Experiment 7 Help

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Experiment 7: Preparation of Oxygen

Page 50: Objectives

1. Prepare oxygen by a decomposition reaction

2. Observe and record some chemical and physical properties of oxygen

3. Dissolve some oxides in water and check whether acids or bases are formed.

This lab is interesting because you are creating oxygen from a solid. The solid is KClO₃ (potassium chlorate). As KClO₃ is heated up it decomposes and O₂ is one of the products.

When you have a way to produce oxygen, there are a lot of possibilities. The first one to come to mind is to increase the rate of combustion. That is why potassium chlorate is used an an oxygen source in rockets and in fireworks. Potassium perchlorate (KClO₄) is also used.

About 47% of the Earth's crust is oxygen. So the mountains you see are about half oxygen. Silicon is the second most abundant element at 28% and aluminum is third at 8%. So 73% of the mountains and soil around you is made from oxygen combined with silicon (makes silica=SiO₂) and aluminum (aluminum oxide=Al₂O₃). It is very hard to drive oxygen away from the silicon and aluminum using heat, but there are other compounds such as the chlorates where heat can cause the oxygen to break loose.

Experiment 7: Page 52

A. Physical Properties of O₂
1. Does oxygen appear to be soluble in water?
This question is similar to lab 5 question D. You can review the help page for that. Here you bubble oxygen into an inverted container. The oxygen will displace the water in the container. If oxygen was real soluble in water, it would simply dissolve in the water and not form a gas that pushes the water out.
2. What observation tells you about the solubility of oxygen in water?
The observation is that oxygen bubbles pass through the water and remains a gas.

Experiment 7: Page 53
3. Stand a small candle on a glass plate and light it. Carefully invert a bottle of oxygen over the candle and record the time that the candle burns in your oxygen. Repeat the experiment using a bottle of air. Record the time that the candle burns.
The father of modern chemistry, Antoine, Lavoisier, did a similar experiment in late 1700's. Instead of a candle, he used a bird and recorded the time the bird lived in a container of pure oxygen compared with just air. He also did an experiment with a guinea pig and realized that respiration of living things was like the combustion of a candle (both produced heat and consumed oxygen).
Reasoning scientifically, would you conclude that all of the oxygen molecules have reacted when the candle goes out? Give a reason for your answer.
A candle can be put out by blowing on the flame. Blowing on it actually brings more oxygen to the flame, yet it goes out. That's because we are cooling the wax which was vaporizing and burning. So a candle wick has to remain hot enough to sustain combustion. We could reason that if the oxygen levels drop low enough, the flame will diminish and there won't be enough heat to maintain combustion. So not all oxygen will be consumed when the flame goes out would be most likely.

3. Hold a 2-5cm strip of magnesium ribbon in crucible tongs and ignite it by heating with the burner flame (Do not look at the burning magnesium because the light is too intense).
Not only is the light too intense, it is also producing ultraviolet light which is damaging to the eyes.

Experiment 7: Page 54
C. Behavior of Oxides plus Water
1. When you burned sulfur, a gas, sulfur dioxide, SO2, was formed by a combination reaction.
Write the correct formulas under the names:

Names	Sulfur	+	oxygen	->	sulfur dioxide
	S	+	O₂	->	SO2

This behavior is typical for oxides of non-metals. Complete the general equation:
Nonmetal oxides + Water --> Oxyacids (also called oxoacids)
The equation for sulfur dioxide and water is:
SO₂ + H₂O --> H₂SO₃ (which is sulfurous acid). So the water provides some hydrogen atoms that easily come off making the solution acidic.
This behavior is what gave oxygen its name. Lavoisier (mentioned above) discovered that non-metal compounds of oxygen such as nitrogen oxides, phosphorus oxides, and sulfur oxides would turn into acids when in mixed with water. For example, nitric acid, phosphoric acid, sulfurous acid, and sulfuric acid would get generated. "Oxy" meant acidic or sharp (like the sharp or sour taste of vinegar). Oxygen means a generator of acids. So that's how oxygen got its name.

