1. Physical Science
Chapter 7
Chemical Reactions

2. Bell ringer 10/11/13
Explain what a chemical reaction means to you.

3. Bell ringer 10/15/13 What is the Law of Conservation of Mass?
Explain physical change and give an example.
Explain chemical change and give an example.

4. Bell ringer 10/16/13 What is the subscript? What are coefficients?
Which do you use to balance equations?

5. Bell ringer 10/17/13 Balance this equation.
____CH ___O2  ___CO2 + ___H2O

7. Law of Conservation of Mass
Matter is not created nor destroyed
Same amount of matter before and after a chemical reaction – it just changes form
So did the results of your reaction support this law? Why or why not?

8. REVIEW on Changes in Matter
Physical Change -A change that alters the form of a substance but not the chemical makeup of the substance, a change of state
Words like: crush, smash, tear, evaporate, slice, breakdown, dissolve, absorb, swell, burst
Chemical Change - One or more substances combine or decompose to form a chemically different substance
Words like: react, burns, forms, decomposed, rusting, sours, rotting, digesting, cooked, molecular change

9. What happens to a piece of charcoal as it burns?

10. stampedes  stampedes Matter & Its Changes
Physical Changes – Alters form or appearance but doesn’t change it into another substance
ie. Water evaporates into water vapor, a rock is broken into pieces
It’s like printing a word in a different font, it’s the same word just looks different!
stampedes  stampedes

11. made + steps  stampedes
Matter & Its Changes
Chemical change- changes the material into a new substance
i.e. hydrogen and oxygen combine to form water.
Chemical reactions take place when chemical bonds are either formed or broken.
Strong chemical bonds resist change: glass
Weak chemical bonds breakdown easily: wood
A chemical change is like scrambling letters to form new words
made + steps  stampedes

12. Observing Chemical Reactions
Chemical reactions produce new substances that can usually be detected by observing the evidence:
Color change
Precipitation
Temperature change
Property change
Gas produced

13. Bill Nye – Chemical Reactions

14. Chemical Reactions
Chemical reactions occur when chemical bonds are formed or broken
Strong chemical bonds resist change: glass
Weak chemical bonds breakdown easily: wood

15. Writing Chemical Reactions
Elements are represented by a one or two letter symbol
a. When symbol is a single letter: always capitalize: Hydrogen=H
b. When symbol is two letters, capitalize first letter & lower case second letter: Sodium = Na

16. Chemical formulas show the ratio of elements found in molecules and compounds
a. Subscript numbers designate how many atoms of each element are present:
H2O2 ; 2 Hydrogen atoms and 2 Oxygen atoms are present in this molecule
b. When no subscript number is shown: it is understood that there is only one atom present:
H2O = 2 Hydrogen atoms and only one Oxygen atom are present in this molecule
C2H6
H2O
C2H4
C2H2

17. Structure of a Chemical Equation:
Law of Conservation of Mass - Matter cannot be created nor destroyed
so there must be the same number of atoms on each side of the equation
Beginning materials are reactants
Ending materials are products
Example of Chemical reaction:
Reactant + Reactant  Product + Product

18. Structure of an Equation:
yields

19. Counting Atoms in an Equation
If no subscript present it is assumed to be 1 atom
If elements in brackets or parenthesis, treat same as in math.
Coefficients multiply the entire molecule atoms
You must add all reactant molecules together & compare w/ all molecules in the products
CaCl2
Ca=1
Cl=2
Ca3(PO4)2
Ca=3
P=2
O=8
2Ca3(PO4)2
Ca=6
P=4
O=16
It’s best to list the # of atoms under the molecules as we are doing in these examples

20. Balancing Equations Remember matter cannot be created or destroyed
Therefore the # of reactant atoms must equal the # of product atoms
This reaction IS NOT balanced
4 Reactant Hydrogens = 4 Product Hydrogens
2 Reactant Oxygens = 2 Product Oxygens
This reaction is balanced!

21. Balance this…
Step 1: Count the atoms on both sides of the equation & compare
** you can only add or change whole # coefficients to balance equations. Never change subscripts
Step 2: apply a coefficient to a molecule to balance an “easy” atom (in this case, we can add a 2 coefficient to the product water to balance the H’s)
Step 3: Recount

22. Balance this… (continued)
Step 4: apply a coefficient to a molecule to balance the next off balanced atom (in this case, the oxygens are not yet balanced. We can add a coefficient of 2 in front of the reactant oxygen to correct this)
Step 5: Recount
Step 6: Continue doing this until all atoms are balanced
BALANCED!!

23. Balancing Equations Use coefficients to balance the equation.
Coefficients: whole numbers placed in front of symbols

24. Rules for Balancing Equations
Count number of atoms of each element in the reactants and products. Use a reactant and product chart to help keep everything straight.
Hint: A polyatomic ion appearing unchanged on both sides of the equation can be counted as a single unit.

25. Rules continued
Balance the elements one at a time using coefficients. DO NOT CHANGE SUBSCRIPTS!!!
Make sure all elements are balanced
Make sure all the coefficients are in the lowest possible ratio.

