A copper penny can be dissolved in nitric acid but not in...

A copper penny can be dissolved in nitric acid but not in hydrochloric acid. Using reduction potentials from the book, show why this is so. What are the products of the reaction? Newer pennies contain a mixture of zinc and copper. What happens to the zinc in the penny when the coin is placed in nitric acid? Hydrochloric acid? Support your explanations with data from the book, and include balanced equations for all reactions.

Key Concepts

Reaction Product Formation

The nature of the reaction products depends on the specific redox interactions taking place in the solution. In the case of copper reacting with nitric acid, the formation of copper nitrate accompanied by the evolution of nitrogen oxides and water is observed. This contrasts with reactions involving hydrochloric acid, where no oxidizing agent is present, and metals that do not react with HCl under typical conditions yield no such products.

Alloy Metal Displacement Reactions

When a metal alloy, such as a penny made of copper and zinc, is exposed to different acids, the behavior of each metal is influenced by its individual reactivity. Zinc, being more reactive than copper, will preferentially oxidize in acid. In nitric acid, both metals might undergo oxidation but yield different products compared to their behavior in hydrochloric acid, where zinc will dissolve to form zinc chloride with the release of hydrogen gas, illustrating how the composition of an alloy affects its overall chemical reactivity.

Redox Reaction Mechanism in Acidic Media

Redox reactions in acidic solutions require careful balancing of electron transfer, hydrogen ions, and water molecules. The oxidation of a metal by an oxidizing acid like nitric acid involves not only the release of metal ions but also the reduction of the acid’s oxidizing component, often resulting in gaseous nitrogen oxides. This mechanism provides a framework for writing balanced equations for the reactions, ensuring conservation of mass and charge.

Activity Series and Metal Reactivity

The metal activity series ranks metals based on their reactivity, or their tendency to lose electrons. In this context, a metal that is lower in the activity series, like copper, does not react with acids that only produce hydrogen gas (as in the case of hydrochloric acid). However, metals such as zinc, which are higher in the activity series, will readily react with both oxidizing acids like nitric acid and non?oxidizing acids like hydrochloric acid, leading to different reaction products.

Oxidizing Ability of Acids

Some acids have oxidizing properties in addition to being proton donors. Nitric acid, for example, is a strong oxidizing acid because the nitrate ion can accept electrons and be reduced to various nitrogen oxides. This oxidizing strength allows it to attack metals such as copper even when the metal itself would not normally react with acids like hydrochloric acid, which is only a proton donor and cannot oxidize copper effectively.

Standard Reduction Potentials

Standard reduction potentials are numerical values that indicate the tendency of a chemical species to be reduced, and they are fundamental to predicting whether a redox reaction will occur spontaneously. By comparing these potentials for different half?reactions, one can determine which species will act as an oxidant and which will act as a reductant, thereby explaining why one acid might oxidize a metal while another may not.

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