1. The oxide from burning magnesium was a powdery white solid. Write the formulas for the magnesium metal combining with elemental oxygen to form magnesium oxide under the names.

Names	Magnesium	+	oxygen	->	magnesium oxide
	Mg	+	O₂	->	MgO

Note the formula is MgO because Mg always has a 2+ charge (Mg²⁺) and oxygen always has a 2- charge (O^2-), so their charges balance with one each.
Complete the general equation:
Metal oxides + Water --> Metal hydroxides (also called alkaline hydroxides)
Here's the equation for MgO and water:
MgO + H₂O --> Mg(OH)₂
Some of the Mg(OH₂) dissolves in water to make Mg²⁺ and 2OH^-. It's the OH^- that the litmus paper reacts to, turning it blue (alkaline).

Experiment 7: Page 55 (skipping questions 1, 2, 3)
4. Circle the compounds that are oxides:

C₂H₅OH The oxygen here is part of the OH group called a hydroxyl functional group. Alcohols have that group.
SiO₂ Yes, this is silicon dioxide (silica mentioned above), so it is an oxide.
Li₂CO₃ Here you need to recognize that CO₃ is the polyatomic ion known as carbonate. So oxygen is attached to the carbon and not the lithium. Li₂O would be an oxide but Li₂CO₃ is a carbonate.
N₂O₅ If you name this you would say "dinitrogen pentoxide" so yes this is an example of a non-metal oxide which forms an oxyacid when reacting with water. It actually produces two nitric acid molecules.
N₂O₅ + H₂O --> 2HNO₃
KClO₃ Here you need to recognize that ClO₃ is a polyatomic ion. So the oxygen is grouped with the chlorine making its behavior that of a chlorate not an oxide. K₂O would be an oxide but this is a chlorate.
HNO₃ Here again you need to recognize O₃ is not alone, it is grouped with nitrogen to make a nitrate (NO₃).
H₂O If you give water its strict chemistry name, you get dihydrogen monoxide. So this is an oxide. Using the name "dihydrogen monoxide" (also referred to as DHMO) was done to cover up the fact that one is referring to water. So it has been used as a prank on many people. Check out this website: http://www.dhmo.org/ and this article at Snopes: http://www.snopes.com/science/dhmo.asp

5. Is combustion a physical or chemical change? This should be easy because we all know that combustion is causing oxygen to combine with what is burning. So it is a chemical change.

Define combustion in terms of reactants and products.

6. Elements may combine under different conditions to form different compounds. At "room temperature", without heating, iron (Fe) combines with oxygen (O₂) to form rust (Fe₂O₃). Water is a catalyst for this reaction, increasing the rate of formation of rust. What is the scientific name for rust?
Our first stab at the name might be "iron oxide". For metal oxides that only have one oxidation state that would be OK, but for metals like iron that have different oxidation states, that isn't enough. We have to indicate the charge (oxidation state) on the iron. In Fe₂O₃ we know that oxygen always has a 2- charge. Since we have O₃, the oxygen atoms have a total of a negative six charge. Since we see two iron atoms, we know that each of them must have a plus three charge (Fe³⁺)in order to balance with the negative 6 charge of the oxygen atoms. So our name must indicate the plus 3 charge on the iron. We do it with Roman Numerals. So what is the correct name?.
Write a word equation and underneath, write the correct formulas for each term.
WORD EQUATION:
water
Iron + oxygen ---> ???
FORMULAS:
H₂O
4Fe + 3O₂ ---> 2???

The catalyst is written above the arrow.

7. Look at the periodic table to classify each element as a metal or non-metal, and predict if a base or an acid would be formed when its oxide reacts with water.

	Name of element	Metal or non-metal	Oxide + water gives
K	potassium	metal	??? [K₂O+H₂O->2KOH]
P	phosphorus	non-metal	??? [PO₃+H₂O->HPO₄]
N	nitrogen	non-metal	??? [2NO+H₂O->2HNO₃]
Ca	calcium	metal	??? [CaO+H₂O->Ca(OH)₂]

You don't need to include the equations I wrote in brackets. I just put them there so you can see how the acid or base got formed.

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