Life 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

Chemistry 2.1 — Group 1 metals and hydrogen
Chemistry 2.2 — The gas laws
Chemistry 2.3 — Preparing and properties of hydrogen
Chemistry 2.4 — Rates of reaction
Chemistry 2.5 — Electrolysis of water
Now Test Yourself

Teacher support documents for enhancement unit Chemistry 2

PowerPoint C2.1

PowerPoint C2.2

PowerPoint C2.3

Chemistry 2.1 - The reaction of active metals with water.

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)

You can see videos of the reactions of each of these metals using the following links

The properties of lithium

The properties of sodium

The properties of potassium

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.

Summary

Group 1 metals react with water to form alkaline hydroxides and hydrogen gas.
Trends in softness, density, and reactivity
Observations (floating, motion, ignition, indicator colour)
Same general equation: 2M + 2H₂O → 2MOH + H₂

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?

Chemistry 2.2 — The gas laws

Avogadro’s law

Avogadro's law states that all gases occupy the same volume for the same number of particles (moles).

n ∝ V (constant T and P)

Avogadro's law

In simple terms, this means that all gases behave the same way as regards their volume compared to the number of particles of gas (moles). So 1 mol of hydrogen gas occupies exactly the same volume as one mol of oxygen gas (under the same conditions of pressure and temperature).

At room temperature, about 25ºC, and normal atmospheric pressure 100kPa, one mol of any gas occupies a volume of about 24 dm³.

This allows us to calculate the number of moles of gas in any given volume at room temperature and pressure.

Example: Calculate the number of particles in 6 dm³ of hydrogen at RTP (room temperature and pressure)

Moles of gas = 6/24 = 0.25 mol

The number of particles = mol x Avogadro's number

The number of particles = 6 x 10²³ x 0.25 = 1.25 x 10²³ hydrogen molecules

Boyle's law

Other scientists, following on from Avogadro’s work, investigated the volume of gases when the pressure varied.

Robert Boyle investigated the relationship between volume and pressure of a gas. He realised that there was an inverse proportionality for all gases.

P ∝ 1/V (constant T and n)

Boyle's law

Charles' law

Jacques Charles investigated the relationship between the volume of a fixed mass of gas and the temperature of the gas.

He found that the volume of all gases is proportional to the temperature when the temperature is measured in Kelvin.

V ∝ T (constant P and n)

Charles' law

The ideal gas equation

The ideal gas equation combines all of the above gas laws and 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

Summary

Avogadro's law relates gas volume and amounts
Boyle's law relates gas volume and pressure
Charles' law relates gas volume and temperature
The ideal gas equation combines all of the above gas laws into PV = nRT

Now test yourself

Chemistry 2.3 — Preparation and properties of 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 (reduction and oxidation)

The reaction between active metals and water (and dilute acids) is an electron transfer reaction.

Metals have loosely held electrons in their outer electron shells. These electrons can be transferred to the hydrogen ions that are present in both water and dilute acids.

The metal atoms lose electrons and become metal ions. This loss of electrons is called oxidation:

Mg(s) → Mg²⁺(aq) + 2e^-

The hydrogen ions in water (or acids) gain electrons and become hydrogen atoms, which then join up to make hydrogen molecules (hydrogen gas).

2H⁺(aq) + 2e^- → H₂(g)

This gain of electrons is called reduction.

Acids react faster than water, because they have a much higher concentration of H⁺ ions.

Equation:

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Summary

Hydrogen can be produced by reacting magnesium, or another active metal with dilute acids.

The ‘squeaky pop’ is a test for hydrogen gas and confirms H₂

The lower density of hydrogen than air is shown by safely ‘pouring’ the gas upwards into an empty test tube.

Gas collection in the laboratory

The gas syringe

1. Using a gas syringe

The gas generation apparatus, usually a conical flask or side-arm tube is connected directly to the gas syringe. It is important that the apparatus is gas tight, with no leaks.

The gas syringe

2. By downward displacement of water using a measuring cylinder.

Gas collection by downward delivery

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.

Culminating activity - Relative mass determination of a reactive metal

A known mass of a reactive metal is allowed to react with water and the hydrogen gas produced is collected by downwards displacement of water. The volume of hydrogen produced is used to determine the moles of hydrogen gas, and from the equation for the reaction, the moles of metal reacted.

The mass of the metal sample and the moles of the metal sample can then be used to determine the relative mass using the equation:

Relative mass = mass/moles

Experiment E01 Relative Mass Determination - experimental details

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?

Chemistry 2.4 — Rates of reaction

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)

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.

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.

Chemistry 2.5 — Electrolysis of water

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.

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.

Now test yourself

Click on the button below to access the self-tests for MYP9 and MYP10.

MYP Self-test