OXIDATION-REDUCTION - CHEM120 Week 5 Concepts: Redox and Organic Chem...

Week 5 Concepts: Redox and Organic Chemistry

CHEM120 Week 5 Concepts: Redox and Organic Chemistry –

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OXIDATION-REDUCTION

Reduction and oxidation are intertwined concepts that are important to understanding some of the most important chemical reactions in the human body. In this lesson, we will explore the relationship between these terms and the how to identify what chemicals in a reduction/oxidation reaction (redox reaction) are playing which parts.

1. Redox reactions involve a transfer of:

▪ Nucleons ▪ Neutrons ▪ Electrons ▪ Protons

• If iron reacts with oxygen gas in the reaction 2 Fe

(s) + O 2(g) →2Fe O (s) , what is oxidized in this reaction?▪ No oxidation is occurring here ▪ FeO

▪ O2

▪ Fe

REDUCTION AND OXIDATION

Reduction and oxidation involves the transfer of electrons between two species within a chemical reaction. Not all chemical reactions involve reduction and oxidation; however, the reactions that involve reduction and oxidation are called Redox reactions. As usual, we need to understand a bit of terminology in order to understand these reactions further.

• Oxidation: loss of electrons

• Reduction: gain of electrons

You may find the word “reduction” a bit of an odd choice for a reaction that involves gaining of electrons.The key is to remember that electrons have a -1 charge, and thus, the more electrons an atom gains, the more negative the charge. To help remember these terms, the mnemonic below can help: LEO the lion goes GER: Loss of Electrons is Oxidation and Gain of Electrons is Reduction In a chemical reaction, oxidation cannot happen without reduction and reduction cannot happen without oxidation.

• Oxidation involves the loss of electrons while reduction involves the gain of electrons. Consider a

reaction where a Li + is converted into Li. Do you think that this Lithium atom gained or lost an electron?▪ gained electron ▪ lost electron

Week 5 Concepts: Redox and Organic Chemistry

IDENTIFYING OXIDATION AND REDUCTION

We know that electrons have a -1 charge, and we understand that +1 -1 = 0. So, if Li + gains an electron, this is what cancels out the positive (+1) charge and makes this atom neutral.The rule for oxidation and reduction, based on the gain or loss of electrons is that:

• As the charge of a species increases in the positive direction, the species is oxidized.• As the charge of a species decreases, becoming more negative, the species is reduced.

A visualization of this relationship appears in the image. Note the direction of the arrows. When the Li + became Li, we see that we moved from +1 to 0, or downwards, telling us that this process was reduction.As we see, if charges are present, determining reduction and oxidation is as easy as looking at how the charges change.

4. In the following reactions, indicate if the species is oxidized or reduced:

▪ Mg 2+ becomes Mg Reduced • becomes O 2 Reduced ▪ Fe 2+ becomes Fe 3+ Oxidized ▪ F becomes F - Reduced ▪ Cu becomes Cu + Oxidized ▪ H becomes H + Oxidized

OXIDATION AND REDUCING AGENTS

Reduction cannot happen without oxidation and oxidation cannot happen without reduction. For example, when a species is oxidized, losing one or more electrons, those electrons must go to another species in the reaction, causing the other species to be reduced.• Oxidizing agents are reduced in a chemical reaction (because oxidizing agents accept electrons) • Reducing agents are oxidized in a chemical reaction (because reducing agents donate electrons)

As seen in the image, species A transfers electrons to species B, and thus species A is the reducing agent. You also see that since species A has lost electrons, species A is oxidized. In other words, the species being oxidized is the reducing agent while the species being reduced is the oxidizing agent (species B in this case).

Week 5 Concepts: Redox and Organic Chemistry

In chemical reactions, a common oxidizing agent is oxygen gas, O2. For example, rust forms when O2 reacts with iron, Fe, causing oxidation.

• Fe(s) + O2(g) → FeO(S)

In this reaction, we would consider the O2 to be the oxidizing agent and Fe to be the reducing agent, as electrons were transferred from Fe to O.A common reducing agent is hydrogen gas, H2. In the reaction below, we see how hydrogen gas can reduce the iron, forming Fe from FeO.FeO(S) + H2(g) → Fe +H2O(g) In this reaction, we would consider H2 to be the reducing agent and FeO to be the oxidizing agent as electrons were transferred from the H2 to the Fe in the FeO.Putting together what you have learned in this lesson, you can identify oxidizing and reducing agents.

In this way, if you can determine whether a species is gaining or losing electrons, you can determine if the species is oxidized or reduced and thus the reducing agent or oxidizing agent. Consider the reaction

between solid zinc and a solution containing copper ions:

Zn(s) + Cu 2+ (aq) → Zn 2+ (aq) + Cu(s) As you can see, the solid Zn donated two electrons to the Cu ion. We know this because you see that Zn went from having no charge (0) to having a positive charge (+2). This tells us that the Zn has lost these electrons. Where did these electrons go? To the Cu ion as the Cu went from a charge of +2 to a charge of 0, indicating a gain of electrons (remember, electrons have a -1 charge, so the more electrons an atom gets, the more negative it becomes). In the image you see that the solid Zn must be the reducing agent as this is the species causing another species, the Cu 2+ , to become reduced.Once we know the reducing agent, we know that the oxidizing agent must be the other reactant, as reduction cannot happen without oxidation.

The key to this is to remember:

• Oxidizing agents are reduced in a chemical reaction (because oxidizing agents accept electrons) • Reducing agents are oxidized in a chemical reaction (because reducing agents donate electrons)

Week 5 Concepts: Redox and Organic Chemistry

5. Choose the correct agent in each of these reactions:

▪ Mg(s) + O2(g) → MgO Select the oxidizing agent: O2(g)

▪ CuO(s) + H2(g) → Cu(s) + H2O(g) Select the reducing agent: CuO(s)H2(g)Cu(s)H2O(g)

▪ 2 Na(s) + Cl2(g) → NaCl(s) Select the reducing agent: 2 Na(s)

REDOX ALL AROUND YOU

Redox reactions are all around you and play important roles in the environment as well as within our own bodies. One important

redox reaction that we all depend on is photosynthesis:

• CO2 + 12 H2O + light → C6H12O6 + 6 H2O + 6 O2

This is the reaction that produces the oxygen gas we breathe as well as glucose (C6H12O6), the simple sugar that fuels life on this planet. The carbon in CO2 is reduced and the oxygen in water is oxidized. We also see the reverse of this

reaction in cellular respiration:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy(ATP) In cellular respiration, the carbon in glucose is oxidized to form CO2 and the oxygen is reduced to form H2O. Cellular respiration is the reaction organisms of all types use to break down glucose for energy in the form of ATP, the chemical energy used by cells.Another important part of cellular energy is nicotinamide adenine dinucleotide, abbreviated as NAD, found in all cells. This important molecule is key to metabolism and carries electrons around the cell to facilitate metabolism.

NAD +

+ 2H → NADH + H

+

As we can see, NAD + is reduced to form NADH.

• If a species accepts electrons from another reactant in a chemical reaction, this species is: (select

all that apply) ▪ Being reduced ▪ Being oxidized ▪ Reducing agent ▪ Oxidizing agent

• Which metal is being oxidized based on the reaction? Cu(s) + Ag

+ (aq) → Cu + (aq) + Ag(s) ▪ Cu(s) ▪ Ag + (aq) ▪ Cu + (aq) ▪ Ag(s)

• What would be the oxidizing agent in the reaction 2 Ag(s) + O2(g) → Ag2O(s)

▪ Ag(s) ▪ O2(g) ▪ Ag2O(s) ▪ This is not a redox reaction