Reduction and Oxidation Help (page 2)
Reduction is the chemical name for a decrease in oxidation number. When hydrogen is heated in combination with metal oxides like copper and zinc, the metal element is separated out and water is formed.
The example below demonstrates the reduction reaction.
CuO + H 2 ⇒ Cu + H 2 O
ZnO + H 2 ⇒ Zn + H 2 O
The metal oxide is reduced to release the uncombined metal. This is called a reduction reaction.
Reduction is what happens when a sample gains one or more electrons provided by the reducing agent. As you might guess, hydrogen can be used as a reducing agent.
Reduction also happens when a compound picks up hydrogen atoms. Methyl alcohol, CH 3 OH, is formed in the reaction of carbon monoxide (CO), hydrogen gas, and a catalyst.
CO + 2 H 2 ⇒ CH 3 OH
An atom is also reduced if it gains electrons directly. This can be seen when Cu 2+ - or Ni 2+ -containing solutions are plated onto an electrode. When electrons are removed from the electrode, the metal ions are reduced.
While studying reduction, it is a good time to look at the opposite reaction, oxidation . Oxidation is the process of a substance combining with oxygen. It is the reverse of reduction.
When silicon (Si) is combined with oxygen and other minerals in the Earth’s crust, sand is formed. In fact, most of the compounds in living organisms on this planet contain oxygen. Nearly 60% of the weight of a human body is oxygen.
Oxidationtakes place when an element gains oxygen, loses hydrogen, or loses electrons.
N 2 + O 2 + (high temperature) ⇒ 2 NO (nitric oxide)
2 H 2 S + 3O 2 ⇒ 2 H 2 O + 2SO 2
Hydrogen sulfide (H 2 S) burns in oxygen to make water and sulfur dioxide. When this is oxidized, O 2 combines with each atom of the products. Oxidation and reduction can be thought of as an “opposite theory.”
Reduction is (+) gain.
Oxidation is (–) loss.
The key to oxidation is to remember that oxidation is not just for oxygen, but it also defines a lot of different reactions with metals.
Figure 8.1 shows oxidation and reduction changes.
Basically, it is a cause and effect system. When an element causes the oxidation of another substance, it is itself reduced in the process.
Redox chemistry is when one sample is reduced and another sample is oxidized at the same time during the same reaction.
Picture the “you scratch my back, I’ll scratch yours,” type of interaction. When all the atoms in a reaction shift around to find balance in filled orbitals, the compound is stable.
Oxidation and reduction are opposite reactions. An oxidizing agent is the element in the reaction that oxidizes another element, while at the same time being reduced itself. A reducing agent is the element in the reaction that reduces another element while at the same time being oxidized itself. In redox reactions, the total charge of the elements’ oxidation numbers will be equal on both sides of the equation.
In electrochemical reactions, you will sometimes see redox reactions written as half-reactions. The charge must be balanced overall and the electrons produced in one part of the reaction must be lost in another part. If this didn’t happen, electrical charges would build up and cause a big problem.
Look at the electrical ion transfer ( electrolysis ) of molten sodium chloride to produce chlorine gas and sodium metal. The sodium (Na + ) and chloride (Cl – ) ions are in the electrolyte solution. When an electrical current is passed through the solution, the chloride ions react at the anode and the sodium ions react at the cathode as shown in the following reactions:
2Cl − ⇒ Cl 2 + 2e − (oxidation)
Na + + e − ⇒ Na (reduction)
The same number of electrons begins at the cathode and leaves at the anode, so the overall reaction is found by multiplying the second half-reaction by two and adding the two reactions to get:
2Na + + 2Cl − ⇒ 2Na + Cl 2
The number used to keep track of the electrons in a reaction is called the oxidation number . Elements, like the halogens, may have different oxidation numbers depending on the specific reaction and environment. Oxidation number also tells us something about the strength or ability of a compound to be oxidized or reduced or to serve as an oxidizing agent or reducing agent. Oxygen has an oxidation number of −2. Using this as a starting place, oxidation numbers are assigned to all other elements.
Since water is a neutral molecule, it is fairly simple to figure out the oxidation numbers for the elements of the H 2 O molecule. Oxygen has an oxidation number of −2, so then each hydrogen must have an oxidation number of +1, to allow the total charge to equal zero. Table 8.1 lists a few hints to help figure out oxidation numbers.
General rules of oxidation and reduction
1. An uncombined element has an oxidation number of zero (K, Fe, H 2 , O 2 )
2. All oxidation numbers added together in a compound must equal zero.
3. In an ion of one atom, the oxidation number is equal to the ion’s charge.
4. In an ion of more than one atom, all the oxidation numbers add up to the ion’s charge.
5. When oxygen is part of a compound, the oxidation number is −2 (except peroxides H 2 O 2 (−1)).
6. Hydrogen’s oxidation number is equal to its +1 charge (except when combined with metals, then it is −1).
Oxidation numbers can be calculated by multiplying the number of elemental atoms present by the oxidation numbers and setting the entire equation equal to zero. The following example shows you how.
What is the oxidation number of chromium in Li 2 Cr 2 O 7 , lithium dichromate?
Li = +1, Cr = x , O = −2
Li 2 (2 × +1) = 2
O 7 (7 × −2) = −14
+2 + 2 x + (−14) = 0
2 x = 12
x = 6
So the oxidation number of chromium is 6.
Figuring out the oxidation and reduction of elements in a sample is fairly simple if you use the Periodic Table and the rules of reaction. Working with redox reactions is basically an accounting task. You need to be able to keep track of all of the electrons as they transfer from one ion form to another. The trick to balancing redox reactions is to balance the charge as well as the elements on each side of the reaction.
Practice problems for these concepts can be found at – The Hydrogen Atom Practice Test
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