When a redox reaction that takes place in an acidic solution involves an oxygen imbalance oxygen should be balanced by adding?

There are many kinds of chemical reactions; Some create explosions, others emit light, and some set off a bunch of other reactions. The type of chemical reaction we will be focusing on today is the redox reaction.

• This article is about redox reactions
• We will learn about oxidation states and how they are important to redox reactions
• Next, we will learn how to use the half-reaction method to balance reactions
• Lastly, we will learn about how photosynthesis is a prime example of a redox reaction

Redox reaction definition

A redox reaction involves the transfer of electrons from one species to another. In a redox reaction, the oxidation state of two species changes, with one being reduced (decreased) and one being oxidized (increased).

The oxidation state (also called oxidation number) corresponds to the number of electrons a species will give or receive during bonding. There are several rules to determine the oxidation state of a species, these are:

• Single elements and single element compounds have an oxidation state of 0
• The sum of the oxidation states in a neutral compound is equal to 0
• The sum of the oxidation states in a charged compound is equal to that charge
• In a compound, the more electronegative species (the species that is more likely to accept an electron) will have a negative oxidation state, while the less electronegative species (the species that is more likely to donate an electron) will have a positive oxidation state.
• For ions, the oxidation state is equal to the charge

Some elements have a usual oxidation state. Transition metals tend to be way more varying.

Now that we know the basics of oxidation states, let's work on a problem.

What is the oxidation state of the metal cation in these compounds?

AgNO3, ZnCl2, Co2O3

1. Let's start with AgNO3. NO3 is an ion with a charge of -1, so Ag has an oxidation state of +1.

2. For ZnCl2, we know that Cl usually has an oxidation state of -1. This is the case here, since there is no O or F in this compound. Since we have 2 mols of Cl to balance out Zn, then the ox. state of Zn is +2.

3. Lastly, for Co2O3, O3 is an ion with a charge of -2. Since we need 2 mols of Co to balance, then the oxidation state of Co is +1.

While we can calculate the oxidation number for every element in a compound, it is easier to calculate the number for just the cation (positively charged species) and anion (negatively charged species) as a whole. Ions like NO3 and SO4 will not be the thing that is being oxidized/reduced, so calculating the oxidation number for each element isn't needed.

Redox reaction example

Now that we know how to calculate oxidation states, let's look at a redox reaction example.

In this reaction, which species is being oxidized, and which is being reduced?

1. Let's start by identifying the oxidation states on the left side. Zn is a lone neutral element, so its oxidation state is 0.

2. SO4 is an ion with a charge of -2, so Cu must have a charge of +2 since this is a neutral compound.

3. Now for the right side. Cu is now alone, so it's oxidation state is 0.

4. Zn is now bonded to SO4, so it's oxidation state is +2.

So, our changes in oxidation state are Cu (+2 → 0) and Zn (0 → +2). Since copper's oxidation state has decreased, it is being reduced. That means that zinc is being oxidized since its oxidation state has increased.

The accepting species is the oxidizing agent, while the species that is donating the electrons is the reducing agent. The wording may be a bit confusing at first, but it can be helpful that the "agent" is the species doing that action and not having that action done to it.

An important component of redox reactions is that they must be balanced by both charge and mass. In the previous example, every species was neutral, but that isn't the case for every redox reaction. We use the half-reaction method to balance these equations, where one or more of the species is charged.

The half-reaction method of balancing is used to balance redox reactions. A redox reaction is split into the oxidation half-reaction and reduction half-reaction. These half-reactions are individually balanced and then recombined into the full balanced equation.

Let's break down how to use this method,

Redox half-reactions

Balance the redox reaction using the skeleton equation below:

We already are given the reaction in its ionic form, so we start by determining the oxidation state for each species.

This step is pretty simple since all of our elements are ions, their oxidation numbers are just their charges.

Now we need to split this reaction into its half-reactions. Let's start with Fe.

This half-reaction is the reduced half-reaction, since Fe's oxidation number is being reduced. Species being reduced gain an electron, so we want to balance our charge by adding an electron to the left side

Now for our oxidation half-reaction.

Species being oxidized lose an electron(s), so we balance by adding 2 electrons to the right side.

Before we combine these reactions, we need to make sure that the electrons will cancel. We can multiply the reduction reaction by 2 so that our electrons will cancel.

Now we can combine our reactions to get the final, balanced reaction

The final reaction shouldn't have any electrons listed as reactants or products, so make sure that you cancel them out. Also, the reduction half-reaction will always have its electrons on the left and the oxidation half-reaction will have them on the right.

When a redox reaction is taking place in an acidic or basic solution, we need to alter the steps just a bit. These steps take place after step 4 in our half-reaction method.

• For acidic solutions: Use H2O to balance oxygen atoms and use H+ to balance hydrogen atoms.
• For basic solutions: Use H2O to balance oxygen atoms and use H+ to balance hydrogen atoms. Add OH- to neutralize any H+ atoms and convert them into H2O molecules. (this step is at the end)

Let's start with an example in an acidic solution.

Write the balanced redox reaction (in acidic solution) using the skeleton formula below:

We start by determining each species' oxidation state:

Fe is simple since its oxidation state is just its charge, so it's oxidation state goes from +2 to +3. This means that Fe is being oxidized. The oxidation state of oxygen is -2, and we have 4 mols of it. This means that Mn must have an ox. state of +7 since the total charge of the compound is -1. Mn is being reduced since its oxidation state goes from +7 to +2.

