36 - Combustion and Photosynthesis (w/ Kathy Yeager!)
36. Combustion and Photosynthesis
Chemical reactions define the world around us with invisible elements and symbols. While atoms can neither be created nor destroyed, the identities of the compounds they make up are constantly changing. What is a chemical reaction? How can it proceed? What is a catalyst? Let’s learn to be scientifically conversational.
General Learning Concepts
1) What is the basic information needed to understand chemical reactions?
i. The Players: Reactants (chemicals or substances that start a chemical reaction), products (chemicals or substances that are produced by the reaction). There is also an arrow that shows the direction in which the reaction occurs instead of an equal’s sign.
ii. Chemical bonds: The Law of conservation of mass says that "Atoms are neither created, nor destroyed, during any chemical reaction." Thus, the same collection of atoms is present after a reaction as before the reaction. The changes that occur during a reaction just involve the rearrangement of atoms. Chemical reactions can do many things, including break chemicals apart and put them together. Some reactions have very tight chemical bonds called covalent bonds that require a terrific amount of energy to break or form, while some are formed from weaker interactions that require less energy.
iii. Energy: Contained within chemical bonds is energy; breaking apart chemical bonds requires energy while forming them often releases energy. In short, chemical bonds contain energy.
b. Potential energy diagram: The lower the potential energy of the system, the more stable it is: for example, if you wanted to play Jenga, would you rather do it with a shaking hand or a steady one? Similarly, chemical reactions can be more stable one they happened. 
i. Catalyst: A chemical or substance that speeds up chemical reactions. Acts by lowering the activation energy required for a chemical reaction. Enzymes, biological catalysts, do this same thing in nature.
2) Chemical Reaction Examples
a. Non-science: Butter + sugar + eggs + flour + chocolate chips à (375 degrees Fahrenheit for eight to ten minutes) chocolate chip cookies. This is a permanent, irreversible reaction: there will never be a way to properly separate out the starting ingredients (therefore, there is not much of a backwards reaction). This reaction is catalyzed; it is unlikely that it would happen in a proper timeframe unless if the environment properly simulated the conditions of the oven.
b. Chemistry: HCH3CO2 (vinegar) + NaHCO3 (sodium bicarbonate) à CH3CO2Na + H2O + CO2 , a reaction commonly associated with making a “baking soda volcano”. This acid-base reaction allows for a transfer of a hydronium ion (proton) and generates sodium acetate, water, and carbon dioxide. The carbon dioxide is the gas that you see pushing out the other liquid products. Even if you were to capture all of the CO2 released from the reaction, you cannot reverse this reaction. 
c. Combustion: Suppose we’re discussing a common car engine. Acting as fuel, octane (a long, greasy carbon chain) is burned in the presence of oxygen to form carbon dioxide and water. The catalyst in this case is a sufficient way to start the reaction: a sitting bottle of octane will not naturally burst into flames until initiated by a spark, provided by the engine. This reaction is definitely not reversible, but it does produce plenty of heat. This heat and small explosion push a piston in each cylinder of the engine which starts a chain reaction of engineering for movement, while the gases and water vapor produced leave the car through the tailpipe.
d. Photosynthesis: While combustion isn’t readily reversible from its own reaction, there are other reactions that are (essentially) the reverse reaction. Photosynthesis tends to be thought of as the inverse reaction, using carbon dioxide and water to form glucose (a sugar) and oxygen. This reaction also requires a catalyst; specifically, the capability of doing photosynthesis. Plants and some bacteria are capable of taking light energy to convert inorganic carbon into organic carbon while simultaneously producing oxygen. While the energy source is different in combustion, sugars can be broken down and made into fats by other enzymes.
3) Fun Tidbits
a. Inorganic versus organic chemistry: While organic chemistry is traditionally thought to the study of carbon, inorganic chemistry also has a variety of important carbon containing molecules. To differentiate between the two, often organic chemistry is explained as “hydrocarbon containing compounds”, which are often associated with life: carbon, oxygen, nitrogen, hydrogen, and choice others. Inorganic carbon is often found in the catalysts that were mentioned previously and will be mentioned in the first solicited question.
b. Energy in nitrogen bonds: A serious explosion at a fertilizer plant in West, Texas during 2013 resulted in injuries and deaths. The explosion that occurred was due to a nitrogen compound that supplements fertilizers called ammonium nitrate. Ammonium nitrate is violently explosive due to the strength of the nitrogen bonds in the compound. 
4) Solicited Questions
a. What are some examples of catalysts? Often, in chemistry, metals can be used (palladium, platinum, etc) to coordinate certain reactions to increase their rates. In biology, typically reactions are sped up by enzymes, a type of protein that is built specifically for doing certain processes. Both have advantages and disadvantages: enzymes are very specific for certain tasks and cannot do all of the different chemistry that humans can, but human chemistry can be expensive and troublesome to do (requiring huge pressure, temperature, or toxic / limited resources).
b. What is an example of a reversible reaction? Once, it was believed that all chemical reactions were irreversible. However, French chemistry Claude Louis Bethollet observed in the lab that sodium carbonate and calcium chloride react to form calcium carbonate and sodium chloride; however, the edges of salt lakes (famously in Egypt at the time) also showed signs of newly formed sodium carbonate (a reverse reaction). There are many examples of reverse reactions known today, though the product and the reactant are not always so different from one another.