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C. Chemical Reactions · Chemical Reactions

Double Displacement: Gas Formation and Neutralization

C2 C3 A1.5 A1.6About 70 minutes
Learning goals
  • I can predict when a double displacement produces a gas (carbon dioxide, sulfur dioxide, hydrogen sulfide or ammonia).
  • I can predict the salt and water formed in a neutralization reaction and write the balanced equation.
  • I can explain neutralization using hydrogen ions and hydroxide ions.
  • I can describe everyday neutralization reactions, such as antacids and baking.

Words to know

acid
A compound that produces hydrogen ions, H⁺, when dissolved in water. Acids taste sour (never taste lab chemicals) and turn blue litmus red.
base
A compound that produces hydroxide ions, OH⁻, in water. Bases feel slippery (never touch lab chemicals) and turn red litmus blue.
neutralization
An acid reacts with a base to form a salt and water.
salt
An ionic compound made of the cation from a base and the anion from an acid, such as NaCl, K₂SO₄ or CaCl₂.
carbonic acid
H₂CO₃, an unstable acid that quickly decomposes into water and carbon dioxide.
indicator
A substance that changes colour depending on whether a solution is acidic, neutral or basic, such as litmus or red cabbage juice.

Fizz and neutral

Drop an antacid tablet into water and it fizzes. Mix vinegar with baking soda and the foam spills over the cup. Swallow an antacid for heartburn and stomach acid is neutralized. All of these are double displacement reactions, but instead of a precipitate, they produce a gas or water.

This lesson completes the double displacement family. Together with the last lesson, you will be able to predict all three outcomes: precipitate, gas or water.

Gas-forming reactions

Some double displacement reactions make a compound that is unstable and immediately breaks down, releasing a gas. Learn these four patterns:

  1. Carbonate (or hydrogen carbonate) + acid → salt + water + carbon dioxide. The swap makes carbonic acid, H₂CO₃, which decomposes into H₂O and CO₂.
  • CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
  • NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
  1. Sulfite + acid → salt + water + sulfur dioxide. The swap makes H₂SO₃, which decomposes into H₂O and SO₂.
  • Na₂SO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g)
  1. Sulfide + acid → salt + hydrogen sulfide. H₂S is a toxic gas that smells like rotten eggs.
  • FeS(s) + 2HCl(aq) → FeCl₂(aq) + H₂S(g)
  1. Ammonium compound + strong base → salt + water + ammonia. The swap makes "NH₄OH", which gives off ammonia gas, NH₃, and water.
  • NH₄Cl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) + NH₃(g)
carbonate + acidNa₂CO₃ + 2HClsalt + carbonic acid2NaCl + H₂CO₃decomposesH₂O + CO₂(g)swapfastOverall: Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)The unstable product breaks down, so a gas bubbles out of the solution.
How a carbonate and an acid make a gas: the partner swap forms carbonic acid, which immediately decomposes into water and carbon dioxide. Write the overall equation with H₂O and CO₂, not H₂CO₃.

Neutralization: acid + base → salt + water

In water, an acid releases hydrogen ions (H⁺) and a base releases hydroxide ions (OH⁻). When they mix, H⁺ and OH⁻ combine to form water, a molecular compound that hardly breaks apart. The other two ions form a salt, which may stay dissolved or precipitate.

To predict the products:

  1. The salt = the cation of the base + the anion of the acid. Build the formula from the charges.
  2. The other product is always water.
  3. Balance. A good check: the number of H⁺ from the acid equals the number of OH⁻ from the base, and equals the number of water molecules.

Examples:

  • HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
  • H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)
  • 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)

Neutralization is exothermic: the solution warms up. In the Solutions and Solubility unit you will define acids and bases more carefully (the Arrhenius theory) and measure them with titrations.

H⁺Cl⁻+OH⁻Na⁺OHHH₂O(l)Na⁺Cl⁻spectators (a saltstays dissolved)acid + base → salt + waterNet change: H⁺(aq) + OH⁻(aq) → H₂O(l)
Neutralization at the particle level. Hydrogen ions from the acid and hydroxide ions from the base form water; the sodium and chloride ions are spectators that make up the salt.
01234567891011121314add baseadd acid
The pH scale (from Grade 10). Vinegar is near pH 2 to 3, pure water is pH 7, and a household ammonia cleaner is near pH 11 to 12. Neutralization moves a solution toward pH 7.
Worked example

A carbonate and an acid

Predict the products and balance: solid magnesium carbonate + hydrochloric acid.

  1. Swap: Mg²⁺ with Cl⁻ → MgCl₂; H⁺ with CO₃²⁻ → H₂CO₃.
  2. H₂CO₃ is unstable: write it as H₂O + CO₂.
  3. Balance: 2 chloride are needed, so 2HCl.

MgCO₃(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) + CO₂(g). Bubbles of carbon dioxide form.

Worked example

An ammonium compound and a base

Predict and balance: aqueous ammonium sulfate + aqueous sodium hydroxide (warmed).

  1. Swap: Na⁺ with SO₄²⁻ → Na₂SO₄; NH₄⁺ with OH⁻ → "NH₄OH" → NH₃ + H₂O.
  2. There are 2 ammonium ions in (NH₄)₂SO₄, so 2NaOH, 2NH₃ and 2H₂O.
  3. Check: N 2 = 2, H 8 + 2 = 6 + 4, Na 2 = 2, S 1, O 4 + 2 = 4 + 2. ✔

(NH₄)₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2NH₃(g) + 2H₂O(l). The ammonia can be detected by its sharp smell (waft carefully, never sniff directly) or by damp red litmus turning blue.