26. ____CH4 + ___O2  ___CO2 + ___H2O
Examples:
____CH ___O2  ___CO2 + ___H2O
Reactants
Products C H O

27. ___NaCl + ___F2  ___NaF + ___Cl2
Reactants
Products Na Cl F

28. ___ Ag2O  __Ag + ___O2

29. ____C3H8 + __O2  __CO2 + __H2O

30. Please remember: Keep trying – a lot just trial and error
If the formulas for your compounds are incorrect, you will not be able to balance the equation correctly.
Pg 195: 1-3

31. Video clip on Balancing Chemical Equations
This is good if you missed my lecture!

32. Let’s try this Interactive Website for Balancing Equations!
Let’s try the EASY ones!
You’ll do the intermediate and hard ones in groups!

33. Balancing Chemical Equations Tutorial – Groups at computers
Click on the link below and choose the intermediate option and select to balance 5 equations
Now, try the advanced option and select to balance 5 more!

34. Balancing Chemical Equations Tutorial
Now try this link:
Make sure you read the directions and hints on each slide.
Attempt to balance 5 equations…just go in order from #1 to #5
Everyone must write their own equations!

35. Bell ringer 10/23/13
Write and balance the following chemical equations
H2O H2O +O2
Mg + HCl H2 + MgCl2

36. Classifying Chemical Reactions
Synthesis
Decomposition
Combustion
Single Replacement
Double Replacement

37. Classifying Chemical Reactions
Synthesis: When two or more substances combine to form a more complex substance
2H2 + O2  2H2O

38. Combination / Synthesis
Types of Reactions
Combination / Synthesis
starts with two or more reactants. can be elements or compounds
ends with single product. compound
A + B  AB
3H2 + N2  2NH3

39. Synthesis Reactions

40. Classifying Chemical Reactions
Decomposition: When a complex substance is broken into two or more simpler substances:
2H2O  2H2 + O2

41. Types of Reactions Decomposition starts with single reactant. compound
ends with two or more products. can be elements or compounds
AB  A B
2HgO  2Hg + O2

42. Decomposition Reactions

43. Classifying Chemical Reactions
Replacement: When one element replaces another or when two elements in different compounds change places:
2CuO + C  2Cu + CO2

44. Replacement Reactions
2 types:
Single Replacement
Double Replacement

45. Types of Reactions Single Replacement
starts with two reactants – one is an element, one is a compound
ends with two products – once again, one is an element, one is a compound
the uncombined element takes the place of the combined element in the compound.
Mg + 2HCl → MgCl2 + H2

46. Types of Reactions Double Replacement
occurs between two ionic compounds
compounds must be in aqueous solution
reactants and products change partners; positive ions of two reacting compounds are switched
CaCl AgNO3 → Ca(NO3) AgCl

47. Types of Reactions Combustion element or compound reacts with oxygen
usually produce energy as heat and light

48. Types of Reactions Combustion
if reactant is a hydrocarbon (contains only carbon, hydrogen, and possibly oxygen), products are always CO2 and H2O
CH4 + O2  CO2 + H2O

49. Types of Reactions Combustion
many combustion rxns are actually a special type of synthesis rxn.
element or compound combines with oxygen to form single product.
2Mg + O2 → 2MgO

50. Video Clip on types of reactions

51. Foldable
Fold paper in half hotdog style then in half hamburger style and in half again
So that you have 4 sections. Cut each section to the midline to make 4 flaps.
Classifying
Synthesis
Chemical
Decomposition
Reactions
Replacement
Single and Double
Combustion
Inside put the definition and an example from your notes or the book.

52. Group Activity
In your groups, you need to design a skit that will show your classmates you can differentiate between the types of chemical reactions:
Synthesis
Decomposition
Single Replacement
Double Replacement
Use paper or note cards, markers, and people in your skit!

53. Another video clip to summarize types of chemical reactions!!!

54. Cute Video on types of rxns

55. Classifying Chemical Reactions Tutorial
Please do the quiz and the worksheet that are at the bottom of the page!

56. List what type the following reactions are:
1)  NaOH + KNO3 --> NaNO3 + KOH
double replacement
2)  CH4 + 2 O2 --> CO2 + 2 H2O
combustion
3)  2 Fe + 6 NaBr --> 2 FeBr3 + 6 Na
single replacement
4)  CaSO4 + Mg(OH)2 --> Ca(OH)2 + MgSO4
5)  Pb + O2 --> PbO2
synthesis
6)  Na2CO3 --> Na2O + CO2
decomposition

57. Controlling Chemical Reactions
Every chemical reaction involves a change in energy.
Some reactions release energy in the form of heat (exothermic)
Ex: potato baking in the oven
Some reactions absorb energy & the container holding the reaction gets colder to the touch (endothermic)
Ex: after potato is removed from the oven, it loses heat to the surroundings as it cools on a plate.

58. Getting Reactions Started
The activation energy is the energy needed by a system to initiate the reaction.
It is the minimum energy needed for a specific chemical reaction to occur.
Once achieved, the reaction continues until reactants are extinguished.

59. Reaction Rates Rate at which reactants change into products over time
Factors that affect it:
Temperature
Surface area (grain explosion p.213)
Stirring
Concentration (dye solution p.214)
Catalysts- speed up reactions

60. Equilibrium
A state in which the forward and reverse reactions take place at the same rate.
A physical change does not go to completion and the reaction goes back and forth equally
H20 (l) H20 (g) (liquid water in equilibrium with water vapor)
Factors affecting Equilibrium:
Temperature
Pressure
Concentration

61. States of Matter Symbols
Solid
(l)
Liquid
(g)
Gas
(aq)
Aqueous (in solution of water)

62. Chemical Equations Practice
See Phy Science File: Chemical Reactions