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C. Chemical Reactions · Chemical Reactions

Predicting Products: Putting It All Together

C2 C3About 70 minutes
Learning goals
  • I can identify the reaction type from the reactants alone.
  • I can choose the right tool (ion charges, activity series, halogen series or solubility table) to predict products.
  • I can predict and balance the products of any reaction from this unit, or decide that there is no reaction.
  • I can explain my prediction in words, including what would be observed.

Words to know

prediction
A statement of what products will form, based on the reaction type and chemical rules, made before the reaction is done.
NR
No reaction: the reactants do not change, because the free element is less reactive, or because no precipitate, gas or water can form.
word equation
An equation that uses names instead of formulas, such as "zinc + hydrochloric acid → zinc chloride + hydrogen".

From reactants to products

Up to now, each lesson focused on one reaction type, so you always knew which rules to use. On a test, in a lab, or in the next chemistry course, you will see only the reactants and must decide everything yourself. This lesson puts all the rules together into one system and gives you lots of mixed practice.

Think of it like hockey: you have practised passing, shooting and skating separately. Now it is game time.

Look at the REACTANTStwo elementssynthesisone compounddecompositionelement + compoundsingle displ.two ionic compoundsdouble displ.fuel + O₂combustionmetal + non-metal→ ionic compoundcarbonate, hydrate,chlorate, binary...higher one?otherwise NR(s), gas or water?otherwise NRplenty: CO₂ + H₂Olimited: CO, COxides with water: metal oxide → base; non-metal oxide → acid (synthesis).Then: write correct formulas from charges, add states, balance, check.
Start from the reactants: their pattern tells you the reaction type, and each type has its own rule. After choosing the products, build every formula from ion charges, add states and balance.

The five-question routine

For every prediction, ask:

  1. What are the reactants? Write correct formulas (check ion charges and diatomic elements).
  2. What pattern do they make? Two elements; one compound; element + compound; two ionic compounds; fuel + oxygen; an oxide + water.
  3. Which rule or table applies? Synthesis families, decomposition families, activity or halogen series, solubility table, gas-forming patterns, neutralization, combustion.
  4. Does it actually happen? In single and double displacement, the answer can be NR.
  5. Write, balance and check the equation, with states. Then describe what you would see.
Ion chargesevery formulastarts hereActivity seriesmetals in singledisplacementHalogen seriesF₂ > Cl₂ > Br₂ > I₂Solubility tableprecipitateor NR?Your prediction toolboxKeep all four beside you for every prediction question.
The four tools for predicting products. Almost every prediction needs ion charges; displacement reactions also need a series or the solubility table.
Worked example

Mixed prediction 1

Predict: a piece of aluminium foil is placed in aqueous copper(II) sulfate.

  1. Pattern: element (Al) + compound (CuSO₄) → single displacement.
  2. Tool: activity series. Aluminium is above copper, so the reaction happens.
  3. Products: aluminium sulfate, Al₂(SO₄)₃ (Al³⁺ with SO₄²⁻), and copper metal.
  4. Balance: 3 sulfate on the right → 3CuSO₄ → 3Cu; 2 Al.
  5. Observation: reddish-brown copper forms on the foil; the blue solution fades (it can be slow at first because of aluminium's oxide layer).

2Al(s) + 3CuSO₄(aq) → Al₂(SO₄)₃(aq) + 3Cu(s)

Worked example

Mixed prediction 2

Predict: solutions of ammonium phosphate and calcium nitrate are mixed.

  1. Pattern: two ionic compounds in solution → double displacement.
  2. Swap: Ca²⁺ with PO₄³⁻ → Ca₃(PO₄)₂; NH₄⁺ with NO₃⁻ → NH₄NO₃.
  3. Solubility: phosphates have low solubility (except group 1 and ammonium), so Ca₃(PO₄)₂(s). Ammonium nitrate is soluble.
  4. Balance: 3 Ca → 3Ca(NO₃)₂; 2 phosphate → 2(NH₄)₃PO₄; then 6 ammonium and 6 nitrate → 6NH₄NO₃.

2(NH₄)₃PO₄(aq) + 3Ca(NO₃)₂(aq) → Ca₃(PO₄)₂(s) + 6NH₄NO₃(aq). A white precipitate forms.

Worked example

Mixed prediction 3

Predict: solid potassium chlorate is heated strongly.

  1. Pattern: one compound + heat → decomposition.
  2. Class: a metal chlorate → metal chloride + oxygen.
  3. Balance: 2KClO₃ → 2KCl + 3O₂.
  4. Observation: the solid melts and gives off a gas that relights a glowing splint.

2KClO₃(s) → 2KCl(s) + 3O₂(g)

Worked example

Mixed prediction 4

Predict: iodine is added to aqueous sodium chloride.

  1. Pattern: element (I₂) + compound (NaCl) → single displacement (halogens).
  2. Tool: halogen series. Iodine is below chlorine, so it cannot displace chloride.

