S5 Chemistry | Ionic Reactions and Precipitation prep guide (for tutors)

S5 Chemistry | Ionic Reactions and Precipitation prep guide (for tutors) | Solubility rules and limiting reagents

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students copy every ion into their ionic equations, and they call any white solid AgCl. This lesson fixes two things: which ions actually change, and which ion runs out first after mixing.

Objectives — Students can: (1) use solubility rules to decide whether AgCl, BaSO4, PbI2 or CaCO3 will precipitate; (2) write a net ionic equation, remove spectator ions and balance charge; (3) compute precipitate mass from concentration and volume; (4) identify the limiting reagent when two solutions are mixed, find the remaining concentrations and check them for electroneutrality.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Moles n = cV	How many moles are in 25.0 cm3 of 0.100 M solution?	2.50e-3 mol: convert cm3 to dm3 first
Balancing equations	In Ag+ + Cl- -> AgCl, is charge balanced?	Yes: +1 -1 = 0 on each side
Molar mass	What is the molar mass of AgCl?	143.32 g/mol

02 Lesson flow (50 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Opening question	NaCl and AgNO3 are poured together: which two ions actually react?	Two ions named
6-12 min	Prerequisite check	n = cV, balancing, molar mass	All three correct before moving on
12-22 min	Act 1	Solubility rules and the four common precipitates	A four-row table
22-30 min	Act 2	Net ionic equations: drop spectators, balance charge	Ag+ + Cl- -> AgCl(s)
30-42 min	Act 3	Limiting reagent: Pb2+ with I-, remaining concentrations	[Pb2+] = 0.075 M
42-47 min	Worked examples	One start-up routine per question type	Students copy the routine
47-50 min	Exit check	Three questions marked on the spot	Answered independently

03 Teaching script (read it out)

Scene 1: Solubility rules: build the table first

All sodium, potassium and ammonium salts dissolve; all nitrates dissolve; chlorides dissolve except AgCl and PbCl2; sulfates dissolve except BaSO4, PbSO4 and the slightly soluble CaSO4; most carbonates are insoluble except those of Na, K and NH4.
Two worked examples: 25.0 cm3 of 0.100 M AgNO3 holds 2.5 mmol of Ag+, so complete precipitation gives 0.358 g of AgCl. For BaCl2, 20.0 cm3 of 0.150 M holds 3.0 mmol of Ba2+, which needs 30.0 cm3 of 0.100 M Na2SO4 and gives 0.70 g of BaSO4.
Teach the order: check solubility first, then concentration after mixing. Never start by writing an equation. Warn about PbCl2 dissolving in hot water, which is a favourite trap.

Task: decide whether mixing KNO3 and NaCl gives a precipitate, and give the reason (no: everything is soluble).

Scene 2: Net ionic equations: keep only what changes

Write Ag+ + NO3- + Na+ + Cl- -> AgCl(s) + Na+ + NO3-, then delete Na+ and NO3- because they appear unchanged on both sides, leaving Ag+(aq) + Cl-(aq) -> AgCl(s).
Three habits: label the precipitate (s); confirm total charge is equal on both sides, here +1 -1 = 0; and split only strong electrolytes, never weak acids, water or solids. Ask students to read the charge balance aloud after every equation they write.

Task: write the net ionic equation for BaCl2 + Na2SO4 and circle the spectator ions (Ba2+ + SO4 2- -> BaSO4(s); spectators Na+ and Cl-).

Scene 3: Limiting reagent: moles first, coefficients second

Mix 50.0 cm3 of 0.200 M Pb(NO3)2 with 50.0 cm3 of 0.100 M KI: 10.0 mmol of Pb2+ and 5.0 mmol of I-. The equation Pb2+ + 2I- -> PbI2(s) needs 20.0 mmol of I- for all the lead, so iodide is limiting, giving 2.50 mmol of PbI2, that is 1.15 g.
Left over: 7.5 mmol of Pb2+ in 100 cm3, so 0.075 M; K+ is a spectator at 0.050 M and NO3- at 0.200 M. Finish with the electroneutrality check: 2 x 0.075 + 0.050 = 0.200, which matches [NO3-]. Make this check routine; it catches errors faster than recalculating.

Task: if 100 cm3 of 0.100 M KI is used instead, what mass of PbI2 forms and what is the remaining [Pb2+]? (2.3050 g and 0.0333 M.)

04 Question types and worked steps

Type	Example	Steps	Answer
Will it precipitate?	Apply solubility rules	Solubility first, then ion pairing	Four-row table
Net ionic equation	Split strong electrolytes, drop spectators	Label (s) and check charge	Ag+ + Cl- -> AgCl(s)
Mass of precipitate	n = cV, then multiply by molar mass	cm3 to dm3 before multiplying	0.70 g
Limiting reagent and leftovers	Compare moles with the coefficient ratio	Divide by total volume, verify electroneutrality	[Pb2+] = 0.075 M

Sanity check (teach this): AgCl from 2.50 mmol gives 0.358 g, about 0.14 g per mmol; BaSO4 from 3.00 mmol gives 0.70 g, about 0.23 g per mmol, so both magnitudes are reasonable. For the PbI2 case three checks must hold: iodide is exhausted, lead remains, and 2[Pb2+] + [K+] = [NO3-]. If a student reports a remaining [Pb2+] above the original 0.200 M, they divided by 50 cm3 instead of the 100 cm3 total.

05 Independent practice (3 questions)

Question	Answer	What to watch
0.050 dm3 of 0.20 M CaCl2 reacts with excess Na2CO3. What mass of CaCO3 forms (M = 100.09)?	1.00 g	0.010 mol x 100.09
Write the net ionic equation for Pb(NO3)2 + 2KI	Pb2+ + 2I- -> PbI2(s)	K+ and NO3- are spectators
0.10 mol Ba2+ meets 0.15 mol SO4 2-. Which is limiting, and how much remains?	Ba2+ limiting; 0.05 mol SO4 2- left	1:1 reaction, subtract directly

06 Common mistakes and what to say

Mistake	Student writes	Say this
Keeping Na+ and NO3- in the ionic equation	Believing every ion must appear	“Na+ and NO3- are spectators from start to finish; delete them.”
Omitting the (s) label	Focusing only on balancing	“A solid needs (s); without it the answer still looks dissolved.”
Judging the limiting reagent by concentration	Assuming the stronger solution must be in excess	“High concentration is not high amount: multiply by the volume.”
Dividing leftover moles by the original volume	Forgetting the volume grows on mixing	“Two beakers make one total volume: 100 cm3, not 50.”

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index (moles, bonding, periodic table, acids, redox, electrolysis) — Do the moles chapter first, then questions 2, 4 and 6 (precipitate mass and limiting reagent)
Secondary prep guides overview (all subjects) — Revise in subject order

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which chloride is the exception to the soluble-chloride rule?	AgCl (and PbCl2)	Wrong means Act 1 again
Which ions are removed from a net ionic equation?	Spectators that appear unchanged on both sides	Wrong means Act 2 again
Do you judge the limiting reagent by concentration or moles?	Moles: concentration times volume	Two or more wrong means back to Act 3

09 Tutor FAQ

Which year is this for?

S5, the reactions and stoichiometry unit of HKDSE chemistry; S4 can meet it in the second term.

Must every solubility rule be memorised?

Five spines: Na K NH4 soluble, nitrates soluble, chlorides minus Ag and Pb, sulfates minus Ba Pb Ca, most carbonates insoluble.

Where do marks usually leak?

Units (cm3 versus dm3) and the total volume; get either wrong and everything after fails.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.