Acidic solutions, balancing redox

C19-0083. Dichromate ions, C r2 0-j, oxidize acetaldehyde, CH3 CHO, to acetic acid, CH3 CO2 H, and are reduced to Cr . The reaction takes place in acidic solution. Balance the redox reaction and determine how many moles of electrons are required to oxidize 1.00 g of acetaldehyde. What mass of sodium dichromate would be required to deliver this many electrons ... 

The following redox reactions occur in acidic solution. Balance the equations using the half-reaction 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 12.1 Sample exercise Balancing a redox equation in acidic solution... 

Balancing Redox Reactions in Acidic and Basic Solutions... 

There are some complications. As we know from Section 13.6, iron(III) ions tend to hydrolyze in aqueous solution unless the pH is very low. Accordingly, it is understood that, unless otherwise stated, E° values refer to measurements in 1.0 mold acid solution, even if the hydrogen ions do not explicitly appear in the balanced redox equation (e.g., reaction 15.12). Second, iodide ion actually reacts with iodine in water to give brown I3- ... 

The key to the oxidation-number method of balancing redox equations is to realize that the net change in the total of all oxidation numbers must be zero. That is, any increase in oxidation number for the oxidized atoms must be matched by a corresponding decrease in oxidation number for the reduced atoms. Take the reaction of potassium permanganate (KMn04) with sodium bromide in aqueous acid, for example. An aqueous acidic solution of the purple permanganate anion (Mn04 ) is reduced by Br- to yield the nearly colorless Mn2+ ion, while Br- is oxidized to Br2. The unbalanced net ionic equation for the process is... 

To summarize, balancing a redox reaction in acidic solution by the oxidation-number method is a six-step process, followed by a check of the answer (Figure 4.3.)... 

An alternative to the oxidation-number method for balancing redox reactions is the half-reaction method. The key to this method is to realize that the overall reaction can be broken into two parts, or half-reactions. One half-reaction describes the oxidation part of the process, and the other half-reaction describes the reduction part. Each half is balanced separately, and the two halves are then added to obtain the final equation. Let s look at the reaction of aqueous potassium dichromate (K2Cr2C>7) with aqueous NaCl to see how the method works. The reaction occurs in acidic solution according to the unbalanced net ionic equation... 

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 ... 

The following redox reaction occurs in an acidic solution Ce4 + Bi — Ce3+ + Bi01+. What is the coefficient before the C e4 when the equation is fully balanced ... 

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 ... 

Equations for redox reactions are sometimes difficult to balance. Use the steps in Skills Toolkit 2 below to balance redox equations for reactions in acidic aqueous solution. An important step is to identify the key ions or molecules that contain atoms whose oxidation numbers change. These atoms are the starting points of the unbalanced half-reactions. For the reaction of zinc and hydrochloric acid, the unbalanced oxidation and reduction half-reactions would be as follows ... 

Using half-reactions, balance the equation for the redox reaction when Cr207 (fl ) and Fe " (fl ) react to form Cr " (a ) and Fe " (fl ) in acidic solution. 

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. 

Use the oxidation-number method to balance this net ionic redox equation for the reaction between the perchlorate ion and the bromide ion in acid solution. 

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. 

Many redox reactions are too difficult to balance by the simple methods of logical reasoning described in Section 2.4. Here, we outline a systematic procedure based on half-reactions and apply it to reactions that occur in acidic or basic aqueous solution. In these reactions, water and (acidic solution) or OH ... 

Balancing redox reactions can be tricky. When you have trouble, follow the steps below to balance a redox reaction that occurs in acidic solution. 

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

We consider the following redox-reaction that we wish to balance in an acid solution ... 

Balancing Redox Reactions in Acidic Solution When a redox reaction occurs in acidic solution, H2O molecules and ions are available for balancing. Even though we ve usually used H30 to indicate the proton in water, we use H" in this chapter because it makes the balanced equations less complex. 

Let s balance the redox reaction between dichromate ion and iodide ion to form chromium(III) ion and solid iodine, which occurs in acidic solution (Figure 21.2). The skeleton ionic reaction shows only the oxidized and reduced species ... 

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... 

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