Resources for Class #2
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Once More For EmphasisOngoing Assignment: Chemistry Everywhere
Ask questions. If you are curious about something chemical in the world around you, please let me know, by stating your question in the form at the bottom of this and every course page. I'll try to include it at an appropriate time in the course. Chemicals and chemical processes are all around you: in the kitchen, the garden, and the garage; in the medicine cabinet and the cleaning closet; in your car's gas tank, engine, and exhaust; in lights, solar collectors, and all kinds of electronic devices; in your body; and all over the night sky. If you want to understand your world better, look for things chemical and ask me about them.
Ask questions. If you are curious about something chemical in the world around you, please let me know, by stating your question in the form at the bottom of this and every course page. I'll try to include it at an appropriate time in the course. Chemicals and chemical processes are all around you: in the kitchen, the garden, and the garage; in the medicine cabinet and the cleaning closet; in your car's gas tank, engine, and exhaust; in lights, solar collectors, and all kinds of electronic devices; in your body; and all over the night sky. If you want to understand your world better, look for things chemical and ask me about them.
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Preparing for Class
Read or watch these pages:
Cloud Chamber (again -- this time we'll talk about it)
An Important Relationship: Mass (easy to measure) versus Numbers of Atoms or Molecules (not so easy).
My first examples in class will be simpler than Hank Green's first examples.
It is inevitable that a new subject entails many terms that are new to you, especially if you have not studied chemistry before or for a long time. To learn more about each subject, look it up at Wikipedia, where the introductory paragraph or two on the subject will usually be all you need to read in order to help you with this class. But if you want to know more, Wikipedia can also take you very deeply into each subject. Many Wikipedia entries start with the simple, and progress to the more complex, so read as far as the material makes sense to you.
• matter
• chemical substance (also called pure substance)
• mixtures, homogeneous and heterogeneous
• atom and element (why pair these words?)
• molecule and compound (why pair these words?)
• phases of matter: solid, liquid, gas
• science -- Do you know what science is?
• chemical substance (also called pure substance)
• mixtures, homogeneous and heterogeneous
• atom and element (why pair these words?)
• molecule and compound (why pair these words?)
• phases of matter: solid, liquid, gas
• science -- Do you know what science is?
In the second class, I will introduce the periodic table briefly (using the video linked above), and then start from scratch with some of the most basic principles of chemistry. We will start building a foundation on which you can build your understanding of chemistry in the world around you. I will review elements (and their components, atoms) and compounds (and their components, molecules), and I will take up the subject of chemical change: reactions.
Chemical reactions are described by chemical equations, and we will talk about just what those equations mean, and how chemists use them to guide their laboratory chemistry. I plan eventually to talk about the reaction of water with oxygen to produce water, reactions involved in leavening during baking, and the class of common reactions called combustion, for example the burning of a candle.
In class, I will explain all of the symbols in the equations that follow. The letters in parentheses indicate the phase of the substance in the reaction: s, l, and g stand for solid, liquid, and gas.
1) Oxygen and hydrogen can react to form water:
Chemical reactions are described by chemical equations, and we will talk about just what those equations mean, and how chemists use them to guide their laboratory chemistry. I plan eventually to talk about the reaction of water with oxygen to produce water, reactions involved in leavening during baking, and the class of common reactions called combustion, for example the burning of a candle.
In class, I will explain all of the symbols in the equations that follow. The letters in parentheses indicate the phase of the substance in the reaction: s, l, and g stand for solid, liquid, and gas.
1) Oxygen and hydrogen can react to form water:
2 H2 (g) + 1 O2 (g) —> 2 H2O (l)
This process is an example of a combination reaction; the reactants simply combine. The hydrogen and oxygen in this reaction are its starting materials or reactants. On the right side of the reaction arrow are products. In words, this equation says that 2 molecules of hydrogen gas (H2) react with 1 molecule of oxygen gas (O2) to produce (-->) 2 molecules of liquid water (H2O).
The equation also tells us that for every 4 grams of hydrogen gas, we would need 32 g of oxygen gas to consume the hydrogen completely and turn it all to 36 grams of water. Can you confirm this from the information in the periodic table? Hint: add up the weights of all the atoms involved to get the weights of the molecules, and confirm that weights of reactants and products are in the ratio 4:32:36.
