Acids, Bases and Salts · Lesson 2 of 12
How Do Acids And Bases React With Metals?
“Add the right metal to acid and hydrogen literally pops in to say hello.”
• Describe the reaction of acids with suitable metals. • Identify hydrogen gas using the characteristic pop test. • Explain how a metal displaces hydrogen from an acid. • Write and balance equations for metal-acid reactions. • Explain the reaction of zinc with sodium hydroxide and recognise that not all metals react similarly with bases.
When certain metals are placed in dilute acids, bubbles appear almost immediately. These bubbles are not trapped air. A new gas is being formed because the metal is chemically reacting with the acid.
Zinc Reacting With Dilute Sulphuric Acid
When zinc granules are added to dilute sulphuric acid, gas bubbles form on the metal surface. The gas can be passed through soap solution so that it becomes trapped inside soap bubbles. Bringing a flame near a gas-filled bubble produces a characteristic pop sound.
Hydrogen gas burns rapidly when ignited and produces a characteristic pop sound. This is commonly used as a test for hydrogen.
The metal replaces hydrogen present in the acid. Zinc becomes part of zinc sulphate, while hydrogen leaves the acid and is released as hydrogen gas. The compound formed along with hydrogen is a salt.
A Salt is an ionic compound formed when the replaceable hydrogen of an acid is substituted by a metal or another positive ion.
Other Acids And Metals
Similar observations can be made when suitable metals react with acids such as hydrochloric acid and acetic acid. The reaction with nitric acid can behave differently because nitric acid has strong oxidising properties, so the simple hydrogen-producing pattern does not apply universally.
Balancing Metal-Acid Equations
A chemical equation must contain the same number of atoms of each element on both sides. The formula of a substance must never be changed during balancing. Only coefficients placed before complete formulae may be adjusted.
Problem
Balance Mg + HCl → MgCl₂ + H₂.
- 1.Write the correct formulae first: Mg + HCl → MgCl₂ + H₂.
- 2.Magnesium is already balanced with one atom on each side.
- 3.The product MgCl₂ contains two chlorine atoms, but HCl supplies only one chlorine atom per molecule.
- 4.Place a coefficient of two before HCl.
- 5.The equation becomes Mg + 2HCl → MgCl₂ + H₂.
- 6.There are now two hydrogen atoms on the left and two hydrogen atoms in H₂ on the right.
- 7.All atoms are balanced.
Problem
Balance Al + H₂SO₄ → Al₂(SO₄)₃ + H₂.
- 1.Keep all formulae unchanged.
- 2.Al₂(SO₄)₃ contains two aluminium atoms, so place a coefficient of two before Al.
- 3.The product contains three sulphate groups. Place a coefficient of three before H₂SO₄.
- 4.Three sulphuric acid molecules contain six hydrogen atoms.
- 5.Place a coefficient of three before H₂ to give six hydrogen atoms on the product side.
- 6.Check aluminium, sulphur, oxygen and hydrogen.
- 7.The balanced equation is 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂.
Problem
Balance Fe + HCl → FeCl₂ + H₂.
- 1.Iron is already balanced with one Fe atom on each side.
- 2.FeCl₂ contains two chlorine atoms.
- 3.Place a coefficient of two before HCl.
- 4.The left side now contains two hydrogen atoms.
- 5.Those two hydrogen atoms form one H₂ molecule on the product side.
- 6.The balanced equation is Fe + 2HCl → FeCl₂ + H₂.
- 7.Always finish by recounting every element.
Reaction Of Zinc With Sodium Hydroxide
Zinc can also react with sodium hydroxide when warmed. Hydrogen gas is again produced, but instead of an ordinary metal salt such as zinc chloride, sodium zincate is formed.
The reaction of a metal with a base is not a universal reaction for all metals. Only certain metals show this behaviour under suitable conditions.
Quiz
Which gas is usually produced when a suitable metal reacts with a dilute acid?
Which observation confirms the presence of hydrogen gas?
What are the products when zinc reacts with dilute sulphuric acid?
Which coefficient correctly balances HCl in Mg + HCl → MgCl₂ + H₂?
Practice Problems
- Write and balance the equation for magnesium reacting with hydrochloric acid.
- Write and balance the equation for zinc reacting with dilute sulphuric acid.
- Balance Al + H₂SO₄ → Al₂(SO₄)₃ + H₂ and explain each coefficient you introduce.
- Describe how you would identify the gas produced when zinc reacts with dilute sulphuric acid.
- Explain why the reaction of metals with bases cannot be written as a universal rule for all metals.
Key Takeaways
• Suitable metals react with dilute acids to form a salt and hydrogen gas. • Hydrogen gas can be identified by its characteristic pop sound on ignition. • During a metal-acid reaction, the metal displaces hydrogen from the acid. • Chemical equations are balanced by changing coefficients, not chemical formulae. • Zinc can react with sodium hydroxide to form sodium zincate and hydrogen.