Physical Science Enhancement - Chemistry 2
Hydrogen – the ‘water maker’
Hydrogen is the most abundant element in the universe and central to chemistry as the ‘water maker’. This unit develops practical skills and chemical understanding through five activities focused on the preparation, properties, and reactions of hydrogen. Balanced equations are a core learning point in every activity.
Content
MYP9PS2.1 — Preparation & properties of hydrogen with Group 1 metals (teacher demo)
Background
The Group 1 elements (alkali metals) include lithium, sodium, and potassium. They are soft, low-density metals that can be cut with a knife and are less dense than water, so they float. Their reactivity with water increases down the group: lithium reacts gently, sodium reacts more vigorously, and potassium can ignite the hydrogen produced. This trend illustrates how periodic position influences chemical behaviour.
Aim
Observe formation of hydrogen when lithium, sodium, and potassium react with water; compare hardness, density, and reactivity; record observations; write balanced equations.
Equations
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Key notes
- All three metals float; hydrogen bubbles cause characteristic ‘skating’ motion.
- Relative softness (ease of cutting): Li < Na < K.
- Phenolphthalein turns pink, confirming alkaline hydroxide formation.
Teaching notes
Students observe the reactions of the three metals with water and record: hardness (when cut), floats? (Y/N), vigour of reaction, ignition/colour, indicator colour, notes.
Safety
Use rice-grain sized pieces kept under oil; blot to remove excess oil before use; conduct in a fume cupboard. Handle with tweezers; have dry sand ready (never water) for fires; ensure goggles for all observers.
LO1: Connect observable properties with reactivity trends down Group 1; link to periodic position.
LO2: Walk through balancing to highlight 2M + 2H2O → 2MOH + H2 mole ratios.
LO3: Emphasise flammability of H2; justify demo-only approach and use of sand, not water.
Pitfalls: Pieces too large; residual oil on metal; students leaning over the beaker; phenolphthalein under-mixed.
For students that wish to see the demonstration again:
Summary
Group 1 metals react with water to form alkaline hydroxides and hydrogen gas. Trends in softness, density, and reactivity are clear (Li < Na < K). Observations (floating, motion, ignition, indicator colour) align with the balanced equations.
Check your understanding
- Why do Li, Na, and K float on water, and how do bubbles affect their motion?
- How does phenolphthalein help confirm the products of the reaction?
- What trend in reactivity is shown down Group 1, and how was this visible in the demo?
MYP9PS2.2 — Avogadro’s law: relative atomic mass by gas collection over water
Avogadro's law states that all gases occupy the same volume for the same number of particles (moles). In other words:
n ∝ V (constant T and P)
Avogadro's law
The ideal gas equation allows calculations to take into account variable temperature and pressure of gases.
PV = nRT
The ideal gas equation
Where 'R' is the universal gas constant = 8.31 J K-1 mol-1.
Hence, if the pressure and temperature of a gas are known, then the number of moles, n, can be determined from the ideal gas equation by rearranging the equation so that:
n = PV/RT
The rearranged ideal gas equation
Activity - Determine the relative mass of a reactive metasl
Measuring the volume of hydrogen released when a known mass of metal reacts with water allows the amount of gas to be calculated using the ideal gas relationship.
For lithium and calcium, the stoichiometry differs: 2 x Li produce 1 x H2, while 1 x Ca produces 1 x H2.
Applying Avogadro’s law and balanced equations links measured gas volume to moles of metal and hence, if the original mass of the metal is known, to the calculated relative atomic mass.
Relative mass = mass/moles
The relationship between moles, mass and relative mass
Apparatus
- Inverted 250 cm3 measuring cylinder in a water trough; clamp/stand.
- Tea-strainer or small mesh to hold metal beneath the cylinder mouth.
- Small pieces of Li and Ca; forceps; watch glass.
- Thermometer/probe; barometer (e.g., PhyPhox) for ambient pressure.
