1. Chemical reactions: particles & energy
Unit 5
Chemical reactions: particles & energy

2. Bell ringer 1/30 What is a chemical reaction?
a process in which one or more substances change into new substance(s) having different physical and chemical properties.
How molecules separate and atoms combine - LR
What is the evidence for a chemical reaction?
Energy Changes: heat, light, sound or electrical energy changes.
New Substances: gas, solid, liquid, water, color changes, odor changes.

3. 6.Remove the nails. Rinse or scrape the precipitate (copper metal) from the nails into your labeled 250 mL beaker. Place the nails in a labeled small beaker. Note the appearance of the nails. Place this beaker on tray labeled nails.
7. Decant solution from the 250 mL beaker. Rinse the precipitate with about 25 mL of distilled water. Try to lose as little of the solid copper as you can when you decant. After a 2nd rinse with distilled water, rinse the copper with 25 mL of 1 M HCl. Rinse one last time with distilled water. Then place the labeled jar in drying oven.

4. Representing reactions in chemistry as an equation
Sodium metal burns in chlorine gas to form solid sodium chloride
Na (s) + Cl2 (g)  NaCl (s)
Reactants: starting substances (left).
Products: ending substances, what's produced (right).
Plus sign (+) means “reacts with.”
Arrow sign () means “yields.”

5. Chemical equation symbols
Coefficients: how many units of each substance are present (whole numbers written before reactants & products).
Subscripts: how many of each atom type are in a substance (subscripts in formulas).
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (water) solution
(ppt)= precipitate
Precipitate is a solid that suddenly appears when that phase was not initially present, i.e. two liquids are mixed and you get a solid.

6. Rearranging atoms activity
the rearrangement process of a chemical reaction requires that all atoms from the reactant molecules MUST become part of one of the products- conservation of mass
coefficients describe how many whole particles of each substance are either consumed or formed
subscripts describe the count of atoms in a substance;
reactions proceed by first breaking bonds between atoms in the reactants, and then forming new bonds between these atoms to make the products.

7. Rearranging atoms
Example
_____O2 + _____H2  _____ H2O

8. Unit 7n Reactions and Chemical Energy
Bell ringer 2/4
L Chedid 2011

9. Unit 7n Reactions and Chemical Energy
Nail Lab
Day 3
8. Mass the dry nails, then discard the nails.
9. Mass the jar + dry copper. Discard the copper in the designated waste container. Wash your jar and let dry.
L Chedid 2011

10. Nail Lab write up
Purpose
Data
Measurements
Mass jar
Mass nails before reaction
Mass nails after reaction
Mass jar + dry copper
The purpose is to determine the ratio of copper produced to iron consumed in a replacement reaction.

11. Nail Lab Day #3 Calculations:
1. Determine the mass of copper produced and the mass of iron used during the reaction.
2. Calculate the moles of copper and moles of iron involved in the reaction.
3.Determine the ratio moles of copper.
moles of iron
Express this ratio as an integer. For example, a ratio of 1.33 can be expressed as 4/3; 0.67 can be expressed as 2/3 , etc.

12. Nail Lab Day #3 Conclusion:
1. Why did the reaction stop? Which reactant was used up? How do you know?
2.Describe what was happening to the atoms of iron and copper during the reaction. What is this type of reaction called?
3. What would happen to the ratio of copper to iron if you had placed more nails in the beaker? If you let the reaction go for less time?
4.What is the accepted ratio of copper atoms to iron atoms in this reaction? Account for differences between your experimental value and the accepted value.
Write the balanced equation for the reaction.

13. Nail Lab –WB Be prepared to:
Present your results from the lab and discuss how changes could be represented symbolically. EQUATION
To help visualize the process, represent the changes using different color circles PARTICLE PICTURE
Verbalize what is happening to the atoms of each substance during the reaction.

14. Write the word equations below as chemical equations and balance:
1) Zinc and lead (II) nitrate react to form zinc nitrate and lead.
2) Aluminum bromide and chlorine gas react to form aluminum chloride and bromine gas.
3) Sodium phosphate and calcium chloride react to form calcium phosphate and sodium chloride.

