Chemistry STSE Connections!
Today’s Goals:
· Read three mini-articles about chemical reactions
· Answer questions on lined paper in FULL sentences, using proper terminology, spelling and grammar
· Hand in work when completed. It will be assessed for Application & Communication marks
How does rust work?
Rust is the common name for a very common compound, iron(III)oxide. Iron oxide, the chemical Fe2O3, is common because iron combines very readily with oxygen -- so readily, in fact, that pure iron is only rarely found in nature. Iron (or steel) rusting is an example of corrosion -- an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust.
For iron to become iron oxide, three things are required: iron, water and oxygen. Here's what happens when the three get together:
When a drop of water hits an iron object, two things begin to happen almost immediately. First, the water, a good electrolyte, combines with carbon dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen. The free oxygen and dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the piece of iron itself.
The chemical compounds found in liquids like acid rain, seawater and the salt-loaded spray from snow-belt roads make them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron and other forms of corrosion on other metals.
- What do you think the (III) means in iron(III)oxide? (Look at the PT in your textbook…)
- Explain what corrosion is.
- Summarize how iron(III)oxide is formed.
How do artificial flavours work?
Many of the processed foods that you buy today come with an ingredient label that lists "artificial flavors" as one of the key ingredients. Artificial flavors are simply chemical mixtures that mimic a natural flavor in some way.
Anything that we smell has to contain some sort of volatile chemical -- a chemical that evaporates and enters a person's nose. The evaporated chemical comes in contact with sensory cells in the nose and activates them. In the case of taste, a chemical has to activate the taste buds. Taste is a fairly crude sense -- there are only four values that your tongue can sense (sweet, salty, sour, bitter) -- while the nose can sense thousands of different odors. Therefore most artificial flavors have both taste and smell components.
Any natural flavor is normally quite complex, with dozens or hundreds of chemicals interacting to create the taste/smell. But it turns out that many flavors -- particularly fruit flavors -- have just one or a few dominant chemical components that carry the bulk of the taste/smell signal. Many of these chemicals are called esters. For example, the ester called octyl acetate (CH3COOC8H17) is a fundamental component in orange flavor. The ester called isoamyl acetate (CH3COOC5H11) is a fundamental component of banana flavor. If you add these esters to a product, the product will taste, to some degree, like orange or banana. To make more realistic flavors you add other chemicals in the correct proportions to get closer and closer to the real thing. You can do that by trial and error or by chemical analysis of the real thing.
There are hundreds of chemicals known to be flavoring agents. It's interesting that they are normally mixed to create "known" tastes. People make artificial grape, cherry, orange, banana, apple, etc. flavors, but it is very rare to mix up something that no one has ever tasted before. But it can and does happen occasionally -- take Juicy Fruit gum as an example!
1. Why must something that smells have volatile chemical compounds in them?
2. What chemical group makes artificial flavourings? Give some examples.
Why do newspapers turn yellow over time?
Paper is made from wood, which is made up mainly of white cellulose. Wood also has a lot of a dark substance in it called lignin, which ends up in the paper, too, along with the cellulose. The exposure of lignin to air and sunlight is what turns paper yellow.
Lignin makes wood stiff and trees stand upright. It eventually turns paper yellow because of oxidation.
Lignin makes wood stiff and trees stand upright. You could say it acts as a glue to bind the cellulose fibers together. It is a polymer, a substance that is formed by the joining of simpler molecules into giant molecules that act differently than the smaller molecules did. Dr. Hou-Min Chang, a professor of wood and paper science at N.C. State University in Raleigh, N.C., compares lignin to the concrete used in buildings, with cellulose as the steel frame. Without lignin, Chang says, a tree could only grow to be about 6 feet tall. Lignin also helps protect the wood from pests and other damage.
Newsprint, which must be produced as economically as possible, has more lignin in it than finer papers. At the mill, the wood that will be turned into newsprint is ground up, lignin and all.
Paper manufacturers utilize the benefits of lignin in some types of paper, though. Brown kraft paper, the dark brown paper used in grocery store bags, and cardboard are stiff and sturdy because they have more lignin in them, and because those kinds of paper aren't treated with bleaching chemicals. It doesn't matter how dark they are because the printing on them is limited.
To make a fine white paper, the mill puts the wood through a chemical solvent process, which separates and discards the lignin. Pure cellulose is white, and the paper made from it will be white and will resist yellowing.
Lignin eventually turns paper yellow because of oxidation. That is, the lignin molecules, when exposed to oxygen in the air, begin to change and become less stable. The lignin will absorb more light, giving off a darker color. Chang says that if newsprint were kept completely out of sunlight and air, it would remain white. After only a few hours of sunlight and oxygen, however, it will start to change color.
1. What is lignin?
2. Explain the difference between brown paper and white paper.
3. Why does newspaper turn brown?