MnSO4  MnO + SO3

8. Types of Reactions
Single-replacement – uncombined element “trades places” with an element in a compound
A +BX  AX + B
Cd + 2 HCl  CdCl2 + H2

9. Types of Reactions
Double-replacement – two binary compounds “trade partners”
AX +BY  AY + BX
CaCO3 + 2 HCl 
CaCl2 + H2CO3

10. Chemical Equations Sentences representing a chemical reaction 2 types
Word equations - gives names of reactants & products
Formula equations – shows chemical formulas and symbols

11. Chemical Equations Formula equations… Symbols used: + reacts/combines
with, and
 yields/produces
 precipitate formed
 gas emitted

12. Chemical Equations Formula equations… Symbols used: (s) solid state
(l) liquid state
(g) gaseous state
(aq) aqueous state

13. Chemical Equations Diatomic molecule reminder… Hydrogen H2 Nitrogen N2
Oxygen O2
Fluorine F2
Chlorine Cl2
Bromine Br2
Iodine I2

14. Chemical Equations Unbalanced Examples:
Calcium reacts with oxygen to produce calcium oxide
Carbon reacts with oxygen to produce carbon dioxide

15. Chemical Equations Unbalanced Examples:
Solid sodium reacts with chlorine gas to produce solid sodium chloride
Nitrogen gas reacts with hydrogen gas to produce ammonia gas

16. Chemical Equations Unbalanced Examples:
Sodium hydroxide reacts with hydrogen chloride to produce water and sodium chloride

17. Chemical Equations How to balance:
Write formula equation with correct formulas & symbols
Count number of atoms on each side of arrow (make table if needed)
Mass must be conserved (number of atoms on each side of arrow are equal), use coefficients to adjust
Work with one atom or ion at a time
Save H & O for last (they tend to be in more than one place & can be difficult)
Recount to ensure that number of atoms are equal on each side (coefficient ratio needs to be lowest possible numbers)

18. Chemical Equations Balancing examples: NaOH + HCl  H2O + NaCl
Ca + O2  CaO

19. Chemical Equations Balancing examples: N2 + 3 H2  2 NH3
CH4 + 2 O2  CO2 + 2 H2O

20. Chemical Equations Balancing examples: C2H6 + O2  CO2 + H2O
FeCl3 + Na2CO3  NaCl + Fe2(CO3)3
C2H6 + O2  CO2 + H2O

21. Chemical Reactions Reaction Rates
Reactions happen if particles have enough energy.

22. Chemical Reactions Reaction Rates Factors Temperature Concentration
Surface area/Particle Size
Catalysts – substances that speed up reaction without being used

23. Chemical Reactions Reactions are reversible
When rate of forward and reverse reaction are equal, system is at equilibrium.

24. Chemical Reactions Reactions are reversible
Equilibrium is described by Le Chatelier’s principle
If stress is applied to a system, the system changes to relieve the stress

25. Chemical Reactions Reactions are reversible
Factors affecting equilibrium:
Concentration
Temperature
Pressure
Increasing pressure shifts equilibrium to whichever side has fewer particles. Decreasing pressure has opposite effect.

26. Chemical Reactions Reactions are reversible
Equilibrium is described by a constant
Keq, which is a ratio of products to reactants.
Keq > 1, products favored
Keq < 1, reactants favored

27. Chemical Equations Can include heat
Endothermic – heat, from surroundings, enters the system, has positive heat of reaction (DH) value, reactants have lower potential energy than products

28. Chemical Equations
Can include heat
Endothermic reaction

29. Chemical Equations Can include heat Endothermic reaction:
C(s) +2 S(s) kJ  CS2(l)
C(s) +2 S(s)  CS2(l)
DH = 89.3 kJ

30. Chemical Equations
Can include heat
Endothermic – heat, from surroundings, enters the system
Exothermic – heat exits the system and goes to surroundings, has negative heat of reaction (DH) value , reactants have higher potential energy than products

