The Reactions of Acids with Metals

Common Acids and the Role of Hydrogen

• There are three primary acids used extensively in the study of chemistry:

  • Hydrochloric acid: $HCl$

  • Sulfuric acid: $H_2SO_4$

  • Nitric acid: $HNO_3$

• A defining characteristic of all acids is that they contain hydrogen in their chemical structure.

The Reactivity Series and Hydrogen Displacement

• The reactivity series is a list of metals ordered by their reactivity.

• Metals that are positioned higher in the reactivity series than hydrogen have the ability to displace hydrogen from acids.

• Displacement involves the more reactive metal "pushing out" the hydrogen from the acid molecule during a chemical reaction.

• Metals lower than hydrogen in the reactivity series will not react with acids in this manner.

General Products of Acid-Metal Reactions

• When an acid reacts with a metal, two distinct products are always formed:

  • A salt: This is a general term for a chemical substance formed in these reactions, such as magnesium sulfate ($MgSO_4$).

  • Hydrogen gas: Represented chemically as $H_2$.

• The specific identity of the salt produced depends entirely on the combination of the acid used and the metal involved.

Salt Naming Conventions Based on Acid Type

• The type of acid used determines the suffix of the resulting salt:

  • Hydrochloric acid ($HCl$) always produces salts ending in the word chloride.

  • Sulfuric acid ($H_2SO_4$) always produces salts ending in the word sulfate.

Specific Reaction Case Studies: Magnesium, Zinc, and Iron

Magnesium ($Mg$)

• Reactivity Profile: Magnesium is significantly more reactive than hydrogen. Consequently, it reacts rapidly with acids because it can easily displace hydrogen.

• Reaction with Hydrochloric Acid:

  • Word Equation: $\text{Magnesium} + \text{Hydrochloric acid} \rightarrow \text{Magnesium chloride} + \text{Hydrogen}$

  • Symbol Equation: $Mg + 2HCl \rightarrow MgCl_2 + H_2$

• Reaction with Sulfuric Acid:

  • Word Equation: $\text{Magnesium} + \text{Sulfuric acid} \rightarrow \text{Magnesium sulfate} + \text{Hydrogen}$

  • Symbol Equation: $Mg + H_2SO_4 \rightarrow MgSO_4 + H_2$

Zinc ($Zn$)

• Reactivity Profile: Zinc is more reactive than hydrogen, leading to a quite rapid reaction, though typically less vigorous than magnesium.

• Reaction with Hydrochloric Acid:

  • Produces the salt zinc chloride and hydrogen gas.

  • Symbol Equation: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

• Reaction with Sulfuric Acid:

  • Produces the salt zinc sulfate and hydrogen gas.

  • Symbol Equation: $Zn + H_2SO_4 \rightarrow Zn_SO_4 + H_2$

Iron ($Fe$)

• Reactivity Profile: Iron is only slightly more reactive than hydrogen. Because the difference in reactivity is small, the reaction proceeds fairly slowly.

• Reaction with Hydrochloric Acid:

  • Produces the salt iron chloride and hydrogen gas.

  • Symbol Equation: $Fe + 2HCl \rightarrow FeCl_2 + H_2$

• Reaction with Sulfuric Acid:

  • Produces the salt iron sulfate and hydrogen gas.

  • Symbol Equation: $Fe + H_2SO_4 \rightarrow FeSO_4 + H_2$

Higher Tier Chemistry Objectives

• Higher tier students are expected to move beyond identifying products to analyzing these reactions in terms of electron transfer.

• This involves identifying which chemicals are oxidized and which are reduced.

• In the context of displacement reactions between metals and acids, the metal atoms typically lose electrons (oxidation) while the hydrogen ions from the acid gain electrons (reduction) to form hydrogen gas.