15. BELL RINGER 2/10 1. Predict the product(s) of the following, if any
a. Ca + HOH 
b. Ca(OH)2 
c. Ca + Br2 
d. Fe + Cu(SO4)2 

16. Reaction Types Lab Combination (synthesis) Decomposition
Single replacement
Double replacement
Combustion

17. A. Combination reactions:
1. Grasp a strip of magnesium ribbon in crucible tongs and ignite it in the burner flame. Hold it over a watch glass. Do not look directly at the flame! Add a few drops of distilled H2O to the ash. Stir with a stirring rod and place a drop of the solution on red litmus paper. Red litmus turning blue is evidence for the presence of a base.
2. Heat a piece of copper metal strongly in the Bunsen burner flame for about 30 s. Remove the copper from the flame and note the change in appearance. Discard the product in the solid waste can.

18. A. Combination reactions:
1a. Observations: 
1a. Mg O2  MgO
Write in words:
1b. Observations:
1b. MgO H2O  Mg(OH)2 (aq)
2. Observations:
2. Cu O2 --> CuO

19. B. Decomposition reactions:
1. Place about 1 scoopful of solid calcium carbonate CaCO3 into a dry test tube and heat strongly for 2 minutes. Toward the end of the heating, light a wood splint and insert the flaming splint into the mouth of the test tube. Note what happens to the splint.

20. B. Decomposition reactions
1. Observations:
1. CaCO3  CaO CO2 (g)
Write in words:

21. C. Single replacement reactions:
1. Place a strip of copper in a test tube with enough 0.1M AgNO3 to cover it. Set this test tube aside, then observe the surface of the metal after 5-10 minutes. 2. Place a couple of pieces of mossy zinc metal in a test tube approximately 1/4 full of 3M HCl. Place a stopper loosely in the tube. After a few minutes, light a wood splint and insert the flaming splint into the mouth of the test tube. Hold the test tube in your hand to feel if the temperature has changed.

22. C. Single replacement reactions
1. Observations:
1. AgNO3 (aq) Cu  Ag Cu(NO3)2 (aq)
Write in words:
2. Observations
2. Zn HCl(aq)  ZnCl2(aq) + H2(g) Write in words:

23. Bell ringer- Classify the following:
H2 + O2  H2O
BaCl2 + Br2  BaBr2 + Cl2
BaCl2  Ba + Cl2

24. D. Double replacement reactions:
1. Add 0.1M AgNO3 to a test tube to a depth of about 1 cm. Add a similar quantity of 0.1M CaCl2 solution. Observe the reaction.
2. Place a scoopful of solid Na2CO3 in a test tube to a depth of about 1 cm. Add a dropperful of 3M HCl. While the reaction is occurring, test with a flaming splint as in part B.Check to see if the temperature of the mixture has changed.

25. D. Double replacement reactions
Observations
1. AgNO3(aq) CaCl2(aq)  AgCl(s) Ca(NO3)2(aq)
Write in words:
2. Na2CO HCl (aq)  NaCl(aq) H2O + CO2(g)

26. E. Combustion reactions:
1. Place about 10 drops of isopropyl alcohol, C3H7OH, in a small evaporating dish. Ignite the alcohol from the top of the liquid with a Bunsen burner. Hold a cold watch glass well above the flame and observe the condensation of water on the bottom. The formation of the mist will be fleeting; watch closely.

27. E. Combustion reactions
Observations
1. C3H7OH(l) O2(g)  CO2(g) H2O(g)
Write in words:

28. Types of Reactions - 1 N2 + 3 H2  2 NH3
Synthesis (combination) - two or more reactants combine to form one product.
A + B  AB
A + 2 B  AB2
N2 + 3 H2  2 NH3

29. Types of Reactions - 2 2 KClO3  2 KCl + 3 O2
Decomposition - one reactant breaks into two or more products.
AB  A + B
2 A2B  4 A + B2
2 KClO3  2 KCl O2

30. Types of Reactions – 3
Single Replacement/Displacement - an element reacts with a compound, and part of the compound is replaced by the element.
(Metals replace metals, and nonmetals replace nonmetals.)
A + BC  AC + B
Cu + H2SO4  CuSO4 + H2(g)­

31. Types of Reactions – 4
Double Replacement/Displacement - one compound reacts with another compound, and they change partners. (Each cation goes with the opposite anion.)
AB + CD  AD + CB
CaCl2 + (NH4)2O  CaO NH4Cl

32. Types of Reactions – 5
Combustion - a substance reacts with oxygen, and everything ends as carbon dioxide or water.
CHx + O2 → CO2 + H2O
CH O2  CO H2O