31. Chemical Equations
Can include heat
Exothermic reaction

32. Chemical Equations Can include heat Exothermic reaction:
CaO(s) + H2O(l)
 Ca(OH)2(s) kJ
CaO(s) + H2O(l)  Ca(OH)2(s)
DH = kJ

33. Chemical Equations Can include heat
For systems with a constant pressure, heat is also called enthalpy (so DH is also change in enthalpy)

34. Chemical Equations Can include heat
Enthalpy diagrams – visual representation of a thermochemical equation

35. Chemical Equations
Enthalpy diagrams – Endothermic reaction

36. Chemical Equations
Enthalpy diagrams – Exothermic reaction

37. Chemical Reactions Generally involve heat
Hess’s Law – summary of all intermediate steps in a reaction, includes enthalpy changes for each step

38. Chemical Reactions Generally involve heat Hess’s Law –
Can be used to determine heat of process that cannot be directly measured

39. Chemical Reactions Generally involve heat Hess’s Law –
Example: conversion of diamond to graphite is too slow to measure, but combustion can be measured

40. Chemical Reactions Generally involve heat Hess’s Law – Example:
C(s, diamond) + O2(g)  CO2(g)
DH = kJ

41. Chemical Reactions Generally involve heat Hess’s Law – Example:
C(s, graphite) + O2(g)  CO2(g)
DH = kJ

42. Chemical Reactions Generally involve heat Hess’s Law –
Example: difference between the two DH values will give the DH for converting diamond to graphite

43. Chemical Reactions
Hess’s Law Diagram –
-1.9 kJ

44. Chemical Reactions Hess’s Law (another look) –
Determine the enthalpy change for the reaction of nitrogen dioxide becoming dinitrogen tetroxide.
2 NO2(g) → N2O4(g)
N2(g) + 2 O2(g) → 2 NO2(g) DH = 67.7 kJ
N2(g) + 2 O2(g) → N2O4(g) DH = 9.7 kJ
DH = kJ

45. Chemical Reactions Will a reaction occur?
A reaction that releases heat is likely to occur

46. Chemical Reactions Will a reaction occur?
A reaction that releases heat is likely to occur
Since endothermic reactions occur, there must be something besides enthalpy to determine if reaction happens

47. Chemical Reactions Will a reaction occur?
Reactions are dependent on a combination of enthalpy and entropy

48. Chemical Reactions Will a reaction occur?
Entropy (S) is the measure of disorder in a system
Law of Disorder - processes move towards maximum disorder (or chaos)

49. Chemical Reactions Will a reaction occur?
Entropy can be determined by comparing entropy of reactants to entropy of products

50. Chemical Reactions Will a reaction occur? Spontaneous reactions
Reactions that occur naturally and favor products (may be slow or fast reactions), energy will be released
Spontaneity or nonspontaneity depends on conditions

51. Chemical Reactions Will a reaction occur?
Spontaneous reaction examples:
Heat released & entropy increased
Increase in entropy is more than amount of heat absorbed
Amount of heat released is more than decrease in entropy

52. Chemical Reactions Will a reaction occur? Nonspontaneous reactions
Reactions that do not favor product formation, so they are not likely to occur (but they can happen)
Spontaneity or nonspontaneity depends on conditions

53. Chemical Reactions Will a reaction occur?
Nonspontaneous reaction examples:
Heat absorbed & entropy decreased
Increase in entropy is less than amount of heat absorbed
Amount of heat released is less than decrease in entropy

54. Chemical Reactions Will a reaction occur?
Spontaneous reactions have energy available to do work called free energy.

55. Chemical Reactions Will a reaction occur?
Spontaneous reactions have energy available to do work called free energy.
The maximum free energy is called Gibbs free-energy change (DG)

56. Chemical Reactions Gibbs free-energy change (DG) is calculated as:
DG = DH – TDS
T is in Kelvins

57. Chemical Reactions More Gibbs free-energy change (DG)…
Spontaneous reactions DG is negative
Nonspontaneous reactions DG is positive