Metals and Non-metals · Lesson 5 of 13
What Happens When Metals React With Acids?
“Acids release hydrogen with reactive metals, except when nitric acid decides to complicate things.”
• State the general reaction between reactive metals and dilute acids. • Compare the observed reactivity of magnesium, aluminium, zinc, iron and copper with dilute hydrochloric acid. • Explain the special behaviour of nitric acid. • Explain why copper does not produce hydrogen with dilute hydrochloric acid. • Describe aqua regia using the composition and properties given in the chapter.
Dilute acids provide another way to compare reactivity. A sufficiently reactive metal can displace hydrogen from a dilute acid, producing a salt and hydrogen gas.
What Happens When Metals React With Acids?
Comparing Reactions With Dilute Hydrochloric Acid
Bubble formation shows hydrogen production and temperature rise shows that the reaction is exothermic. The observed reactivity decreases in the order Mg > Al > Zn > Fe. Copper shows no bubbles and no significant temperature change with dilute hydrochloric acid.
Why Copper Does Not React With Dilute Hydrochloric Acid
Copper lies below hydrogen in the reactivity series and therefore does not displace hydrogen from dilute hydrochloric acid.
Nitric Acid
Nitric acid is an important exception. Hydrogen is generally not evolved because nitric acid is a strong oxidising agent. It oxidises the hydrogen that would otherwise form to water and is itself reduced to nitrogen oxides. Magnesium and manganese can evolve hydrogen with very dilute nitric acid.
Do not automatically predict hydrogen whenever a metal reacts with an acid. Nitric acid usually prevents hydrogen evolution because of its oxidising action.
Aqua Regia
Aqua Regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio three to one.
Aqua regia is highly corrosive and fuming. It can dissolve gold and platinum.
Problem
Copper and magnesium are separately added to dilute hydrochloric acid. Predict the observations.
- 1.Magnesium is above hydrogen and can displace it.
- 2.Hydrogen bubbles form with magnesium and heat is released.
- 3.Copper is below hydrogen.
- 4.Copper does not displace hydrogen, so no hydrogen bubbles are expected.
Quiz
What gas is generally produced when a reactive metal reacts with dilute hydrochloric acid?
What is the observed decreasing order of reactivity?
Why is hydrogen generally not evolved with nitric acid?
Which metal does not react with dilute hydrochloric acid in the comparison?
What is aqua regia?
Practice Problems
- Write balanced equations for magnesium, aluminium, zinc and iron reacting with dilute hydrochloric acid.
- Explain how bubble formation and temperature rise can compare metal reactivity.
- Use Mg > Al > Zn > Fe to compare any two of these metals.
- Explain why copper does not evolve hydrogen with dilute hydrochloric acid.
- Explain the nitric acid exception.
- Describe aqua regia, including composition and the metals mentioned that it can dissolve.
Key Takeaways
• Reactive metals can react with dilute acids to form salts and hydrogen. • The observed reactivity with dilute hydrochloric acid decreases as Mg > Al > Zn > Fe. • Copper cannot displace hydrogen from dilute hydrochloric acid. • Nitric acid usually does not evolve hydrogen with metals because it is a strong oxidising agent. • Aqua regia is a corrosive mixture of concentrated hydrochloric and nitric acids that can dissolve gold and platinum.