SCH3U · Unit 2 test review

Chemical Reactions: Test Review

Everything for the Unit 2 test on one page: the key facts from all 7 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

Writing & Balancing Chemical Equations

Word equation (names) → skeleton equation (formulas) → balanced equation (coefficients added).
Law of conservation of mass: atoms are conserved — the same count of each element on both sides.
Only coefficients may be changed to balance an equation; subscripts define the substance and must never change.
Balancing order that usually works well: metals, then other nonmetals, then hydrogen, then oxygen last.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).

Synthesis & Decomposition Reactions

Synthesis: $A + B \to A B$ — multiple reactants combine into a single product.
Decomposition: $A B \to A + B$ — a single reactant breaks into multiple products.
Metal oxide + water → metal hydroxide (base); nonmetal oxide + water → acid.
Metal carbonates decompose (with heat) into a metal oxide + carbon dioxide gas.
Metal hydroxides decompose (with heat) into a metal oxide + water.

Combustion Reactions

Complete combustion (excess O₂): hydrocarbon + O₂ → CO₂ + H₂O.
Incomplete combustion (limited O₂): hydrocarbon + O₂ → CO + H₂O, and possibly soot (C) in severe oxygen shortage.
Carbon monoxide (CO) is toxic because it binds to hemoglobin more strongly than oxygen, blocking oxygen transport in the blood.
A blue flame indicates efficient, complete combustion; a yellow/orange sooty flame indicates incomplete combustion.
Combustion reactions release energy as heat and light.

Single Displacement Reactions

Single displacement: $A + BC \to A C + B$ , but only if A is more reactive than B.
The activity series ranks metals from most to least reactive; a metal can only displace a less reactive metal below it.
Metals above hydrogen in the activity series react with acids to release H₂ gas; metals below hydrogen do not react with acids.
HOFBrINCl elements (H₂, O₂, F₂, Br₂, I₂, N₂, Cl₂) exist as diatomic molecules in their elemental form.
Halogen reactivity order: F₂ > Cl₂ > Br₂ > I₂ (decreases down the group, opposite to metals).

Double Displacement Reactions

Double displacement: $A B + C D \to A D + CB$ — cations and anions trade partners.
A double displacement reaction "happens" (is worth writing) only if a precipitate, gas, or water forms.
Group 1 and ammonium compounds are always soluble; nitrates and acetates are always soluble.
Silver, lead(II), and mercury(I) halides (Cl⁻, Br⁻, I⁻) are the classic insoluble exceptions to the halide rule.
Carbonates, phosphates, and hydroxides are insoluble except with Group 1 metals and ammonium.
Neutralization, acid + base → salt + water, is a double displacement driven by $H^{+}$ and $OH^{-}$ forming water.

Total & Net Ionic Equations

Formula equation → total ionic equation (dissociate all soluble aqueous ionic compounds) → net ionic equation (cancel spectator ions).
Only (aq) ionic compounds are split into ions; (s), (l), and (g) substances are kept as whole formulas.
Spectator ions appear identically on both sides and are removed in the net ionic equation.
A net ionic equation must be balanced for both atoms and total charge.
The net ionic equation shows only the ions/substances that actually undergo chemical change.

Recognizing Reaction Patterns & Predicting Products

Acid + base → salt + water (neutralization; technically a double displacement).
Acid + metal carbonate → salt + water + CO₂ gas (recognized by fizzing).
Acid + reactive metal (above H) → salt + H₂ gas (a single displacement reaction).
Hydrocarbon + O₂ → combustion products (CO₂/H₂O if excess O₂; CO or soot/H₂O if limited O₂).
General strategy: identify reactant types, match to a known pattern, predict products, check states, then balance.

