Thermometers
The laboratory thermometers are different than thermometers you may be
used to using to take your temperature. The liquid in a laboratory
thermometer responds quickly to the surroundings, therefore, you should
NEVERshake down a
laboratory thermometer. When you measure the temperature of a
substance, always read the thermometer while it is immersed in the
substance. The entire bulb of the thermometer should be
immersed in order to get an accurate reading.
Chemical & Physical Changes
In any chemical change, one or more of the substances
are used up and one or more new substances are formed. Some chemical
changes can be detected by observing one or more of the following
occurrences: formation of gas (bubbles), formation of a solid
(precipitate), disappearance of
a solid (dissolving), change in color, change in pH (acidity) or if
heat
is given off or absorbed.
A physical change is different from a chemical change in that no change in chemical composition occurs. Some physical changes can be characterized by a change in state, change in size, tearing, breaking, or grinding. The original substance is still the same substance only in a new form.
Heats of Reaction
When heat is given off by a reaction, the temperature increases and the reaction is exothermic. When the temperature decreases, heat is absorbed and the reaction isendothermic . In an exothermic reaction, heat is considered a product and in an endothermic reaction, heat is a reactant. This can be shown in the balanced equation as seen in the examples below:
Exothermic Reaction: N 2(g) + 3H2(g) -----> 2NH3(g) + heat
Endothermic Reaction: H 2O(s) + heat ---> H2O (l)
Notice that the first example above involves a chemical change, but
the second example involves a physical change. So while the observence
of heat being given off or absorbed during a reaction is a good
indicator that
a chemical change has occured, heat is also associated with physical
changes.
Balancing Chemical Equations
The Law of Conservation of Matter states that matter is neither created
or destroyed. So in a chemical reaction, all of the atoms on the
reactant side must be equal to the same atoms on the product side of
the equation. When this occurs, the reaction equation is said to be a
"balanced equation". To balance an equation, we place numbers (called coefficients)
in front of the substances until the atoms on the reactants side of the
equation are equal to the products. Let's use the reaction below as an
example:
Step 1
N2(g) + H2(g) ---> NH3(g) (unbalanced) Reactants Products
N = 2 N = 1
H = 2 H = 3
Step 2
To balance the nitrogens, we place a 2 in front of the NH 3 and recount the number of each atom.
N2(g) + H2(g) ---> 2NH3(g) (unbalanced) Reactants Products
N = 2 N = 2
H = 2 H = 6
Step 3
To balance the hydrogens, we place a 3 in front of the H 2 and recount the number of each atom.
N2(g) + 3H2(g) ---> 2NH3(g) (balanced) Reactants Products
N = 2 N = 2
H = 6 H = 6
Now the number of each atom is equal in both the reactants and
products so the equation is balanced. You will be asked to balance the
equation for the reactions that you perform in lab today.
Laboratory Procedures
Thermometers
Obtain a laboratory thermometer from your instructor. Observe the
markings on the thermometer and answer the following questions:
What temperature scale is represented on the thermometer? __________
What is the highest temperature that can be measured using this thermometer? ________
What is the lowest temperature that can be measured using this thermometer? ________
The Bunsen Burner
Your instructor will demonstrate the proper technique for using a
Bunsen burner. Leave space in your notebook to take notes and to draw
a picture of a Bunsen burner. Label the gas adjustment, air
vent, bench-top gas source, blue inner flame, and the hottest part of
the flame.
Practice Makes Perfect...
1. The air vent, the gas adjustment, and the gas source should all be off.
2. Have your match ready to strike and open the gas source.
3. Strike your match and hold the flame to the top of the burner
while slowly opening the gas adjustment valve until your
burner is lit.
4. Slowly open the air vent. Opening the air vent too far may blow out the flame.
5. Adjust the air vent and gas adjustment until you have a flame
that is
3-5 cm high with two distinct parts, an inner cone
and an outer flame.
6. Close the air vent and gas adjustment valve and then turn off the gas source to extinguish the flame.
7. Repeat steps 1-6 until you are comfortable with this procedure.
Observations:
When the air vent is closed, what color is the flame? _______________
When the air vent is opened, what color is the flame?
