# Predicting Spontaneous Direction of a Redox Reaction

## The direction of a redox reaction depends on the relative strength of the oxidants and reductants in a solution.

#### Key Points

• Sometimes, the direction of a redox reaction can be determined by estimating the relative strengths of reductants and oxidants.

• In situations where an electrochemical series is not sufficient to determine absolutely the direction of a redox reaction, the standard electrode potential, Eo, can be used.

• A large magnitude of Eo indicates that the reaction is far displaced from equilibrium. A negative value indicates a reducing environment, while a positive value indicates an oxidative environment.

#### Terms

• An electrode potential measured under standard conditions; a temperature or 298K, 1 atmosphere pressure and at 1 mole of the activity of redox participants of the half-reaction.

#### Figures

1. ##### Electrochemical Series

In order to predict if two reactants will take part in a spontaneous redox reaction, it is important to know how they rank in an electrochemcial series, as that shown here.

Generally, the direction of a redox reaction depends on the relative strength of oxidants and reductants in a solution. In simple situations, electrochemical series, as seen in Figure 1, can be very useful for determining the direction of the reaction.

However, some reactions cannot be "eyeballed" in this manner. The reactions require a more mathematical method to determine the direction. To achieve this, it is important to consider the standard electrode potential, which is a measure of the driving force behind a reaction. The larger the value of the standard electrode potential, the further the reaction is from equilibrium. The sign of the standard electrode potential indicates the direction in which the reaction must shift to reach equilibrium.

Consider the reaction between zinc and acid:

$Zn(s) + 2 H^+(aq) \leftrightarrow Zn^{2+}(aq) + H_2(g)$

Eo = 0.76 V

The positive Eo value indicates that when this system is present at standard-state conditions, it has to shift to the right to reach equilibrium. Reactions for which Eo is positive, therefore, have equilibrium constants that favor the products of the reaction.

What happens to the standard electrode potential with a reversal of the direction in which a reaction is written? Turning the reaction around doesn't change the relative strengths of the oxidizing or reducing agents. The magnitude of the potential must remain the same. However, turning the equation around changes the sign of the standard electrode potential, and can therefore turn an unfavorable reaction into one that is spontaneous, or vice versa.

The relative reactivities of different half reactions can be compared to predict the direction of electron flow. Half reaction equations can be combined if one is reversed to an oxidation in a manner that cancels out the electrons.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
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electrode
the terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit. In electrolysis, the electrodes (cathode and anode) are placed in the solution separately.
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electrode potential
The potential difference of a half-reaction that occurs across a reversible cell made up of any electrode and a standard hydrogen electrode.
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electron
The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.
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equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
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equilibrium constant
referring to a numerical value derived from the ratio of concentrations of products to reactants of a reversible reaction
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oxidation
a reaction in which the atoms of an element lose electrons and the valence of the element increases
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Oxidation
Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
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product
a chemical substance formed as a result of a chemical reaction
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reactivity
Relative susceptibility to chemical reaction.
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redox
a reversible chemical reaction in which one reaction is an oxidation and the reverse is a reduction
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reducing agent
any substance that donates electrons to another; in so doing, it becomes oxidized
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solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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standard
something used as a measure for comparative evaluations
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standard electrode potential
An electrode potential measured under standard conditions; a temperature or 298K, 1 atmosphere pressure and at 1 mole of the activity of redox participants of the half-reaction.
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standard state
In chemistry, a reference point used to calculate a material's (pure substance, mixture, or solution) properties under different conditions.
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state
The physical property of matter as solid, liquid, gas or plasma
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system
the part of the universe being studied, arbitrarily defined to any size desired
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zinc
a chemical element (symbol Zn) with an atomic number of 30.