Where marks get lost

Changing a subscript to try to balance an equation (e.g. changing H₂O to H₂O₂) — this creates a completely different substance and is never allowed. (Writing & Balancing Chemical Equations)
Balancing atoms one at a time in a random order instead of following metals → other nonmetals → hydrogen → oxygen, leading to needing constant re-balancing. (Writing & Balancing Chemical Equations)
Forgetting to balance a polyatomic ion that appears unchanged on both sides as a single group, and instead balancing its individual atoms separately (more error-prone, though not technically wrong if done carefully). (Writing & Balancing Chemical Equations)
Leaving coefficients that share a common factor instead of reducing to the lowest whole-number ratio (e.g. leaving 4, 2, 2 instead of 2, 1, 1). (Writing & Balancing Chemical Equations)
Omitting state symbols (s), (l), (g), (aq) — required for a fully correct, complete equation. (Writing & Balancing Chemical Equations)
Forgetting that decomposition reactions usually require an energy input (often shown as $Δ$ , heat, above the arrow) to begin. (Synthesis & Decomposition Reactions)
Predicting the wrong product for metal oxide + water (a base/hydroxide) versus nonmetal oxide + water (an acid) by mixing up which is which. (Synthesis & Decomposition Reactions)
Assuming any reaction with two reactants must be synthesis — synthesis specifically requires the two reactants to combine into exactly one product. (Synthesis & Decomposition Reactions)
Forgetting to balance the resulting equation after correctly predicting the reaction type and products. (Synthesis & Decomposition Reactions)
Not double-checking that a predicted ionic product has correctly balanced charges (e.g. predicting KCl₂ instead of KCl). (Synthesis & Decomposition Reactions)
Assuming all combustion automatically produces CO₂ — incomplete combustion (limited oxygen) produces CO or soot instead. (Combustion Reactions)
Forgetting that combustion equations must still be balanced using the metals/nonmetals/H/O order like any other equation. (Combustion Reactions)
Underestimating the danger of carbon monoxide because it can't be seen or smelled — this is exactly why it's so dangerous. (Combustion Reactions)
Confusing soot (unburned solid carbon, from severely limited oxygen) with carbon monoxide (a toxic gas, from moderately limited oxygen) — they occur under different degrees of oxygen shortage. (Combustion Reactions)
Forgetting that hydrocarbons must contain only carbon and hydrogen — combustion of a fuel that also contains oxygen (like ethanol) follows the same idea but starts from a slightly different formula. (Combustion Reactions)
Forgetting to check the activity series before writing a product — not every single displacement "reactant pairing" actually reacts. (Single Displacement Reactions)
Writing elemental hydrogen, oxygen, or a halogen as a single atom (H, O, Cl) instead of correctly as a diatomic molecule (H₂, O₂, Cl₂). (Single Displacement Reactions)
Assuming a nonmetal (like a halogen) follows the metal activity series — halogens have their own separate reactivity order. (Single Displacement Reactions)
Forgetting to write "NR" when the displacing element is less reactive — leaving the question blank or forcing an incorrect product. (Single Displacement Reactions)
Mixing up which element ends up as the free element versus which stays combined — the more reactive element always ends up combined in the compound, displacing the less reactive one into its free form. (Single Displacement Reactions)
Forgetting to check BOTH new possible products against the solubility rules — only one needs to be insoluble for a reaction to occur. (Double Displacement Reactions)
Swapping only the elements instead of correctly identifying and swapping the cation/anion pairs (especially error-prone with polyatomic ions). (Double Displacement Reactions)
Forgetting to rebalance charges within each newly formed formula unit (e.g. writing BaCl instead of BaCl₂). (Double Displacement Reactions)
Assuming any two ionic solutions mixed together must react — many combinations produce no visible change (NR) because both products are soluble. (Double Displacement Reactions)
Mixing up the sulfate exception list (Ba, Sr, Pb, and slightly soluble Ca) with the halide exception list (Ag, Pb, Hg) — they are different exception sets. (Double Displacement Reactions)
Dissociating a solid, liquid, or gas into ions — only (aq) ionic compounds are split apart; a precipitate (s) is written as a whole formula in every stage. (Total & Net Ionic Equations)
Forgetting to distribute the coefficient when dissociating a compound (e.g. 2NaCl(aq) becomes 2Na⁺(aq) + 2Cl⁻(aq), not Na⁺ + Cl⁻). (Total & Net Ionic Equations)
Cancelling an ion that isn't actually identical on both sides (different coefficient, different charge, or different physical state) — only truly unchanged spectator ions may be cancelled. (Total & Net Ionic Equations)
Forgetting to check that charge is balanced in the net ionic equation, not just atom count. (Total & Net Ionic Equations)
Skipping straight from the formula equation to the net ionic equation without writing out the total ionic equation, which increases the risk of missing a spectator ion. (Total & Net Ionic Equations)
Trying to memorize every possible reaction instead of recognizing the small number of underlying patterns that cover almost everything you'll be asked. (Recognizing Reaction Patterns & Predicting Products)
Forgetting that acid-base neutralization and acid-carbonate reactions are technically double displacement reactions, and still need solubility/state logic applied to the salt. (Recognizing Reaction Patterns & Predicting Products)
Applying the acid + metal pattern even when the metal is below hydrogen in the activity series (forgetting to check reactivity first). (Recognizing Reaction Patterns & Predicting Products)
Mixing up the products of acid + carbonate (salt + water + CO₂) with acid + base (salt + water only, no gas). (Recognizing Reaction Patterns & Predicting Products)
Not checking physical states carefully at the end — forgetting water is (l), gases like CO₂ and H₂ are (g), and the salt's state depends on its own solubility. (Recognizing Reaction Patterns & Predicting Products)