Now we need to draw our half-reactions. Let's start with Mn

First, we must balance our oxygen by adding 4 mols of H2O to the right side

Next, we balance our hydrogen by adding 8 mols of H+ to the left side

Mn goes from +7 to +2, meaning it gains 5 electrons, so we need to add those 5 electrons to the equation

Now that this half-reaction is balanced, we can move on to the oxidation half-reaction

Fe loses an electron in this reaction, so we can add that electron to the right side

When we combine these reactions, we want the electrons to cancel, so we have to multiply the whole equation by 5.

Lastly, we can combine these two reactions to get the final redox reaction.

Reactions in acidic solution will always have H+ present, and reactions in basic solution will always have OH-. Now onto a basic solution problem.

Balance the redox reaction (in basic solution) given the skeleton equation:

The oxidation states for each species are the same as their charge.

Now onto the half-reactions.

Zn is being oxidized, since its oxidation number is increasing. This also means we need to add our 2 electrons to the right side.

Now for our reduction reaction,

Next, we add 2H+ to the left to balance H

Since this is a reduction, we add an electron to the left side

For our electrons to cancel, we multiply this reaction by 2

Now we can combine our reactions

Now for the fun part. We need to add 4OH- to both sides, so we can make water to cancel our hydrogen molecules

We combine the H+ and OH- to make water

Lastly, we can subtract the 2 water molecules on the right from the left. So, our final reaction is:

For basic reactions, there should not be any H+ atoms in the final reaction.

Photosynthesis redox reaction

Probably the most famous of the redox reactions is photosynthesis.

There are two main reactions in photosynthesis: a light-dependent and a light-independent reaction. Both of these reactions are redox reactions. Let's look at the light-dependent reaction.

Which species are oxidized and reduced in this reaction?

First things first, we have a bunch of things that, while they are a part of the reaction as a whole, aren't directly a part of the redox. We can simplify this reaction to be:

Now on to giving each species its oxidation number. H has an oxdation state of +1 on both sides, O starts with -2, then changes to 0. NADP is a bit trickier. Instead of calculating the oxidation state, we can look at the half-reaction.

We see that 2e- are added on the left side to balance the reaction. Since NADP+ is gaining electrons, it is being reduced. Oxygen goes from an oxidation state of -2 to 0, so it is being oxidized.

Redox Reactions - Key takeaways

• A redox reaction involves the transfer of electrons from one species to another. In a redox reaction, the oxidation state of two species changes, with one being reduced (decreased) and one being oxidized (increased).
• The oxidation state (also called oxidation number) corresponds to the number of electrons a species will give or receive during bonding.
• By calculating oxidation states, we can determine which species is being reduced and which species is being oxidized
• The species that is accepting the electrons is the oxidizing agent, while the species that is donating the electrons is the reducing agent.
• We use the half-reaction method to balance redox reactions
• We use H+ and H2O to balance acidic redox reactions, and those species plus OH- for basic ones
• Photosynthesis is an important example of a redox reaction

A redox reaction involves the transfer of electrons from one species to another. In a redox reaction, the oxidation state of two species changes, with one being reduced (decreased) and one being oxidized (increased).  

You can balance redox reactions using the half-reaction method. This method splits the equation into the oxidized half-reaction and reduction half-reaction. Redox reactions need to be balanced by both mass and charge, so this method makes it simpler to do so. 

A redox reaction will have one species be oxidized (lose electrons and oxidation number increases) and one species be reduced (gain electrons and oxidation number decreases). To identify a redox reaction, calculate the oxidation number for each species. If the number increases for one species and decreases for another, then it is a redox reaction. 

In a redox reaction, electrons are swapped between two species. One species is being reduced, meaning its oxidation number decreases, and it gains electrons. The other species is being oxidized, meaning its oxidation number increases, and it loses electrons.  

A) 2H2 + O2 → 2H2O

B) ZnCl2  + MgSO4 → ZnSO4 + MgCl2

The answer is A. Hydrogen's ox. number goes from 0 to +1 (oxidized), while oxygen's ox. number goes from 0 to -2 (reduced). For B, none of the element's ox. numbers/charges are changing, so no redox reaction is taking place

Question

What is a redox reaction?

Answer

A redox reaction involves the transfer of electrons from one species to another. In a redox reaction, the oxidation state of two species changes, with one being reduced (decreased) and one being oxidized (increased).  

Question

What is the oxidation number of the anion and cation in these compounds?

NaH, Co(OH)2, and VO2

Answer

NaH: Na(+1), H(-1) 

Co(OH)2: Co(+2), OH(-1)

VO2: V(+4), O(-2)

Question

True or False: Photosynthesis is the work of 2 redox reactions

Answer

Question

What is the oxidation number of Cl is KClO3?

Question

What is the difference between an oxidizing agent and a reducing agent?

Answer

The species that is accepting the electrons is the oxidizing agent, while the species that is donating the electrons is the reducing agent. 

Question

What is an oxidation state?

Answer

The oxidation state (also called oxidation number) corresponds to the number of electrons a species will give or receive during bonding. 

Question

What are the steps in the half-reaction method of balancing? (not including those related to acidic and basic conditions)

Answer

1. Write the unbalanced reaction in its ionic form2. Determine the oxidation state of every species in the reaction3. Determine which species is being oxidized, and which is being reduced4. Split the reaction into the two half-reactions5. Use electrons to balance the charge for each half-reaction. 6. Multiply the necessary half-reactions so that the number of electrons cancels7. Combine each half-reaction and cancel like terms

Question

What are the special steps taken when using the half-reaction method in acidic and basic solutions?

Answer

For acidic solutions: Use H2O to balance oxygen atoms and use H+ to balance hydrogen atoms.  

For basic solutions: Use H2O to balance oxygen atoms and use H+ to balance hydrogen atoms. After the two half-reactions are combined, add OH- to neutralize any H+ atoms and convert them into H2O molecules.