1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
The top diagram represents a mixture of SO₂(g) and O₂(g) in a sealed container. The gases react according to the equation 2 SO₂(g) + O₂(g) → 2 SO₃(g). Which numbered diagram best represents the contents of the container after the reaction has gone to completion?
Answer and reasoning
ADiagram 1 A student who reads the coefficients as the actual number of particles that react picks this diagram: 2 SO₂ and 1 O₂ have formed 2 SO₃, and the other 2 SO₂ and 1 O₂ are left unreacted. Coefficients give a ratio, so the remaining SO₂ and O₂, which are in the same 2 : 1 ratio, react as well.
BDiagram 2 A student who multiplies the coefficient into the formula picks this diagram, in which the product is drawn as S₂O₆ particles. The atoms are conserved, but 2 SO₃ means two separate SO₃ molecules, each with one S atom, not one particle that contains two S atoms.
CDiagram 3Correct The coefficients give the ratio 2 SO₂ : 1 O₂ : 2 SO₃. The box holds 4 SO₂ and 2 O₂, exactly this ratio, so the reaction uses all of both reactants and forms 4 separate SO₃ molecules. The 4 S atoms and 12 O atoms present before the reaction are all present afterward.
DDiagram 4 A student who thinks the number of molecules is conserved picks this diagram, which has 6 molecules after the reaction as there were 6 before. Atoms are conserved, not molecules: 6 SO₃ would need 6 S atoms and 18 O atoms, but only 4 S atoms and 12 O atoms are present.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
4.3.A.1 Particulate representation of a reaction Fix
Particulate representation of a reaction
A drawing in which symbols (usually circles) stand for the atoms, molecules and ions present before and after a change. A representation that is consistent with a balanced equation shows the correct particles, in numbers that match the ratios given by the coefficients, with every atom present before the change also present after it.
Coefficient
The number written in front of a formula in a balanced equation. Coefficients give the ratio in which particles react and form, not a fixed number of particles: 2 NH₃ means two separate NH₃ molecules for each N₂ molecule that reacts.
Subscript
The number written after a symbol in a formula; it gives the number of atoms of that element in one particle of the substance. Changing a subscript changes the identity of the substance, so 2 NH₃ (two molecules) is not the same as N₂H₆ (one particle).
Conservation of atoms in a representation
In a chemical change atoms are rearranged, not created or destroyed, so a particulate diagram must show the same number of atoms of each element before and after the change. The number of molecules does not have to stay the same.
Particulate meaning of state symbols
A soluble ionic compound labeled (aq) is represented as separate cations and anions dispersed among water molecules; an ionic solid labeled (s) is represented as ions packed together in a fixed, ordered arrangement; a molecular gas labeled (g) is represented as separate molecules that are far apart.
Forms of an equation and what each represents
A molecular equation shows complete formulas for every substance; a complete ionic equation shows dissolved ionic substances as separate ions; a net ionic equation shows only the particles that change. Spectator ions are omitted from the net ionic equation but are still present in the mixture and belong in a particulate diagram of it.
Students often think The coefficients in a balanced equation are the actual numbers of particles that react: the reaction happens once as written, and any further reactant particles in the diagram are left unreacted. In fact No. Coefficients give the ratio in which particles react and form. If twice as many reactant particles are present in the correct ratio, twice as many product particles form, and no reactant is left over.
Students often think A coefficient can be multiplied into the formula, so 2 NH₃ can be written or drawn as one particle, N₂H₆, with the same atoms joined together. In fact No. A coefficient counts separate particles, and a subscript counts atoms within one particle. 2 NH₃ means two separate molecules, each with one N atom and three H atoms; N₂H₆ would be a single particle of a different substance.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
The particle diagrams represent the substances present in a sealed container before and after a reaction in the gas phase. Which equation is the best symbolic representation of the reaction shown?
Answer and reasoning
AC₂H₄(g) + 3 O₂(g) → C₂O₄(g) + H₄O₂(g) A student who multiplies a coefficient into the formula picks this, writing the two CO₂ molecules as C₂O₄ and the two H₂O molecules as H₄O₂. The atom totals are right, but the diagram shows the products as separate CO₂ and H₂O molecules, which are written 2 CO₂ and 2 H₂O.
BC₂H₄(g) + 3 O₂(g) → 2 CO₂(g) + 2 H₂O(g)Correct The diagram shows one C₂H₄ molecule and three O₂ molecules before the reaction, and two separate CO₂ molecules and two separate H₂O molecules after it. Each formula matches a particle in the diagram and each coefficient is the number of those particles; there are 2 C, 4 H and 6 O atoms on each side.
CC₂H₄(g) + 6 O(g) → 2 CO₂(g) + 2 H₂O(g) A student who writes a diatomic element as separate atoms picks this. The diagram shows the six O atoms joined in pairs, as three O₂ molecules; 6 O would mean six separate O atoms.
DC₂H₄(g) + 6 O(g) → C₂O₄(g) + H₄O₂(g) A student who treats an equation as a tally of the atoms of each element in each substance picks this: 6 O atoms before, and C₂O₄ and H₄O₂ for the atoms in the carbon-containing and hydrogen-containing products. An equation must also show how the atoms are grouped into particles: 3 O₂, 2 CO₂ and 2 H₂O.
Working Before: 1 molecule with 2 C and 4 H atoms (C₂H₄) and 3 molecules of two O atoms (3 O₂). After: 2 molecules of one C and two O atoms (2 CO₂) and 2 molecules of one O and two H atoms (2 H₂O). Equation: C₂H₄(g) + 3 O₂(g) → 2 CO₂(g) + 2 H₂O(g). Check: C 2 = 2; H 4 = 4; O 6 = 4 + 2.
