CDS Chemistry · Formula sheet
Chemical Reactions formulas
1 formula, 9 reference tables and 12 common traps for CDS Chemistry Chemical Reactions, grouped by subtopic.
Types of Reactions and Energy Changes
Learn this subtopic in the notesState functions and Gibbs energy
Gibbs energy
- enthalpy change (heat at constant pressure)
- entropy change
- temperature in kelvin
Physical and chemical changes
| Event | Kind of change |
|---|---|
| Freezing of water | Physical |
| Rusting of iron | Chemical |
| Cooking food | Chemical |
| Burning candle | Both physical and chemicalQ |
Combination, decomposition, displacement and double displacement
Common traps
The candle is the 'both' answer
Freezing is only physical and rusting only chemical. A burning candle is the standard example of both at once: melting wax plus burning vapour.
Swapping partners is double displacement
In K₂SO₄ + BaCl₂ both compounds exchange ions. That is double displacement, not a single displacement or an addition reaction.
Lead nitrate gives PbO, not PbO₂
The solid left is lead monoxide, PbO, and the brown gas is NO₂. Options with PbO₂ or with NO are wrong.
q and w are path functions, their sum is not
Heat and work each depend on the route taken, but q + w = ΔU, the change in internal energy, depends only on the start and end states.
Both negative means 'below', not 'above'
When ΔH and ΔS are both negative, the reaction is spontaneous below T = ΔH ÷ ΔS. That temperature is a maximum, not a minimum.
Oxidation and Reduction
Learn this subtopic in the notesOxidation and reduction in everyday changes
The thermite reaction
| Statement | True or false |
|---|---|
| Iron oxide reacts with aluminium to join railway tracks | TrueQ |
| The heat given out is used for welding | True |
| Aluminium acts as an oxidising agent | False — it is the reducing agent |
| Molten iron and molten aluminium are formed | False — molten iron and aluminium oxide |
The standard hydrogen electrode
| Statement about the SHE | True or false |
|---|---|
| Its standard potential is taken as 0 V by convention | True |
| Its absolute electrode potential is not zero | TrueQ |
| Its potential is zero only at 25 °C | False |
Common traps
A precipitate is not an oxidation
Forming BaSO₄ from barium chloride and sodium sulphate changes no oxidation state, so it is not redox. Rusting, rancidity and souring wine are.
The reducing agent is oxidised
The substance that reduces something else gives up electrons, so it is itself oxidised. Don't confuse the agent with what happens to it.
Aluminium reduces, it does not oxidise
In thermite, aluminium takes oxygen from iron oxide, so aluminium is the reducing agent. 'Aluminium acts as an oxidising agent' is false.
The hydrogen electrode's zero is agreed, not measured
The SHE is assigned 0 V so that other electrodes can be compared with it. Its absolute potential is not zero, and the zero is not limited to 25 °C.
Reactions in Daily Life
Learn this subtopic in the notesQuicklime, slaked lime and whitewashing
Tests for common gases
| Gas | Test | Made by |
|---|---|---|
| Carbon dioxide | Limewater turns milky | Carbonate + dilute acid; breathing outQ |
| Hydrogen | Burns with a pop | Zinc or magnesium + dilute acid |
| Oxygen | Relights a glowing splint | Heating potassium permanganate |
Electrolysis of brine
| Product | Where it forms |
|---|---|
| Chlorine gas | Anode |
| Hydrogen gas | Cathode |
| Sodium hydroxide | Left in the solutionQ |
Common traps
Whitewash is slaked lime, not quicklime
Walls are coated with Ca(OH)₂. Quicklime (CaO) is what you start from, and it is the slaked lime that reacts with CO₂ on the wall.
The shine is calcium carbonate
The shiny finish is CaCO₃, formed from Ca(OH)₂ and CO₂. Oxygen plays no part.
Zinc and acid give hydrogen, not CO₂
Only a carbonate (or hydrogen carbonate) with acid gives CO₂, which turns limewater milky. A metal with acid gives hydrogen, which does not.