______________
Reactions
A. Combustion of Magnesium
1. Obtain a small strip of magnesium ribbon and record its appearance.
2. Using a pair of tongs to hold the end of the magnesium strip, ignite the magnesium using a Bunsen burner. As soon as the magnesium catches fire, remove it from the flame and do not look directly at the strip as it burns. Record your observations of the reaction in the table below. What does the product look like compared to the reactant (the original strip of magnesium)? Use the following unbalanced equation to balance the equation for the reaction:
Mg(s) + O2(g) ---> MgO(s) (unbalanced)
Combustion of Magnesium Observations:
| Appearance of Magnesium Strip | ||
| Observation of Reaction | ||
| Appearance of Product | ||
| Balanced Reaction Equation: _____________________________________ | ||
| Did the magnesium undergo a chemical or physical change? _________________ | ||
B. Sublimation of Iodine
1. Your instructor will demonstrate this experiment to the class by placing a small amount of iodine crystals in a beaker and covering it with a watch glass cooled with ice. The beaker is then warmed on a hot plate in the hood. Record your observations in the table below.
Sublimation of Iodine Observations:
| Appearance of Iodine Crystals | |
| Observation of Reaction | |
| Appearance of Product | |
| Did the iodine undergo a chemical or physical change? _________________ | |
1. Obtain a small piece of copper wire. Record its appearance.
2. Place about 2 mL of 1 M AgNO3 in a small test tube. Record its appearance. Note: AgNO 3will temporarily turn your skin yellow, so avoid contact with skin.
3. Place the copper wire into the test tube containing the AgNO 3. Record your observations after approximately 5 minutes. Set this test tube aside and record your observations again at the end of the lab period.
4. Use the following unbalanced equation to balance the equation for the reaction:
Cu(s) + AgNO3(aq) ---> Cu(NO3 )2 (aq) + Ag(s) (unbalanced )
Reaction of Copper and Silver Nitrate Observations:
| Appearance of Copper Wire | ||
| Appearance of AgNO3 Solution | ||
| Observation of Reaction
After 5 Minutes |
||
| Observation of Reaction
At End of Lab Period |
||
| Balanced Reaction Equation: _________________________________________ | ||
|
Did the copper undergo a chemical or physical change? _________________ |
||
D. Reaction of Metals with Acid
1. Obtain 3 small test tubes and label them as A, B, and C. Place about 2 mL of 1 M HCl in each test tube. Caution : HCl is corrosive - handle carefully and avoid contact with skin.
2. Obtain a small piece of each of the following metals and note their appearance: copper (Cu), zinc (Zn), and magnesium (Mg).
3. Add the Cu to the acid in test tube A and observe the metal to determine if any bubbles of gas (H2) are given off from any of the pieces of metal. Record your observations. After the reaction is observed, carefully pour the acid into the sink with the water running. Rinse the remaining metal pieces with water and place them in the waste container provided by the instructor.
4. Repeat step 3 with Zn by adding the Zn sample to test tube B.
5. Repeat step 3 with Mg by adding the Mg sample to test tube C.
6. Complete the balanced equation for each sample that showed a chemical reaction. If no reaction occurred, cross out the products and write "NR" (no reaction).
A. Cu(s) + HCl(aq) ---> CuCl2(aq) + H 2 (g) (unbalanced)
B. Zn(s) + HCl(aq) ---> ZnCl2(aq) + H 2 (g) (unbalanced )
C. Mg(s) + HCl(aq) ---> MgCl2(aq) + H 2 (g) (unbalanced)
Reactions of Metals with Acid Observations
|
|
|
|
|
|
|
|||
|
|
|||
| Balanced Equations:
Copper: _____________________________________________________________________ Zinc: ________________________________________________________________________ |
|||
1. Label two test tubes A and B. Place 5 mL of distilled water in each test tube. Measure and record (to 0.1 degree Celsius) the temperature of the water in each test tube using a thermometer.
2. Add a small scoop of NH4NO 3 crystals to the water in test tube A. Stir and record the temperature of the solution.
3. Add a small scoop of anhydrous CaCl2 to the water in test tube B. Stir and record the temperature of the solution.
4. Using your initial and final temperature measurements, determine whether each reaction is exothermic or endothermic.
5. From the following reactions, balance the equations for the formation of each solution. Include the term heat on the side of the reactants if the reaction is endothermic and on the side of the products if the reaction is exothermic.
A. NH4NO3(s) ---> NH 4+(aq) + NO3 - (aq)(balanced )
B. CaCl2(s) ---> Ca2+ (aq) + Cl -(aq) (unbalanced )
Endothermic and Exothermic Reaction Observations
|
|
|
|
| Initial Temperature | ||
| Final Temperature | ||
| Change in Temperature
(final - initial = change) |
||
| Endothermic or Exothermic? | ||
| Balanced Equations:
NH4NO3 ____________________________________________________________________ CaCl2______________________________________________________________________ |
||
Which of the reactions in these lab activities involved a chemical change? What evidence do you have to support your answer in each case?
Which of the reactions in these lab activities involved a physical change? What evidence do you have to support your answer in each case?
How is a balanced equation related to the law of conservation of mass?
(updated 1-19-04 P Powers)