- Beaker, spatula, paper towel; PPE: goggles, gloves.
Procedure
- Weigh out between 0.05 g and 0.10 g of lithium that has had the oil removed from the surface.
- Place the metal sample in a tea strainer and quickly place it under an inverted 250 cm3 measuring cylinder in a water trough.
- Record volume of hydrogen gas, water temperature and atmospheric pressure (Phyphox).
- Use consistent units (e.g. kPa and dm3 with R = 8.314 J K-1 mol−1).
Teaching notes
- Use very small metal masses; keep faces away from the cylinder mouth.
- Handle Li and Ca with tweezers; keep pieces dry until use.
- Solutions become alkaline (LiOH/Ca(OH)2); neutralise and dispose per school policy.
- Carefully dry the tea strainer between trials.
Worked example (typical values)
0.10 g of lithium = 0.10/6.99 = 0.0143 mol
2Li + 2H2O → 2LiOH + H2
There is a 2:1 ratio between lithium reacted and hydrogen formed
Expected gas approximately equal to 0.00715 x 24 = 0.171 dm3 = 171 cm3.
Teaching notes
LO1: Apply Avogadro’s law with vapour-pressure correction; reinforce consistent units (J K-1 mol−1).
LO2: Link balanced equations to mole ratios (Li: 2→1 H2; Ca: 1→1 H2).
LO3: Model uncertainty propagation (mass, volume, pressure, temperature) and discuss the dominant contributors.
Possible systematic errors: Reading meniscus above/below eye level; assuming room T = water T without checking. Discuss the difference between systematic and random inaccuracies.
Summary
Gas collection over water allows calculation of moles of H2 and hence the relative atomic mass of the metal via balanced stoichiometry. Careful measurement and unit consistency are essential for accurate results.
Check your understanding
- How does the Li : H2 mole ratio differ from Ca : H2, and why?
- Which measurement (mass, V, p, or T) is most likely to dominate the uncertainty, and how could you reduce it?
MYP9PS2.3 — Preparing, testing, and ‘pouring’ hydrogen
Background
In the previous lesson, hydrogen gas was produced when very reactive metals reacted with water, forming alkaline solutions and H2. In this lesson, we switch to less reactive metals and use dilute acids to prepare hydrogen. In both cases the metal donates electrons and hydrogen ions are reduced to H2:
With acids, H+(aq) accepts electrons; with water, H2O acts as a (weak) proton (hydrogen ion) source.
This is because water is a much weaker acid than H+(aq), only the more reactive metals release hydrogen from water at room temperature, whereas many metals will react with dilute acids.
Redox view (what’s really happening)
Metal oxidation: Mg(s) → Mg2+(aq) + 2e-
Hydrogen reduction (acid): 2H+(aq) + 2e- → H2(g)
Hydrogen reduction (water): 2H2O(l) + 2e- → H2(g) + 2OH-(aq)
Acids usually give a faster, cleaner reaction because they provide a high concentration of H+ and dissolve any oxide coating on the metal surface.
Equation (this lesson)
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Activity - To investigate the properties of hydrogen gas
Apparatus and chemicals
- Side-arm boiling tube or conical flask with bung and delivery tube
- Magnesium ribbon; dilute sulfuric acid (~0.5–1.0 mol dm−3)
- Water trough; test tubes for collection over water; test-tube rack
- Splints; lighter/matches
- PPE: goggles, gloves
Procedure
- Assemble the generator with Mg ribbon. Add dilute H2SO4 and connect the delivery tube to the water-filled collection setup.
- Collect gas in test tubes over water; cap each tube immediately after filling.
- Discard the first tube (air contamination). Test the next with a lighted splint — a ‘squeaky pop’ confirms H2.
- For density: hold a tube of H2 mouth-up beneath an empty test tube. Then test the 'empty' test tube with a lighted splint.
Safety
- Keep flames well away from the generator; test gases away from the apparatus.
- Vent a small amount of gas before testing; never test directly at the delivery tube.
