IB Chemistry HLTopic 6 — Electron TransferPaper 1 & 2Practical skill~9 min read
Acids with Reactive Metals
Drop a piece of zinc into hydrochloric acid and it fizzes. That fizzing is redox happening right in front of you: the metal is handing electrons to hydrogen ions, no wires or electrodes required. This page is the bridge between the reactivity series and everything electrochemical that follows.
📚 What you need to know
The general reaction is acid + metal → salt + hydrogen.
The metal is oxidised; the H+ ions from the acid are reduced to hydrogen gas.
Only metals above hydrogen in the reactivity series react. Copper, silver and gold do not.
The anion of the acid is a spectator — it appears unchanged on both sides.
A more reactive metal or a stronger acid gives a faster reaction: more fizzing, metal disappears sooner.
A salt is what you get when the hydrogen of an acid is replaced by a metal ion.
What is actually happening
Keep this picture in mind for the next page. A voltaic cell is this same reaction, but with the two halves separated so the electrons have to travel through a wire.
From full equation to ionic equation
Start with the full equation and strip out everything that does not change.
🧩 Getting to the ionic equation
Full equation: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Solids, liquids and gases never split. Only aqueous ionic compounds do.
The ionic equation is the honest version. It shows you that swapping hydrochloric acid for sulfuric acid changes nothing about the chemistry — you would still get the same ionic equation, just with a different spectator ion. Only the name of the salt changes.
Which metals react, and how hard
Metal
With dilute acid
What you observe
Why
Potassium, sodium
Dangerously violent
Not done in school labs
Far above hydrogen; loses electrons extremely easily
Magnesium
Fast
Vigorous fizzing, metal gone in under a minute
Well above hydrogen
Zinc, iron
Steady
Gentle stream of bubbles
Above hydrogen, but not by much
Copper, silver
No reaction
Nothing at all
Below hydrogen; cannot reduce H+
Strong versus weak acid: at the same concentration, hydrochloric acid fizzes faster than ethanoic acid with the same metal. A strong acid is fully ionised, so it has a much higher H+ concentration and the collisions happen more often. The total gas produced can still be the same — only the rate differs.
Worked examples
WORKED EXAMPLE
0.240 g of magnesium is added to excess dilute hydrochloric acid. Calculate the volume of hydrogen produced at STP. (Ar Mg = 24.31, molar volume = 22.7 dm3 mol−1)
Step 1: Write the equationMg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)Step 2: Moles of magnesiumn = 0.240 ÷ 24.31 = 9.87 × 10−3 molStep 3: Ratio is 1 : 1, so the same moles of H2n(H2) = 9.87 × 10−3 molStep 4: Convert to volumeV = 9.87 × 10−3 × 22.7 = 0.224 dm3224 cm3 of hydrogenthe acid is in excess, so magnesium is the limiting reactant
WORKED EXAMPLE
25.0 cm3 of 2.00 mol dm−3 hydrochloric acid reacts with excess zinc. Find the volume of hydrogen at STP and the mass of zinc used. (Ar Zn = 65.38)
Step 1: Moles of acidn = 0.0250 × 2.00 = 0.0500 molStep 2: The ratio is 2 HCl to 1 H2n(H2) = 0.0500 ÷ 2 = 0.0250 molStep 3: Volume of gasV = 0.0250 × 22.7 = 0.5675 dm3Step 4: Zinc is also 1 : 1 with H2m = 0.0250 × 65.38 = 1.635 g568 cm3 of hydrogen; 1.63 g of zinc reactedthis time the acid is limiting, because the zinc is in excess
WORKED EXAMPLE
Explain why copper does not react with dilute sulfuric acid, but magnesium does.
Step 1: Compare each metal with hydrogenMagnesium is above hydrogen in the reactivity series; copper is below it.Step 2: Say what that means in electron termsMagnesium loses electrons more readily than hydrogen does, so it can reduce H+ to H2.Mg + 2H+ → Mg2+ + H2Step 3: Apply it to copperCopper holds its electrons more tightly than hydrogen does, so no transfer happens.Copper cannot reduce H+, so there is no reactionthis is why copper pipes can carry slightly acidic water safely
💡 Exam tip
Learn the general pattern acid + metal → salt + hydrogen and name the salt properly: hydrochloric gives a chloride, sulfuric gives a sulfate, nitric gives a nitrate.
Give both half-equations if the question mentions redox, oxidation or electron transfer.
Check which reactant is limiting. “Excess” in the question tells you which one is not.
Use 22.7 dm3 mol−1 at STP for IB, and watch whether the answer is wanted in cm3 or dm3.
Describe observations like a person in the lab: bubbles, metal disappearing, the tube getting warm.
For rate comparisons say higher H+ concentration, not just “stronger acid”.
⚠ Common mix-up
Splitting solids or gases into ions. Only aqueous ionic compounds split up.
Leaving spectator ions in the ionic equation.
Forgetting hydrogen is diatomic. H2, so you need 2H+ and 2 electrons.
Confusing strong with concentrated. Strong means fully ionised; concentrated means a lot of it dissolved.
Assuming a faster reaction gives more gas. The total depends only on the limiting reactant.
Expecting hydrogen from nitric acid. Nitric acid is an oxidising acid and gives nitrogen oxides instead — it is the exception to the general pattern.
Up next: Primary Cells — take this exact reaction, pull the two halves apart into separate beakers, and force the electrons to travel through a wire. Now you have a battery.
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