Chemical Reactions: Changes, Rates, Reversibles, and Redox
What changed, how fast, can it go back, and where did the oxygen go ⚡
Introduction
1. Introduction
Okay, this chapter is about what actually happens when substances react: whether anything new is made, how fast it happens, whether it can run backwards, and where the oxygen goes.
Four questions and it all clicks: a change is chemical if a new substance forms, a reaction's rate is how fast it goes (and you can change it), it's reversible if it can run backwards, and it's redox when oxygen is gained and lost. This is your quick refresh, not the full textbook: one key idea per topic, one worked example, and the exact move the marker is paying for. Rate of reaction is the biggest area of the chapter, so we spend the most time there. Let's go. ⚡
Four questions and it all clicks: a change is chemical if a new substance forms, a reaction's rate is how fast it goes (and you can change it), it's reversible if it can run backwards, and it's redox when oxygen is gained and lost. This is your quick refresh, not the full textbook: one key idea per topic, one worked example, and the exact move the marker is paying for. Rate of reaction is the biggest area of the chapter, so we spend the most time there. Let's go. ⚡
2. Physical vs Chemical Changes
Every change is one of two types, and it all rests on one question: did any new substance get made? A physical change makes no new substance (melting, boiling, dissolving, crushing): the substance is still the same substance, and physical changes are usually easy to reverse. A chemical change makes brand-new substances with new properties, and the evidence is a colour change, a gas, a precipitate, or heat or light given out. Burning, rusting, neutralising and electrolysis are all chemical. Careful with "easy to reverse": it's a clue, not the test. Blue copper(II) sulfate turns white on heating and blue again with water, and that's chemical and reversible. Always go back to: is there a new substance?
Key idea🔑 Key idea: No new substance = physical. New substance with new properties = chemical. Reversibility is only a clue: some chemical changes reverse too. Dissolving is physical: the stuff is still the same stuff. 🎯
Worked example
Worked Example: Spot the Physical Change
Four Test-Tubes 🧪
Four test-tubes: A salt added to water; B an iron nail in water; C zinc added to acid; D calcium carbonate added to acid. Which one is a physical change?
- 1Tube A — the salt dissolves. Nothing new is made and you could evaporate the water to get the salt back. That's physical.
- 2The rest are chemical: B the nail rusts, C gives off hydrogen, D gives off carbon dioxide — all new substances.
So it's test-tube A. Dissolving is the classic trap that looks like a reaction but isn't. ✅
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3. What Changes the Rate
Rate just means how fast a reaction goes. At Core you describe the effect of each condition. Faster with: a more concentrated solution, a higher pressure (only when the reactants are gases), a bigger surface area (smaller pieces or powder, same mass), a higher temperature, and adding a catalyst, including enzymes. To slow a reaction down, do the opposite: dilute it, lower the pressure, use bigger lumps, cool it, or take the catalyst away. These change how fast the product is made, not how much you get in the end.
Key idea🔑 Key idea: Speed it up with: more concentrated, higher gas pressure, smaller pieces, hotter, or a catalyst. Slow it down by doing the opposite. Faster = shorter time, slower = longer time. ⚡
Worked example
Worked Example: Two Ways Faster
Pick the Right Pair ✅
Heating speeds up a metal-and-acid reaction. Name two other ways to speed it up.
- 1The other rate-boosters are: more concentrated acid, smaller pieces (more surface area), and a catalyst.
- 2So pick "increase the concentration" and "use smaller pieces of metal". Anything that says cooler, more dilute, or bigger lumps is wrong.
So: increase the acid concentration and use smaller pieces. If a pair has even one wrong-direction change in it, the whole pair is wrong. ✅
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4. Following and Calculating a Rate
You can follow a rate by measuring the volume of gas (gas syringe), the loss of mass (balance, as gas escapes), or the time for a fixed change (like a cross disappearing behind cloudy liquid) — always with a stop-watch and a steady temperature for a fair test. Two sums come up a lot: mean rate = change ÷ time (units like cm³/s or g/s), and relative rate = 1 ÷ time (no units, and the shortest time wins).
Key idea🔑 Key idea: Mean rate = change ÷ time (keep the units the right way up!). Relative rate = 1 ÷ time, no units, shortest time = greatest rate. 🎯
Worked example
Worked Example: Mean Rate
Mind the Units ⏱️
A 5 cm coil of magnesium disappears in 20 s. Mean rate = length ÷ time. Work out the rate and its unit.
- 1Mean rate = 5 cm ÷ 20 s = 0.25.
- 2The unit is cm over s, so cm/s.
So it's 0.25 cm/s. Watch the trap: it's change ÷ time, not time ÷ change — doing 20 ÷ 5 gives 4.0 with the units upside-down. ✅
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5. Reading a Rate Graph
A graph of gas against time tells you two separate things. Steepness = the rate: it's steepest at the start (most reactant left) and goes flat when the reaction has finished. Final height = the total amount of product, which depends on how much reactant you started with. So a faster reaction is steeper and goes flat sooner, but can end at the same height as a slower one — that's the tell-tale shape of a catalyst or higher concentration with the same amounts.
Key idea🔑 Key idea: Steepness = rate (steepest at the start, flat = finished). Final height = total product. Highest point ≠ fastest point. 📈
Worked example
Worked Example: Where's the Rate Greatest
Find the Steepest Bit 🔍
A curve of gas against time rises steeply then flattens. Points: A at the start, B partway up, C just after the start, D near the top. Where is the rate greatest?
- 1The curve is steepest at the very start, because that's when the most reactant is present.
- 2Point A is at the start, so it's the steepest = fastest. By D it's nearly flat, so the rate there is almost zero.