Practice test

21 questions from across the unit. Each one shows the lesson it comes from.

Writing & Balancing Chemical Equations
1
Why is it never allowed to change a subscript to balance an equation?
Writing & Balancing Chemical Equations
2
Balance $C_{2} H_{6} + O_{2} \to CO_{2} + H_{2} O$ and report the sum of all four coefficients.
Writing & Balancing Chemical Equations
3
Match each part of an equation to what it means.
the 2 in $2 H_{2} O$
the 2 in $H_{2} O$
(aq)
$Δ$ above the arrow
$\to$
Synthesis & Decomposition Reactions
4
Which general pattern describes a synthesis reaction?
Synthesis & Decomposition Reactions
5
Put these steps for predicting the product of a synthesis reaction in order.
1. 1Balance with coefficients and recount
2. 2Add state symbols
3. 3Write the product formula from ion charges
4. 4Identify the reactants (e.g. metal + nonmetal)
5. 5Choose the matching product pattern
Synthesis & Decomposition Reactions
6
Match each reaction to its product pattern.
$Na + Cl_{2}$
$Na_{2} O + H_{2} O$
$SO_{3} + H_{2} O$
$CuCO_{3}$ heated
$NaClO_{3}$ heated
Combustion Reactions
7
What are the products of the complete combustion of a hydrocarbon?
Combustion Reactions
8
Which product can form during severely oxygen-limited combustion, in addition to or instead of CO?
Combustion Reactions
9
Write the formula of the carbon-containing product when methane burns in a severe oxygen shortage: $CH_{4} + O_{2} \to ? + 2 H_{2} O$ .
Single Displacement Reactions
10
Iron is placed into a solution of lead(II) nitrate. According to the activity series (Fe above Pb), what happens?
Single Displacement Reactions
11
Write the correct diatomic formula for elemental nitrogen gas.
Single Displacement Reactions
12
Match each single displacement to its result.
Zn + CuSO₄
Cu + ZnSO₄
Mg + HCl
Cl₂ + KI
Br₂ + KCl
Double Displacement Reactions
13
In a double displacement reaction $A B + C D \to A D + CB$ , what happens?
Double Displacement Reactions
14
Mixing aqueous NaCl and aqueous KNO₃ produces no visible reaction. Why?
Double Displacement Reactions
15
Which pair of solutions produces a precipitate when mixed?
Total & Net Ionic Equations
16
Which substances are dissociated into separate ions when writing a total ionic equation?
Total & Net Ionic Equations
17
Put these three equation types in the order you would normally write them.
1. 1Total ionic equation
2. 2Formula (molecular) equation
3. 3Net ionic equation
Total & Net Ionic Equations
18
Aqueous $NH_{4} Cl$ and aqueous $NaNO_{3}$ are mixed. What is the net ionic equation?
Recognizing Reaction Patterns & Predicting Products
19
What are the two products of any acid-base neutralization reaction?
Recognizing Reaction Patterns & Predicting Products
20
For an unfamiliar reaction, put these problem-solving steps in the correct order.
1. 1Balance the final equation
2. 2Check product states (solubility, acid/base rules)
3. 3Predict the products using that pattern
4. 4Identify the type of each reactant
5. 5Match the reactants to a known reaction pattern
Recognizing Reaction Patterns & Predicting Products
21
What are the products when copper metal is added to dilute hydrochloric acid?

Endless drill

Fresh numbers every time, for the calculation questions in this unit.

Endless drillnew numbers every time

Generating a problem…