Lecture 5

 Watch Faraday's Lecture 5, Respiration and the Burning of a Candle

• Chemical Equations in Faraday's Lecture 5

    Taking carbon dioxide "asunder" (separating its carbon from its oxygen):

    Respiration (in short); 

    (see the bottom of this page to glimpse the full complexity of respiration of sugars)

Read or Watch

• A Broad Overview of Chemistry

What do chemists do? What are the various fields of chemistry?

The following video has a few minor errors (listed in the text below the video at YouTube), but it gives a very concise and clear overview of the ideas and activities that constitute chemistry. I thought it might be more effective now,  near the end of this course, after you had encountered many basic terms and concepts of chemistry.

I hope it stimulates questions!

The central tenet of chemistry is that function comes from structure.
 
In chemical terms, molecular structure gives rise to function. Molecular structure is the basis of the properties of substances - their colors, textures, shapes, and tendencies to change. As we learn to make these connections, we are drawn into a world of imagination that enriches the world we see. We study chemistry because many interesting and important things we see -- the intricacy of a snow flake, the symptoms of an cancer sufferer -- are caused by things we cannot see, but must understand.

Structure-function relationships are the heart and soul of chemistry.

••••••

Some of the basic operations of practicing chemists are

1. purification: the separation of mixtures into "pure"* substances
2. analysis: detecting and identifying substances (qualitative analysis), and measuring their quantities (quantitative analysis)
3. structure determination: composing and testing structural models that fit all available data on a substance, in order to learn its molecular structure
4. synthesis: making complex molecules of defined structure using reactions between simpler substances
5. studying reactions, particularly their rates (kinetics) and energy changes (thermodynamics), in order to  establish relationships between structure and reactivity.

These operations are interdependent. For example, synthesis entails analysis to find out whether the desired product was obtained, and what impurities it contains. The findings from these activities then will guide attempts to purify the product. With the product free of other substances, the chemist can study its reactions and be assured that its measured properties are those of the desired product and not of impurities.

Which of these operations have we encountered in this course?

••••••

* What Does Pure Mean?

No substance is completely pure.

Once more, for emphasis: no substance is completely pure.

In practice, a pure substance is one whose level of impurities is too low to have a detectable effect on what you are using the substance to do. The question is never whether there are impurities in a natural or synthetic substance. The question is how much. A liter of "pure" water, with impurities at less than one part per trillion (only 1 part in 10¹² !) could still harbor trillions of impurity molecules in each liter. We are surrounded by impurities. There are likely to be atoms of every element in any sample taken on the earth, though most are at undetectable levels. There are likely to be some atoms of radioactive plutonium in our classroom.

But come to class anyway.

As scientists develop the technology to detect ever smaller numbers of atoms and molecules, it becomes more important to determine what level of a specific impurity actually is significant, because all imaginable impurities are probably present in some minuscule amount, but perhaps of no significance to the chemical goal (or safety concern) at hand.

• Read these poems

Telescope, Louise Glück

When I Heard the Learn'd Astronomer, Walt Whitman

Question: Why do you think I used poetry in this course?

Submit Your questions using the instructions at the bottom of this page.
Your questions and comments help me to keep the course at your level.

Additional Resources (optional)

Commentary on Lecture 5
by the producers of the video series

• The Reactions in a Candle Flame (greatly simplified)

(Remember: As you read DOWN this chart, you are traveling UP the candle flame.)

• Respiration in All Its Glory

    First, the sugar glucose is transformed to pyruvate by a process called glycolysis:

    Then pyruvate is oxidized to carbon dioxide in a cyclic process called the citric acid cycle:

Each step in these two processes is catalyzed (promoted, without heating) by enzymes, whose names are shown either in a list (glycolysis) or with the reaction arrows for each step (citric acid cycle). 

As you can see, metabolism is much more complicated than combustion !! However, the results are the same, the carbons of an organic compound (glucose in this case) are oxidized to carbon dioxide.

So why don't living organisms just burn sugar and turn it into carbon dioxide all at once?