33. Combustion
Combustion – Form of oxidation that releases a large amount of energy (HIGHLY EXOTHERMIC).
Three requirements for a combustion:
Fuel – reacts with oxidizing agent to release energy
Oxidizing agent – substance that causes a fuel to react (usually oxygen)
Ignition temperature – the required temperature needed to combust the fuel
Three types of combustions:
Rapid – Release a lot of energy in a short period of time
Spontaneous – Fuel ignites without the need for outside energy
Slow – Occurs of long period of time

34. Keeping Track of Energy During Chemical Change
A coherent way to treat energy in chemical reactions

35. Modeling Approach
Use energy bar diagrams to represent energy accounts at various stages of reaction
Provide mechanism for change
Reconnect kinetic and chemical potential energy
Focus on what is happening during the course of the reaction

36. Endothermic reaction
A system of reactants that absorbs heat from the surroundings in an endothermic reaction has a positive ΔQ, because the enthalpy of the products is higher than the enthalpy of the reactants of the system.
How do you know on which side to write the energy term?
If you had to supply energy to the reactants, the products store more energy
energy + 2H2O 2H2+ O2 (g)
If uncertain, use analogy from algebra
If 3 + y = x, which is greater, y or x?
Consistent with generalization that separated particles have more energy

37. Endothermic reaction
This is the standard energy diagram found in most texts.
But it doesn’t tell the whole story.

38. Endothermic Reaction +DH Endothermic Energy Reaction progress
Energy + Reactants  Products
Activation
Energy
Products
Energy
+DH Endothermic
Reactants
Reaction progress

39. Energy Bar Graphs- Endothermic
Show energy transfers between surroundings and system
Allow you to consider other energy accounts
energy + 2H2O 2H2+ O2 (g)

40. Endothermic Consider role of Ek
How does heating the reactants result in an increase in Ech?
Energy to rearrange atoms in molecules must come from collisions of molecules
Low energy collisions are unlikely to produce molecular rearrangement

41. Endothermic Heating system increases Ek
Hotter, faster molecules (surroundings) transfer energy to colder, slower molecules (system)
Now reactant molecules are sufficiently energetic to produce reaction
Note intermediate stage

42. Now reaction proceeds
During collisions, particles trade Ek for Ech as products are formed
After rearrangement, resulting particles move more slowly (lower Ek).

43. Consider all steps in process
1.Heating system increases Ek of reactant molecules 2.Energy is transferred from Ek to Ech now stored in new arrangement of atoms 3. Resulting system is cooler - requires continued heating to bring Ek back up to level required to sustain reaction energy + 2H2O 2H2+ O2 (g)

44. Exothermic Contrast Conventional Diagram
This is the standard energy diagram found in most texts.
But, again,it doesn’t tell much of the story.

45. Exothermic Reaction Reactants  Products + Energy Energy Reactants -DH
10 energy = energy energy
Energy of reactants
Energy of products
Energy
Reactants
-DH
Products
Reaction Progress

46. Exothermic reaction
 A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔQ by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system 
How do you know on which side to write the energy term?
If energy flows from system to surroundings, then the products must store less Ech than the reactants
CH4 + 2O2  CO2 + 2H2O + energy

47. Exothermic reaction
CH4 + 2O2  CO2 + 2H2O + energy

48. Exothermic reaction Now take into account changes in Ek
When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly
Note intermediate stage

49. Exothermic reaction
System is now hotter than surroundings; energy flows out of system until thermal equilibrium is re-established

50. Consider all steps in process
1. Decrease in Echresults in increased Ek 2. System is now hotter than surroundings 3. Energy is transferred from system to surroundings via heating

51. What about a spontaneous endothermic process?
When NH4Cl dissolves in water, the resulting solution gets much colder
What caused the Ek to decrease?
Some Ek of water required to separate ions in crystal lattice.
Resulting solution has greater Ech than before

52. Reaction useful for cold-packs
The system trades Ek for Ech
Eventually energy enters cooler system from warmer surroundings (you!)

53. Steps to drawing energy bar graphs
Write the balanced chemical equation with the energy term on the correct side of the equation.
Energy on right – exothermic – start with a high Ech and go from initial to heated final to final (low Ech) increase Ek
Energy on left – endothermic – start with a low Ech and go from initial to heated initial to final (high Ech) decrease Ek