A positive standard cell potential indicates a spontaneous reaction.
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The sign of the standard cell potential indicates whether the redox reaction is spontaneous under standard conditions. The relation delta G = -nFE links the two, so a positive standard potential corresponds to a negative standard free energy change. Whether a metal dissolves in an acid can be predicted by comparing its reduction potential with that of hydrogen ions, which is defined as zero. A metal with a negative reduction potential will reduce hydrogen ions and dissolve, while one with a positive potential will not. This simple criterion makes the electrochemical series a predictive tool.
Key ideas: If E cell is positive the reaction is spontaneous, delta G is negative, and K exceeds 1. If E is negative the reverse reaction is spontaneous. If E is zero the system is at equilibrium. Under nonstandard conditions apply the sign test to E (Nernst), not E standard.
Common mistakes and connections: Students use E standard to predict outcomes at unusual concentrations, where a nearly zero E standard can easily reverse. Another mistake is claiming spontaneous means fast. A general rule: E standard above +0.3 V is strongly product-favored (K above about 1e5 for n equals 1). Connects to free energy and equilibrium.
A metal with a negative reduction potential reduces H+ and dissolves in acid, whereas copper, silver and gold at positive potentials do not. The interactive lesson shows this as a live animation or chart.
Copper does not dissolve in hydrochloric acid, because its reduction potential is positive compared with the zero of the hydrogen electrode.
A positive standard cell potential indicates a spontaneous reaction.
Spontaneity from Cell Potential is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Acids, Bases & Electrochemistry first: Standard Hydrogen Electrode, Cell Potential, Electrochemical Series.
The next topics in Acids, Bases & Electrochemistry are Nernst Equation and Cell Potential and Free Energy.
Yes. The interactive lesson includes a self-check question on Spontaneity from Cell Potential, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Spontaneity from Cell Potential is topic 54 of 105 in Acids, Bases & Electrochemistry.