So it's point A. Don't fall for D — that's where the most gas is, but the rate there is basically zero. Highest isn't fastest. ✅
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6. Catalysts
A catalyst is a substance that increases the rate of a reaction and is unchanged at the end of the reaction, so you can reuse it (manganese(IV) oxide for breaking down hydrogen peroxide). Enzymes are biological catalysts: catalase in potato or liver speeds up the breakdown of hydrogen peroxide, and amylase in saliva speeds up the breakdown of starch. The big "do not forget": a catalyst changes only the speed, never the amount of product, and its mass is the same before and after. It gets you to the same finish line faster; it never moves the finish line.
Key idea🔑 Key idea: A catalyst speeds things up and comes out unchanged (same mass before and after). It never changes how much product forms. "Catalyst makes more product" is a classic wrong answer. 🚫
Worked example
Worked Example: What a Catalyst Does
Spot the Truth ✅
Adding a catalyst makes hydrogen peroxide give off oxygen faster. Which is correct? (A) the mass of catalyst is the same before and after; (B) it makes the reaction more exothermic; (C) it increases the final volume of oxygen; (D) all the catalyst is used up.
- 1A catalyst isn't used up (D wrong) and its mass stays the same (A correct).
- 2It only changes the speed, not the energy (B wrong) and not the amount of oxygen (C wrong).
So the answer is A. A catalyst makes a reaction faster, never bigger. ✅
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7. Reversible Reactions
Some reactions can run both ways: the products react together to re-make the reactants. That's shown by the double arrow (a single arrow means one-way only). The two you must know both involve water locked in crystals: hydrated means it contains that water, anhydrous means the water's been removed. Cobalt(II) chloride is pink when hydrated and blue when anhydrous; copper(II) sulfate is blue when hydrated and white when anhydrous.
Key idea🔑 Key idea: means reversible — it can go both ways. Hydrated = has water (cobalt pink, copper sulfate blue); anhydrous = water removed (cobalt blue, copper sulfate white). 🔄
Worked example
Worked Example: Cobalt(II) Chloride
Hydrated or Anhydrous? 💧
Heating pink cobalt(II) chloride gives a blue solid and water; adding water turns it pink again. Is the reaction reversible, and is the pink solid hydrated or anhydrous?
- 1The colour change goes both ways (pink → blue → pink), so the reaction is reversible.
- 2Heating the pink solid drives off water, so the pink form is the one that contains water — it's hydrated.
So: reversible, and the pink form is hydrated. Quick rule — the hydrated form is the one that loses water when heated. ✅
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8. Hydrated vs Anhydrous: Test for Water
This reversible colour change isn't just a party trick — it's the chemical test for water. Add the unknown liquid to white anhydrous copper(II) sulfate: if it turns blue, water is present. The same idea works with cobalt(II) chloride (blue anhydrous → pink with water). Heating drives the water off, adding water brings the colour back, and it gives out heat as it does.
Key idea🔑 Key idea: White anhydrous copper(II) sulfate + water → turns blue = the test for water. It's not oxidation (no oxygen involved) — it's water going in and out of the crystals. 🟦
Worked example
Worked Example: Copper(II) Sulfate
What's the Word? 🔵
Blue copper(II) sulfate turns white when heated, and the white solid turns blue again when water is added. Which word best describes this reaction?
- 1Heat turns it white (water off); water turns it blue (water back). It goes both ways.
- 2A change you can drive both ways is reversible — not neutralisation, oxidation or reduction.
So the word is reversible, and that blue ⇄ white cycle is the test for water. ✅
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9. Redox: It's All About Oxygen
Redox means oxidation and reduction happening together. At Core you define both with oxygen: oxidation = GAIN of oxygen, reduction = LOSS of oxygen. When one substance grabs oxygen, another must lose it, so the two always come as a pair. You see this all the time in extracting metals: the metal oxide loses oxygen (reduced) while carbon or carbon monoxide grabs it (oxidised).
Key idea🔑 Key idea: Oxidation = GAIN of oxygen. Reduction = LOSS of oxygen. They always happen together = redox. Don't flip them! 🔥
Worked example
Worked Example: Which Is Reduced
Follow the Oxygen ⛏️
Zinc is extracted by . How does this show the zinc oxide is reduced?
- 1On the left the zinc has oxygen (ZnO); on the right it has none (Zn). It lost oxygen.
- 2Losing oxygen = reduction, so the zinc oxide is reduced. (Meanwhile the carbon gains oxygen, so it's oxidised — both at once = redox.)
So: the zinc oxide loses oxygen to form zinc. To prove "reduced", point to the oxygen disappearing. ✅
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10. Oxidation Numbers & Roman Numerals
Some metals form ions of more than one charge (iron can be Fe²⁺ or Fe³⁺), so the name carries a Roman numeral in brackets to say which — and that numeral is the oxidation number of the metal. So iron(II) is +2, iron(III) is +3, copper(II) is +2, lead(IV) is +4. It's not the number of atoms and it's not an energy — just the oxidation number, written with a plus sign.
Key idea🔑 Key idea: The Roman numeral in a name = the metal's oxidation number. iron(III) → +3, copper(II) → +2. Just rewrite the Roman numeral as a positive number. 🔢
Worked example
Worked Example: Oxidation Number
Read It Off 🧮
What is the oxidation number of iron in iron(III) oxide, ?
- 1The name says iron(III), and that Roman numeral is the oxidation number: read it and put a + in front, so +3.
- 2It is not the number of atoms: has two iron atoms but is iron(III) oxide. Check with charges: three = 6−, shared by two iron ions, so each iron is 3+.
So the oxidation number of iron is +3. ✅
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