Question

Pennies in the United States consist of a zinc core that is electroplated with a thin coating of copper. Zinc dissolves in hydrochloric acid, but copper does not. If a small scratch is made on the surface of a penny, it is possible to dissolve away the zinc core, leaving only the thin shell of copper. Identify which element is oxidized and which is reduced in the reaction for the dissolving of the zinc by the acid. $$\mathrm{Zn}(s)+2 \mathrm{HCl}(a q) \rightarrow \mathrm{ZnCl}_{2}(a q)+\mathrm{H}_{2}(g)$$

Short answer

In the given reaction \(\mathrm{Zn}(s)+2 \mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq)+\mathrm{H}_{2}(g)\), zinc (Zn) is the element that gets oxidized, as its oxidation state increases from 0 to +2, while hydrogen (H) is the element that gets reduced, as its oxidation state decreases from +1 to 0.

Step-by-step solution

Determine the initial oxidation states

We can determine the initial oxidation states for the elements in the reactants. For the zinc metal (Zn), the initial oxidation state is 0 since it is an uncombined element. For HCl, hydrogen (H) has an oxidation state of +1 and chlorine (Cl) has an oxidation state of -1.

Determine the final oxidation states

Now, we'll determine the final oxidation states for the elements in the products. In ZnCl₂, zinc (Zn) has an oxidation state of +2 and chlorine (Cl) remains with an oxidation state of -1. In the hydrogen gas (H₂), the oxidation state of hydrogen (H) is 0, as it is in its uncombined elemental state.

Compare oxidation states to identify the oxidized and reduced elements

Comparing the initial and final oxidation states of the elements will allow us to see which element gained or lost electrons. The oxidation state of zinc (Zn) increased from 0 to +2, meaning it lost electrons, and therefore, it's the element that was oxidized. The oxidation state of hydrogen (H) decreased from +1 to 0, meaning that it gained electrons, and therefore, it's the element that was reduced. The oxidation state of chlorine (Cl) remained the same throughout the reaction, so it was neither oxidized nor reduced. In conclusion, zinc (Zn) is the element that gets oxidized and hydrogen (H) is the element that gets reduced in the given reaction: \[\mathrm{Zn}(s)+2 \mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq)+\mathrm{H}_{2}(g)\]

Key concepts

Oxidation States

Oxidation states are essential for understanding redox reactions like the one between zinc and hydrochloric acid. They act as indicators showing how many electrons an element has gained or lost. By assessing these states, we can track electron movement in a reaction.
The initial oxidation state of zinc (Zn) in its elemental form is 0. Elements in their pure form always have an oxidation state of zero. For hydrochloric acid (HCl), hydrogen (H) has an oxidation state of +1, because hydrogen often forms compounds by donating electrons.
Chlorine (Cl) in HCl has an oxidation state of -1, taking electrons from the hydrogen.

• Zn (initial): 0
• H in HCl (initial): +1
• Cl in HCl (initial): -1

When the reaction proceeds, zinc transforms into zinc chloride (ZnCl₂), adopting an oxidation state of +2 as it loses electrons. Hydrogen, on the other hand, forms H₂ gas with a new oxidation state of 0, indicating it gains electrons by pairing up to become neutral. Understanding these changes helps identify which substances are oxidized and which are reduced.

Electron Transfer

Electron transfer is at the heart of redox reactions. In these reactions, electrons move from one element to another, which leads to changes in oxidation states. Identifying the element losing and the one gaining electrons is key to understanding the entire process.
In the reaction \[\mathrm{Zn}(s)+2 \mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq)+\mathrm{H}_{2}(g)\] zinc (Zn) serves as the reducing agent. It loses two electrons, as seen from its oxidation state changing from 0 to +2. This loss is why we say Zn is oxidized.
On the flip side, hydrogen ions in HCl accept those electrons, decreasing their oxidation state from +1 to 0 to form H₂ gas. This electron gain means hydrogen is reduced.

• Zinc loses electrons: \[\mathrm{Zn} \rightarrow \mathrm{Zn}^{2+} + 2e^-\]
• Hydrogen gains electrons: \[2\mathrm{H}^{+} + 2e^- \rightarrow \mathrm{H}_{2}\]

Thus, inclusively understanding electron transfer helps students grasp which elements are oxidized or reduced.

Chemical Reactions

Chemical reactions like the one described for zinc and hydrochloric acid involve transformation and reorganization of molecules. In essence, such reactions display how substances change over time, primarily through breaking and forming bonds.
In this specific redox reaction, zinc metal and hydrochloric acid interact to yield zinc chloride and hydrogen gas. Observing this chemical process helps us understand why some elements can trade electrons while others cannot.
For zinc, being a metal that actively participates in reactions, it reacts with acids by displacing hydrogen ions, hence its oxidizing behavior. It forms stronger bonds with chlorine to produce zinc chloride (ZnCl₂), while the hydrogen ions pair up to form H₂ gas, showing the conversion from ionic to gaseous states.
Understanding such chemical reactions allows us to see how electron exchange drives the transformation of substances, showcasing real-world applications like the corrosion of metals.

• Zinc and acid interaction
• Formation of ZnCl₂ and H₂
• Real-world applications

Therefore, comprehending these basic chemical reactions provides insights into everyday chemical processes that occur around us.