The mole is a common unit of quantity in chemistry. (Another example of a unit of quantity is the dozen). So the mole is a number, not a mass or volume. The value of that number is about a million billion billion, or 1 followed by 24 zeros. Just as a dozen large eggs weighs more than a dozen small eggs, a mole of oxygen weighs more than a mole of hydrogen. Oxygen atoms are bigger eggs than hydrogen atoms.
The mass in atomic units of H2 is 2 units. A million billion billion hydrogen molecules weigh 2 g. It's easier to say that a mole of H2 weighs 2 grams. The mass of a water molecule is 18 atomic units. So a mole of water weighs 18 grams. Why is this handy? Because a mole of any substance contains the same number of molecules as a mole of any other substance. In other words, there are the same number of H2 molecules in a mole of hydrogen gas as there are water molecules in 18 grams of water.
So the above equation says that 2 moles of hydrogen reacts with 1 mole of oxygen gas to give two moles of water. Mols are lab-sized units -- less than 100 grams for simple compounds. We can't count the atoms or molecules in a sample, but we can weigh them, and if we know their formulae, we can convert their weight to numbers of atoms or molecules.
Thinking back to Class 1, we looked at the nature of the phases of matter at the molecular level, and talked about the structure of water molecules and their arrangement in ice. We saw why snowflakes have their hexagonal symmetry.
The equation also tells us that for every 4 grams of hydrogen gas, we would need 32 g of oxygen gas to consume the hydrogen completely and turn it all to 36 grams of water. Can you confirm this from the information in the periodic table? Hint: add up the weights of all the atoms involved to get the weights of the molecules, and confirm that weights of reactants and products are in the ratio 4:32:36.
The mole is a common unit of quantity in chemistry. (Another example of a unit of quantity is the dozen). So the mole is a number, not a mass or volume. The value of that number is about a million billion billion, or 1 followed by 24 zeros. Just as a dozen large eggs weighs more than a dozen small eggs, a mole of oxygen weighs more than a mole of hydrogen. Oxygen atoms are bigger eggs than hydrogen atoms.
The mass in atomic units of H2 is 2 units. A million billion billion hydrogen molecules weigh 2 g. It's easier to say that a mole of H2 weighs 2 grams. The mass of a water molecule is 18 atomic units. So a mole of water weighs 18 grams. Why is this handy? Because a mole of any substance contains the same number of molecules as a mole of any other substance. In other words, there are the same number of H2 molecules in a mole of hydrogen gas as there are water molecules in 18 grams of water.
So the above equation says that 2 moles of hydrogen reacts with 1 mole of oxygen gas to give two moles of water. Mols are lab-sized units -- less than 100 grams for simple compounds. We can't count the atoms or molecules in a sample, but we can weigh them, and if we know their formulae, we can convert their weight to numbers of atoms or molecules.
Thinking back to Class 1, we looked at the nature of the phases of matter at the molecular level, and talked about the structure of water molecules and their arrangement in ice. We saw why snowflakes have their hexagonal symmetry.
Questions to Think About
• Do you know why it's called a cloud chamber?
• In the cloud-chamber video, can you spot any clouds that do not appear to come from the rod?
• What is an element? What is a compound?
• What three forms -- or phases -- of water do you encounter frequently?
• What does density mean?
• Think about the arrangement of water molecules in ice; can you infer why ice floats in liquid water?
• In your kitchen, look for examples of pure substances. Look for examples of homogeneous mixtures and heterogeneous mixtures.
More Resources (optional)
• Recognizing chemical changes, which produce new substances (as opposed to physical changes, such as the melting of a ice to form liquid water)
• Build your own cylindrical periodic table. Building instructions are in section 12 of this very long page about periodic relationships. If you are building it, the figures at the top of the article will help you see how you are doing.
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Other recommended reading
Poem: The Abacus and the Rose
In this poem, what is "the spring"?
Is there more than one sensible answer to this question?
What do I mean by "sensible"?
