Basic solutions, balancing redox

The following redox reactions occur in basic solution. Balance the equations using the oxidation number method. 

Balance the following redox reactions, and calculate the standard-state potential and the equilibrium constant for each. Assume that the [H3O+] is 1 M for acidic solutions, and that the [OH ] is 1 M for basic solutions. 

EXAMPLE 12.2 Sample exercise Balancing a redox equation in basic solution... 

Redox reactions do not always take place under neutral conditions. Balancing half-reactions is more complicated for reactions that take place in acidic or basic solutions. When an acid or base is present, or OH ions must also be considered. However, the overall approach is similar. This approach involves writing the correct formulas for the reactants and products, balancing the atoms, and adding the appropriate number of electrons to one side of the half-reaction to balance the charges. 

Balancing Redox Reactions in Acidic and Basic Solutions... 

To balance a redox reaction which occurs in a basic solution is a very similar to balancing a redox reaction which occurs in acidic conditions. First, balance the reaction as you would for an acidic solution and then adjust for the basic solution. Here is an example using the half-reaction method ... 

Redox reactions can be balanced in basic solutions as well. This task will require an additional step but it can still be done following the steps shown above. To balance the redox reaction H202 + C1031 — C1021- + 02 in a basic solution, do as follows ... 

Balancing oxidation-reduction reactions depends on whether the solution is acidic or basic. The method for balancing redox reactions in an acidic solution is as follows ... 

The method for balancing redox reactions in a basic solution is as follows ... 

In this case, the hydrogen ion and the water molecule are eliminated because neither is oxidized nor reduced. The only additional information needed is that the reaction takes place in acid solution. In acid solution, hydrogen ions (H+) and water molecules are abundant and free to participate in redox reactions as either reactants or products. Some redox reactions can occur only in basic solution. When you balance equations for these reactions, you may add hydroxide ions (OH ) and water molecules to either side of the equation. Basic solutions have an abundance of OH ions instead of H30" ions. 

Redox reachons that take place in acidic soluhon often use water molecules and hydrogen ions in the process. Redox reactions that take place in basic solution often use water molecules and hydroxide ions in the process. Therefore, it is appropriate to add these species to the equahon in order to balance the numbers of oxygen and/or hydrogen atoms. 

Before you attempt to balance the equation for a redox reaction, why do you need to know whether the reaction takes place in acidic or basic solution (20.3)... 

Use the half-reaction method to balance these equations for redox reactions. Add water molecules and hydrogen ions (in acid solutions) or hydroxide ions (in basic solutions) as needed. 

An alternative procedure for basic solutions is to balance the redox equation for acid solutions first. Then, add... 

Even though there is only a minor difference between the balance of redox reactions in acid and basic solutions the principles differ in that the equation are balanced with OH" ions in stead of H. this we will look into in the following example ... 

We ll balance a redox reaction that occurs in acidic solution first and then go through Sample Problem 21.1 to balance one in basic solution. 

Balancing Redox Reactions in Basic Solution As you just saw, in acidic solution, H2O molecules and H ions are available for balancing. As Sample Problem 21.1 shows, in basic solution, H2O molecules and 0H ions are available. Only one additional step is needed to balance a redox equation that takes place... 

Some redox reactions can occur only in basic solution. When you balance equations for these reactions, you can add hydroxide ions (OH ) and water molecules to either side of the equation. 

Compare and contrast balancing redox equations in acidic and basic solutions. 

A redox reaction consists of two parts or half-reactions. These are the oxidation reaction in which a substance loses or donates electrons and the reduction reaction in which a substance gains or accepts electrons. An oxidation reaction and a reduction reaction must always be coupled because "free" electrons cannot exist in solution and electrons must be conserved. The coupling between the two half-reactions is by the electrons that are either generated (by oxidation) or consumed (by reduction). We will use this fact in our technique for balancing redox reactions, which basically is a stepwise stoichiometric (mass) balancing of each constituent followed by a balancing of charge (electroneutrality). 

If a redox reaction occurs in basic solution, the equation must be balanced by using OH and H2O rather than and H2O. One approach is to first balance the halfreactions as if they occurred in acidic solution and then count the number of in each... 

What is being oxidized and what is being reduced in this reaction (d) Gold miners then react the basic aqueous product solution from part (c) with Zn dust to get gold metaL Write a balanced redox reaction for this process. What is being oxidized, and what is being reduced ... 

In the half-reaction method, we usually begin with a skeleton ionic equation showing only the substances undergoing oxidation and reduction. In such cases, we assign oxidation numbers only when we are unsure whether the reaction involves oxidation-reduction. We will find that (for acidic solutions), OH (for basic solutions), and H2O are often involved as reactants or products in redox reactions. Unless H, OH , or H2O is being oxidized or reduced, these species do not appear in the skeleton equation. Their presence, however, can be deduced as we balance the equation. 

If a redox reaction occurs in basic solution, the equation must be balanced by using OH and H2O rather than and H2O. Because the water molecule and the hydroxide ion both contain hydrogen, this approach can take more moving back and forth from one side of the equation to the other to arrive at the appropriate half-reaction. An alternate approach is to first balance the half-reactions as if they occurred in acidic solution, count the number of in each half-reaction, and then add the same number of OH to each side of the half-reaction. This way, the reaction is mass-balanced because you are adding the same thing to both sides. In essence, what you are doing is neutralizing the protons... 

EXERCISE 20.3 Balancing Redox Equations in Basic Solution... 

Complete and balance this equation for a redox reaction that takes place in basic solution CN (aq) + Mn04 (oq) --------> CNO (oq) + Mn02(s) (basic solution)... 

Balance these ionic redox equations using the ion-electron method. These reactions occur in basic solution. 

Problem Permanganate ion is a strong oxidizing agent, and its deep purple color makes it useful as an indicator in redox titrations. It reacts in basic solution with the oxalate ion to form carbonate ion and solid manganese dioxide. Balance the skeleton ionic equation for the reaction between NaMn04 and Na2C204 in basic solution ... 

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