- Wear goggles; handle acids carefully; rinse spills and neutralise as required.
- Ensure bungs/tubing are secure to avoid leaks and flashback risk.
Teaching notes
LO1: Reinforce safe gas testing: small volumes, away from source, discard first tube.
LO2: Link the ‘squeaky pop’ and the upward ‘pour’ to particle model and relative density vs air.
LO3: Contrast H2 vs air mixtures (flammability limits) to justify gentle, separate testing.
Pitfalls: Loose bungs/leaks; saturated splints; slow collection (air ingress); bringing flame too close to generator.
Summary
Hydrogen is produced by reacting Mg with dilute sulfuric acid. The ‘squeaky pop’ confirms H2, and its lower density than air is shown by safely ‘pouring’ the gas upwards into an empty test tube.
Check your understanding
- Why is the first test tube of gas discarded before testing?
- What does the ‘squeaky pop’ reveal about the gas collected?
- How does the ‘pouring’ demonstration provide evidence about hydrogen’s density?
MYP9PS2.4 — Rates of reaction: magnesium with different acids
Background
Building on Lesson 3 (hydrogen from metals + dilute acids), we now compare how fast hydrogen is produced when magnesium reacts with different acids at the same formal concentration. Differences in rate arise from acid strength (extent of ionisation → [H+]) and, for sulfuric acid, its diprotic nature. We’ll measure gas volume vs time and estimate the initial rate fairly by controlling variables.
Equations
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
Mg(s) + 2CH3COOH(aq) → (CH3COO)2Mg(aq) + H2(g)
- Fair test: keep Mg surface area, acid concentration, and temperature the same.
- Initial rate: use the early, near-linear part of the V–t curve (or a tangent at t = 0).
- Expectation: stronger acids (greater [H+]) give faster rates than weak acids of the same molarity.
Activity - Measure and compare initial rates of H2 production for Mg with different acids
Apparatus and chemicals
- Gas syringe (100 mL) with low-friction plunger or water displacement setup
- Conical flask with side-arm (or bung + delivery tube), clamp/stand, stopwatch
- Magnesium ribbon (cut to equal lengths, e.g., 2.0 cm) and fine emery paper
- Acids at the same formal concentration (e.g., 0.50 mol dm−3): HCl, H2SO4, CH3COOH
- Thermometer/probe, measuring cylinder(s), PPE: goggles, gloves
Procedure
- Cut identical Mg pieces; lightly clean with emery paper; keep pieces dry.
- Add a measured volume of acid to the flask; note the temperature.
- Start timing as soon as the Mg is added and the bung secured.
- Record H2 volume every 5 s for 30–60 s (higher density early on gives the best estimate of the initial slope).
- Repeat for each acid (same Mg length and acid volume). Keep room and solution temperature as constant as possible.
- Plot V (mL) vs t (s) for each acid on the same axes and estimate the initial rate from the early slope (see worked example).
Safety
- Wear goggles; handle acids carefully; neutralise and wipe spills immediately.
- Do not bring flames near the apparatus; vent a small amount of gas before any test.
- Ensure good seals (bung/tubing) to avoid leaks and plunger blow-out.
Worked example — estimating an initial rate
From a V–t graph, take two earliest reliable points on the straight region, e.g., at 10 s: 14 mL and at 25 s: 32 mL.
Initial rate ≈ ΔV/Δt = (32 − 14) mL / (25 − 10) s = 18 mL / 15 s = 1.2 mL s−1
Use the steep, early section; avoid curved portions or plateaus.
Teaching notes
LO1: Enforce control of variables (Mg size, acid concentration/volume, temperature).
LO2: Link rate differences to acid strength and [H+] (HCl & H2SO4 > CH3COOH at same molarity). Note that H2SO4 is diprotic; discuss fairness if students propose “equal normality”.
LO3: Teach the “initial slope” method (tangent at t = 0 or first linear segment) and commenting on anomalies/outliers.
