Redox reactions, balancing

We have seen how analytical calculations in titrimetric analysis involve stoichiometry (Sections 4.5 and 4.6). We know that a balanced chemical equation is needed for basic stoichiometry. With redox reactions, balancing equations by inspection can be quite challenging, if not impossible. Thus, several special schemes have been derived for balancing redox equations. The ion-electron method for balancing redox equations takes into account the electrons that are transferred, since these must also be balanced. That is, the electrons given up must be equal to the electrons taken on. A review of the ion-electron method of balancing equations will therefore present a simple means of balancing redox equations. 

Example 8.1. Balancing Redox Reactions Balance the following redox reactions (1) oxidation of Mn to Mn04 by Pb02 and (2) oxidation of S2O3 to S4O4 by O2. 

Finding out about redox reactions Balancing redox equations Taking a look at electrochemical cells... 

Separating a redox reaction into its half-reactions is useful if you need to balance the reaction. One method for balancing redox reactions is reviewed in Appendix 4. 

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. 

Balancing a redox reaction is often more challenging than balancing other types of reactions, because we must balance both electrons and elements. Perhaps the simplest way to balance a redox reaction is by the half-reaction method, which consists of the following steps. 

Adding together the two half-reactions and simplifying gives us the final balanced redox reaction. 

Reactions. The SCR process is termed selective because the ammonia reacts selectively with NO at temperatures >232° C in the presence of excess oxygen (44). The optimum temperature range for the SCR catalyst is determined by balancing the needs of the redox reactions. 

When the potassium permanganate Is added, a redox reaction occurs (the equation for the reaction Is derived and balanced In the text). 

Consider the following balanced redox reaction in basic medium. 

Because E° is a positive quantity, +0.165 V, a redox reaction should occur. The balanced half-equations are... 

Only one method for balancing redox reactions, the half-equation method introduced in Chapter 4. 

Because electrons can be neither lost nor created in a chemical reaction, all the electrons lost by the species being oxidized must be transferred to the species being reduced. Because electrons are charged, the total charge of the reactants must be the same as the total charge of the products. Therefore, when balancing the chemical equation for a redox reaction, we have to balance the charges as well as the atoms. 

Some redox reactions, particularly those involving oxoanions, have complex chemical equations that require special balancing procedures. We meet examples and see how to balance them in Chapter 12. 

When balancing the chemical equation for a redox reaction involving ions, the total charge on each side must be balanced. 

K.l Write a balanced equation for each of the following skeletal redox reactions ... 

J 3 Write and balance chemical equations for simple redox reactions (Self-Test K.4). 

K.25 The following redox reactions are important in the refining of certain elements. Balance the equations and in each case, write the name of the source compound of the element (in bold face) and the oxidation state in that compound of the element that is being extracted ... 

We need to be able to write balanced chemical equations to describe redox reactions. It might seem that this task ought to he simple. However, some redox reactions can be tricky to balance, and special techniques, which we describe in Sections 12.1 and 12.2, have been developed to simplify the procedure. 

The key to writing and balancing equations for redox reactions is to think of the reduction and oxidation processes individually. We saw in Section K that oxidation is the loss of electrons and reduction the gain of electrons. 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

The chemical equation for a reduction half-reaction is added to the equation for an oxidation half-reaction to form the balanced chemical equation for the overall redox reaction. 

Balance chemical equations for redox reactions by the halfreaction method (Toolbox 12.1 and Examples 12.1 and 12.2). 

Write the balanced chemical equation for (a) the thermal decomposition of potassium chlorate without a catalyst (b) the reaction of bromine with water (c) the reaction between sodium chloride and concentrated sulfuric acid, (d) Identify each reaction as a Bronsted acid—base, Lewis acid—base, or redox reaction. 

Balance each of the following skeletal equations and classify the reaction as Bronstcd-I. owry acid-base, Ix wis acid-base, or redox. For both types of acid-base reactions, identify the acid and the base. For redox reactions, identify the reducing agent and the oxidizing agent. 

Reservoirs in which Acid-Base and/or Redox Reactions and Balances Occur... 

Redox reactions are more complicated than precipitation or proton transfer reactions because the electrons transferred in redox chemishy do not appear in the balanced chemical equation. Instead, they are hidden among the starting materials and products. However, we can keep track of electrons by writing two half-reactions that describe the oxidation and the reduction separately. A half-reaction is a balanced chemical equation that includes electrons and describes either the oxidation or reduction but not both. Thus, a half-reaction describes half of a redox reaction. Here are the half-reactions for the redox reaction of magnesium and hydronium ions ... 

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