A student is drawing a particulate representation of the reaction represented by the equation 2 Al(s) + 3 Cl₂(g) → 2 AlCl₃(s). The student's 'before' box contains 6 Cl₂ molecules. How many Al atoms should the 'before' box contain, and how many AlCl₃ formula units should the 'after' box contain, if no reactant is to be left over?
Answer and reasoning
A2 Al atoms and 2 AlCl₃ units A student who reads the coefficients as the actual numbers of particles picks this, copying the 2 in front of Al and of AlCl₃. Two Al atoms react with only 3 Cl₂ molecules, so 3 of the 6 Cl₂ molecules would be left over.
B6 Al atoms and 6 AlCl₃ units A student who thinks reactant particles combine one-to-one picks this, matching each Cl₂ molecule with one Al atom. The ratio is 2 Al to 3 Cl₂, and 6 AlCl₃ units would need 18 Cl atoms when only 12 are present.
C9 Al atoms and 9 AlCl₃ units A student who applies the ratio of coefficients upside down picks this: 6 × 3/2 = 9. The ratio is 2 Al for every 3 Cl₂, so 6 Cl₂ molecules require 6 × 2/3 = 4 Al atoms.
D4 Al atoms and 4 AlCl₃ unitsCorrect The coefficients give the ratio 2 Al : 3 Cl₂ : 2 AlCl₃. Six Cl₂ molecules is twice the 3 Cl₂ of the equation, so twice the 2 Al and twice the 2 AlCl₃ are needed: 4 Al atoms and 4 AlCl₃ units. Check: 12 Cl atoms before, 4 × 3 = 12 Cl atoms after.
Working Ratio from the coefficients: 2 Al : 3 Cl₂ : 2 AlCl₃. Al atoms = 6 Cl₂ × (2 Al / 3 Cl₂) = 4. AlCl₃ units = 6 Cl₂ × (2 AlCl₃ / 3 Cl₂) = 4. Atom check: 4 Al and 12 Cl before; 4 AlCl₃ contain 4 Al and 12 Cl. Distractors: coefficients copied, 2; one-to-one, 6; ratio inverted, 6 × 3/2 = 9.
A student places a conductivity probe in a solution of Ba(OH)₂ and observes a high conductivity. The student then stirs in a solution that contains an equal number of moles of H₂SO₄, and the reaction represented by the equation Ba(OH)₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2 H₂O(l) occurs. Based on a particulate representation of the equation, which prediction of the conductivity of the final mixture is correct?
Answer and reasoning
AIt falls to nearly zero, as the products are a solid and a liquid made of molecules.Correct Before mixing, each solution is represented as separate, mobile ions, which carry the current. In the reaction every Ba²⁺ and SO₄²⁻ ion becomes part of solid BaSO₄, where the ions are held in place, and every OH⁻ and H⁺ ion becomes part of an H₂O molecule. With the reactants in a 1 : 1 mole ratio, almost no mobile ions are left.
BIt stays high, as the ions of the BaSO₄ that forms can still carry a current. A student who represents an ionic precipitate as free ions picks this. BaSO₄(s) is a solid: its ions are packed together in fixed positions and cannot move through the liquid to carry a current.
CIt stays high, as a reaction between two solutions leaves spectator ions behind. A student who thinks every reaction between ionic solutions leaves spectator ions picks this. In this reaction both products remove ions from the solution: neither product is labeled (aq), so no dissolved ions are left when the reactants are mixed in a 1 : 1 mole ratio.
DIt rises, as the water that forms in the reaction is a good conductor of electricity. A student who thinks water itself conducts well picks this. H₂O(l) is represented as neutral molecules; a solution conducts only because of the mobile ions dissolved in it, and this reaction removes them.
A student draws a particulate diagram of the mixture that results from the reaction represented by the equation AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). The diagram shows Ag⁺ and Cl⁻ ions packed together in a solid at the bottom of the beaker and, in the water above it, NaNO₃ units in which each Na⁺ ion is joined to an NO₃⁻ ion. Which evaluation of the diagram is best?
Answer and reasoning
AIt is consistent with the equation: NaNO₃ is written as one unit, so its ions stay joined in pairs. A student who thinks a dissolved ionic compound exists as joined pairs of ions, like molecules, picks this. The formula NaNO₃ in a molecular equation gives the ratio of the ions; the (aq) label means the ions are separated in the solution.
BIt is inconsistent with the equation: Na⁺ and NO₃⁻ are spectator ions, so they should not appear. A student who thinks spectator ions are no longer present because they cancel from the net ionic equation picks this. Na⁺ and NO₃⁻ do not change in the reaction, but they remain in the solution and belong in a diagram of the mixture, as separate ions.
CIt is inconsistent with the equation: the (aq) label means Na⁺ and NO₃⁻ exist as separate ions.Correct The state symbol (aq) on a soluble ionic compound means that it is present as separate ions dispersed in the water, so the diagram should show Na⁺ ions and intact NO₃⁻ ions that are not joined to each other. The solid AgCl is drawn correctly.
DIt is inconsistent with the equation: NaNO₃(aq) should be drawn as separate Na, N and O particles. A student who thinks a dissolved compound separates into all of its atoms picks this. Dissolving separates the Na⁺ ions from the NO₃⁻ ions; each NO₃⁻ ion stays together as one particle.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
