Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Redox Reaction and Oxidation Number State of Electrochemistry. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
To approach the topic of electrochemistry, we must first understand redox reactions, and oxidation number/state. A redox reaction is any reaction where the oxidation states of atoms change. First we will determine how to calculate the oxidation state of an atom, and then we will apply that knowledge to redox reactions.
The term oxidation state, sometimes referred to as oxidation number, describes the number of electrons that an atom has gained or lost. The process of an atom gaining electrons is called reduction, whereas the process of an atom losing electrons is called oxidation. An easy way to remember this is through the mnemonic “Oil Rig” (Oxidation is loss, reduction is gain). The value of oxidation state can be positive, negative, or zero. If the oxidation state of an atom is negative, it has gained electrons. If it’s oxidation state is positive, it has lost electrons. If it’s oxidation state is zero, it has neither gained nor lost electrons. For example, every free element (e.g. Na or Cl2) has an oxidation state of zero because it is not participating in any reactions for which it could gain or lose electrons. However, in the compound NaCl, Na has an oxidation state of +1, having given an electron to Cl, which has an oxidation state of -1.
The oxidation state of any free atom is zero. If an atom is not free, and is instead part of a compound, we determine its oxidation state in the context of the compound it is participating in. First, we would look at the compound it is in, and note that the sum of the oxidation states of the individual atoms in the compound has to equal the net charge of the molecule. Next, we would compare the atoms that are bonded to one another in the compound. In a bond, the more electronegative atom will take electrons from the less electronegative atom. One important caveat is that in a double bond, the more electronegative atom will gain two electrons, and three in a triple bond, etc.
Example: Water
For example: what is the oxidation state of H2O? H2O is not a free atom, and it is composed of elements with different electronegativities, so its individual atoms have an oxidation state. The net charge of water is zero, so the oxidation states must sum to 0. Looking at water’s bonds, we can count that water has two bonds between hydrogen and oxygen. Oxygen is more electronegative than hydrogen, so in each bond we can predict that oxygen is gaining an electron and hydrogen is losing an electron. Therefore, for each bond, oxygen gains “-1” to its oxidation state, whereas the corresponding hydrogen gains “+1” to its oxidation state. Overall, the oxygen has an oxidation state of -2 and each hydrogen has an oxidation state of +1, the sum of which is 0.
Carbon Containing Compounds
Glucose is a carbon containing compound that is the primary substrate for energy in cellular respiration. Glucose is oxidized, losing electrons and releasing energy. Therefore, one important relation between redox reactions and biochemistry is that in a biochemical context, we can think of reduced carbon compounds as high in energy, and ripe for energy-releasing oxidation. For instance, consider CH4 and CO2. Carbon has an oxidation state of -4 in CH4, but it has an oxidation state of +4 in CO2. Carbon is much more reduced in CH4 than CO2. Therefore, a biochemical pathway might take advantage of this difference and seek to oxidize CH4 to release energy into a biological system. Metabolic reactions, which we cover in another video, largely involve the oxidation of highly reduced carbon compounds.
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