Precipitation Reactions

Equations and Reaction Types

What is a precipitation reaction?

A reaction in which an insoluble solid is formed when two solutions are mixed.

What is a precipitate?

An insoluble solid formed during a reaction in solution.

What state symbol is used for a precipitate?

(s).

What are precipitation reactions commonly used for?

Chemical tests for identifying ions and working out chemical equations.

What is the test for carbon dioxide?

Bubble the gas through calcium hydroxide solution (limewater).

What is limewater?

Calcium hydroxide solution.

What happens when carbon dioxide is bubbled through limewater?

A white precipitate forms and the limewater turns milky/cloudy.

What is the white precipitate formed in the carbon dioxide test?

Calcium carbonate, CaCO₃.

What is the equation for carbon dioxide reacting with limewater?

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

Why does limewater turn cloudy when carbon dioxide is added?

Insoluble calcium carbonate forms as a white precipitate.

How can sulfate ions be tested for?

Add barium ions, usually using barium chloride or barium nitrate solution.

What is the positive result for sulfate ions?

A white precipitate forms.

What precipitate forms in the test for sulfate ions?

Barium sulfate, BaSO₄.

Why does a precipitate form in the sulfate test?

Barium sulfate is insoluble.

What is the full equation for sodium sulfate reacting with barium chloride?

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).

What is the simplest ionic equation for the sulfate test?

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

Which ions actually react in the sulfate test?

Ba²⁺ and SO₄²⁻ ions.

How can halide ions in solution be detected?

Add silver ions using silver nitrate solution.

What forms when silver ions react with halide ions?

An insoluble silver halide precipitate.

What happens when silver nitrate is added to sodium chloride solution?

A precipitate of silver chloride forms.

What is the formula of silver chloride?

AgCl.

What is the full equation for sodium chloride reacting with silver nitrate?

NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq).

What is the simplest ionic equation for the chloride test?

Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

Which ions actually react to form silver chloride?

Ag⁺ and Cl⁻ ions.

What happens when aqueous lead nitrate and potassium iodide are mixed?

A yellow precipitate of lead iodide forms.

What colour are lead nitrate and potassium iodide solutions before they react?

Colourless.

What is the precipitate produced from lead nitrate and potassium iodide?

Lead iodide, PbI₂.

What colour is lead iodide precipitate?

Yellow.

What is the word equation for lead nitrate reacting with potassium iodide?

Lead nitrate + potassium iodide → lead iodide + potassium nitrate.

What is the balanced equation for lead nitrate reacting with potassium iodide?

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).

What is the simplest ionic equation for the formation of lead iodide?

Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).

Which ions form the lead iodide precipitate?

Pb²⁺ and I⁻ ions.

Which ions are spectator ions in the lead nitrate and potassium iodide reaction?

K⁺ and NO₃⁻ ions.

How can a precipitation experiment be used to determine the reacting ratio?

Vary the amount of one reactant while keeping the other constant and measure the amount of precipitate formed.

Why is the same volume of potassium iodide used in each test tube in the experiment?

To keep the amount of potassium iodide constant.

What is changed between the test tubes in the lead nitrate–potassium iodide experiment?

The volume of lead nitrate solution.

Why are the test tubes centrifuged for the same length of time?

To allow a fair comparison of the depth of precipitate.

What does the depth of precipitate indicate?

The amount or mass of precipitate formed.

What happens to the amount of PbI₂ precipitate as more lead nitrate is initially added?

It increases.

Why does the amount of precipitate eventually stop increasing?

All of the potassium iodide has been used up, so it becomes the limiting reactant.

What is the reacting mole ratio of lead nitrate to potassium iodide?

1:2.

How many moles of iodide ions react with one mole of Pb²⁺ ions?

2 mol.

How many moles of PbI₂ form from one mole of Pb²⁺ when iodide is in excess?

1 mol.

What equation is used to calculate amount in moles from concentration and volume?

n = cV.

What units must volume have when using n = cV with concentration in mol dm⁻³?

dm³.

How do you convert cm³ to dm³?

Divide by 1000.

How many dm³ is 5.0 cm³?

0.0050 dm³.

How many moles are in 5.0 cm³ of a 1.0 mol dm⁻³ solution?

0.0050 mol.

How many moles are in 2.5 cm³ of a 1.0 mol dm⁻³ solution?

0.0025 mol.

What mole ratio is shown by 0.0025 mol lead nitrate reacting with 0.0050 mol potassium iodide?

1:2.