NR. The solution keeps the light brown colour of the dissolved iodine; no new substance forms.

Hands-on activity

Prediction card game

You need: 20 small cards or slips of paper, a pen, a timer, and a partner (a family member can be the "referee" using the answer key below).

  1. Write each of these reactant sets on a card (one per card): calcium + oxygen; heating lithium carbonate; zinc + copper(II) nitrate solution; copper + zinc nitrate solution; chlorine + potassium iodide solution; silver nitrate + potassium bromide solutions; sodium carbonate + nitric acid; potassium hydroxide + sulfuric acid; propane + plenty of oxygen; carbon dioxide + water. Make 10 more cards of your own from this unit's lessons (with answers on the back).
  2. Shuffle. Draw a card and set a 2-minute timer. Name the reaction type, predict the products, and write the balanced equation (or NR).
  3. Your partner checks against the key. Score 1 point for the type, 1 for correct products, 1 for correct balancing.

Answer key for the first ten cards:

  • calcium + oxygen: 2Ca(s) + O₂(g) → 2CaO(s) (synthesis)
  • heating lithium carbonate: Li₂CO₃(s) → Li₂O(s) + CO₂(g) (decomposition)
  • zinc + copper(II) nitrate solution: Zn(s) + Cu(NO₃)₂(aq) → Zn(NO₃)₂(aq) + Cu(s) (single displacement)
  • copper + zinc nitrate solution: NR (copper is below zinc)
  • chlorine + potassium iodide solution: Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(aq) (single displacement)
  • silver nitrate + potassium bromide solutions: AgNO₃(aq) + KBr(aq) → AgBr(s) + KNO₃(aq) (double displacement)
  • sodium carbonate + nitric acid: Na₂CO₃(aq) + 2HNO₃(aq) → 2NaNO₃(aq) + H₂O(l) + CO₂(g) (double displacement, gas)
  • potassium hydroxide + sulfuric acid: 2KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(l) (neutralization)
  • propane + plenty of oxygen: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g) (combustion)
  • carbon dioxide + water: CO₂(g) + H₂O(l) → H₂CO₃(aq) (synthesis)

What to look for: quick, correct identification of the type before any writing. If one type keeps costing points, revisit that lesson.

Just for fun

Riddle: what do you call a reaction that refuses to show up?

An NR. Chemists are very polite about it: they do not write "rude" or "boring", just "no reaction".

Your turn

1

Predict and balance (or NR): magnesium + nitrogen.

2

Predict and balance (or NR): lead + hydrochloric acid.

3

Predict and balance (or NR): silver + copper(II) nitrate solution.

4

Predict and balance (or NR): sodium sulfide + copper(II) chloride solutions.

5

Predict and balance (or NR): heating nickel(II) hydroxide.

6

Predict and balance (or NR): complete combustion of heptane, C₇H₁₆.

7

Predict and balance (or NR): potassium chloride + sodium nitrate solutions.

More practice: warm-up

1

Element + compound: which type?

2

Two aqueous ionic compounds: which type?

3

Which tool decides whether a double displacement makes a precipitate?

4

Which tool decides whether a metal displaces another metal?

5

Predict: H₂(g) + Cl₂(g) →

More practice: on your own

1

Predict and balance: lithium + water.

2

Predict and balance: aqueous iron(III) nitrate + aqueous sodium hydroxide.

3

Predict and balance: barium oxide + carbon dioxide.

4

Predict and balance: hydrochloric acid + solid calcium hydrogen carbonate.

5

Predict and balance: electrolysis of molten potassium bromide.

Challenge

1

A student mixes solutions of sodium carbonate and calcium chloride, filters the white solid, and then adds hydrochloric acid to the solid. Write the two equations and describe the observations.

2

Predict the products of chlorine gas bubbled into aqueous magnesium bromide, and then explain what would happen if the resulting solution were mixed with silver nitrate.

3

Iron reacts with chlorine gas to form iron(III) chloride, but iron reacts with hydrochloric acid to form iron(II) chloride. Write both equations and classify them. What does this tell you about predicting products of multivalent metals?

Check your understanding

Quiz

1.

What is the first thing to look at when predicting products?

2.

What forms when zinc is placed in silver nitrate solution?

3.

Mixing aqueous K₂SO₄ and NaCl gives:

4.

What are the products of heating Mg(OH)₂?

5.

Which reaction gives NR?

6.

The coefficient of O₂ in the complete combustion of C₄H₁₀ (lowest whole numbers) is:

For parents and teachers

This is a consolidation lesson. If your teen does well here, the unit review will be quick.

How to help: play the card game with them; you only need the answer key in the activity. Insist on the routine: name the type out loud before writing products.

Common difficulties:

  • Forgetting to check for NR in displacement reactions.
  • Writing products before choosing the type, which leads to strange mixtures.
  • Multivalent metals: if the question does not say, it is fair for your teen to ask which ion forms.

If one reaction type causes most errors, go back to that lesson's worked examples.