Pitfalls: Leaks or sticking plungers; unequal Mg surface area; warm solutions midway; mixing delays after adding Mg.
Summary
With Mg held constant, stronger acids (greater effective [H+]) produce hydrogen faster than a weak acid of the same molarity. Plotting V–t and comparing initial slopes gives a fair, quantitative comparison of rates.
Check your understanding
- Why must the Mg pieces be the same size and similarly cleaned?
- How do you estimate the initial rate fairly from a V–t graph?
- Explain why ethanoic acid gives a slower rate than hydrochloric acid at equal molarity.
MYP9PS2.5 — Electrolysis of water: gas volumes and tests
Background
Earlier in the unit, hydrogen was produced chemically from metals. Here we make hydrogen (and oxygen) by passing an electric current through acidified water. The gases form at different electrodes via redox half-reactions, giving a characteristic volume ratio close to 2:1 (H2:O2).
Equations (acidic solution)
Cathode (−): 2H+(aq) + 2e- → H2(g)
Anode (+): 2H2O(l) → O2(g) + 4H+(aq) + 4e-
Overall: 2H2O(l) → 2H2(g) + O2(g)
Activity - Demonstrate electrolysis of water and test the gases
Apparatus and chemicals
- Hoffman voltammeter (demo) or beaker with two graphite electrodes fixed in place
- Low-voltage DC power supply (~6 V) and leads
- Distilled water with a small amount of electrolyte (e.g., a few drops of H2SO4 or ~0.1 mol dm-3 Na2SO4)
- Two test tubes (inverted) or collection arms for the gases; trough; clamps/stand
- Wooden splints; matches/lighter; PPE: goggles, gloves
Procedure
- Fill the apparatus with electrolyte; remove bubbles from the electrode surfaces.
- Invert a test tube over each electrode (or use the voltammeter arms) and switch on the supply.
- Observe that gas at the cathode (negative) collects roughly twice the volume of the anode gas.
- Test gases separately, away from the apparatus: vent a little first, then
- H2 (cathode): lighted splint gives a ‘squeaky pop’.
- O2 (anode): glowing splint relights.
Safety
- Use low voltage; keep hands dry; check leads and connections.
- Keep flames away from the apparatus during collection; test small samples separately.
- Handle acid/electrolyte carefully; neutralise and clean spills.
- Avoid chloride electrolytes (NaCl) to prevent Cl2 formation at the anode.
Worked note — why 2:1?
From 2H2O(l) → 2H2(g) + O2(g), every 2 mol of H2 form with 1 mol of O2.
At equal temperature and pressure, gas volumes are proportional to moles, so V(H2):V(O2) ≈ 2:1.
Teaching notes
LO1: Stress separate, small-volume gas tests; never test at the apparatus.
LO2: Use half-equations to link electrode polarity, oxidation/reduction, and gas identity.
LO3: Troubleshoot low O2 yield (anode kinetics, solubility, leaks, electrode area), and link to energy topics (H2 as a fuel).
Pitfalls: Using NaCl electrolyte (Cl2 hazard), bubbles stuck on electrodes, electrodes touching, over-acidification causing corrosion of clips.
- Expected volume ratio H2:O2 ≈ 2:1 (same T and p → volume ∝ moles).
- Student mini-rigs may show less obvious O2 because: oxygen production is slower at the anode, O2 is more soluble than H2, small leaks, or unequal electrode areas.
- Polarity: cathode = negative (reduction of H+); anode = positive (oxidation of H2O).
Summary
Electrolysis of acidified water produces hydrogen at the cathode and oxygen at the anode. The gases collect in an approximate 2:1 volume ratio, confirmed by separate and safe tests with a lighted/glowing splint.
Check your understanding
- State which gas forms at each electrode and the tests used to identify them.
- Explain why the H2:O2 volume ratio is close to 2:1.
- Give two reasons why a student mini-rig might produce a smaller visible volume of O2.