Worked example

Neutralization with a multivalent ratio

Write the neutralization of phosphoric acid by calcium hydroxide.

  1. Salt: Ca²⁺ (from the base) and PO₄³⁻ (from the acid) → Ca₃(PO₄)₂.
  2. 3 Ca need 3Ca(OH)₂; 2 phosphate need 2H₃PO₄.
  3. H⁺ count: 2 × 3 = 6. OH⁻ count: 3 × 2 = 6. So 6H₂O.

2H₃PO₄(aq) + 3Ca(OH)₂(aq) → Ca₃(PO₄)₂(s) + 6H₂O(l). (Calcium phosphate has low solubility, so it precipitates as well.)

Worked example

An antacid

Milk of magnesia contains magnesium hydroxide. Stomach acid is mostly hydrochloric acid. Write the reaction.

  1. Acid + base → salt + water. Salt: Mg²⁺ with Cl⁻ → MgCl₂.
  2. Mg(OH)₂ has 2 OH⁻, so 2HCl and 2H₂O.

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

Hands-on activity

Red cabbage indicator: neutralize vinegar drop by drop

You need: a quarter of a red cabbage, a pot or kettle, a strainer, white vinegar, baking soda, three clear glasses, a medicine dropper or a teaspoon, a measuring spoon, and a grown-up for the hot water.

Safety: a grown-up handles the boiling water. Everything here is food-safe, but this is an experiment, so do not drink the mixtures.

  1. Chop the cabbage, cover it with boiling water and let it steep for 15 minutes. Strain. The purple liquid is your indicator (store leftovers in the fridge).
  2. Put 50 mL of indicator in glass 1 (your neutral reference). Note the colour.
  3. Put 50 mL of indicator in glass 2 and add 1 teaspoon of vinegar. It turns pink-red (acidic).
  4. In glass 3, dissolve 1 teaspoon of baking soda in 50 mL of water.
  5. Add the baking soda solution to glass 2 one dropper-full at a time, swirling and counting. Watch for fizzing (CO₂) and for the colour to return to purple like glass 1. Keep going and it turns blue or blue-green (basic).
  6. Repeat with fresh vinegar to check your count.

Analysis: the reaction is NaHCO₃(aq) + CH₃COOH(aq) → NaCH₃COO(aq) + H₂O(l) + CO₂(g) (sodium acetate is the salt). How many dropper-fulls did it take to reach purple each time? Were your two trials close? This is a simple version of a titration, which you will do with real lab equipment in the Solutions and Solubility unit.

Just for fun

Every classic "baking soda and vinegar volcano" is really a gas-forming double displacement. Its "lava" is carbon dioxide foam in a solution of sodium acetate, the same salt used in some reusable hand warmers. Possibly the least dangerous volcano on Earth.

Your turn

1

Predict and balance: K₂CO₃(aq) + HNO₃(aq) →

2

Predict and balance: HCl(aq) + KOH(aq) →

3

Predict and balance: H₂SO₄(aq) + NaOH(aq) →

4

Predict and balance: Na₂S(aq) + HCl(aq) →

5

Predict and balance: NH₄NO₃(aq) + KOH(aq) →

6

Name the salt formed when nitric acid neutralizes aluminium hydroxide, and write its formula.

More practice: warm-up

1

What are the two products of every neutralization?

2

What gas forms when a carbonate reacts with an acid?

3

Which ion do acids produce in water?

4

Which ion do bases like NaOH produce in water?

5

Why do we write H₂O + CO₂ instead of H₂CO₃ as products?

More practice: on your own

1

Predict and balance: calcium carbonate (limestone) + sulfuric acid.

2

Predict and balance: barium hydroxide + hydrochloric acid.

3

Predict and balance: sodium sulfite + sulfuric acid.

4

An unknown white powder fizzes with vinegar. A student concludes that it must be baking soda. Is this a valid conclusion? Explain.

5

Write the net ionic equation for any neutralization between a strong acid and a strong base such as NaOH, and explain why it is the same each time.

Challenge

1

Sodium hydrogen carbonate is often kept near acid storage in labs to clean up spills. Write the equation for it reacting with a hydrochloric acid spill, and give two reasons it is a better choice than sodium hydroxide.

2

A student mixes 2 drops of an ammonium chloride solution with sodium hydroxide solution in a test tube, warms it gently and holds damp red litmus paper at the mouth. Predict the observation and write the equation.

3

Predict all products for aqueous barium hydroxide + sulfuric acid. Explain why this neutralization is unusual and how you could show the ions are being removed.

Check your understanding

Quiz

1.

The products of an acid + base neutralization are:

2.

What forms when sodium carbonate reacts with hydrochloric acid?

3.

Which gas forms when an ammonium compound is warmed with sodium hydroxide?

4.

What is the salt in H₂SO₄ + 2KOH?

5.

How many water molecules form in the balanced equation for H₃PO₄ + NaOH?

6.

Which gas, released when a sulfide reacts with acid, is toxic and smells like rotten eggs?

For parents and teachers

This lesson finishes double displacement. Your teen should now be able to look at two ionic reactants and say precipitate, gas, water or no reaction.

How to help: the red cabbage activity is safe, colourful and a great conversation starter. Ask your teen to explain what each colour means and why fizzing stops at the neutral point.

Common difficulties:

  • Writing H₂CO₃, H₂SO₃ or NH₄OH as final products instead of their decomposition products.
  • Getting the salt formula wrong. Remind them: cation from the base, anion from the acid, then balance charges.
  • Not matching the number of H⁺ and OH⁻.

Safety: boiling water for the cabbage is for a grown-up. Never taste lab chemicals; the sour and slippery descriptions in textbooks are general properties, not instructions.