Everything from the 6 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A physical change does not produce a new substance: the particles are only rearranged or moved apart, as in melting, boiling, dissolving or separating a mixture, and it is usually easy to reverse. A chemical change produces one or more new substances with different properties, as in burning, rusting or neutralisation. Chemical changes are often accompanied by an energy change and are usually difficult to reverse.
Dissolving is a physical change: evaporate the water from salt solution and the salt is recovered unchanged. Do not confuse "it disappeared" with "it reacted". And a colour change alone is not proof: heating anhydrous copper(II) sulfate is a chemical change, but a solution simply getting paler when diluted is not.
The rate of a reaction increases when the concentration of a solution or the pressure of a gas increases, when a solid is broken into smaller pieces (larger surface area), when the temperature increases, and when a catalyst is added. A catalyst increases the rate of a reaction and is unchanged at the end. Rate is measured by following a product or reactant against time, for example the volume of gas given off or the loss in mass.
Changing the rate does not change the amount of product if the same amounts of reactants are used. On the graph, a faster reaction is steeper at the start but levels off at the same final volume. Only changing the amount of the limiting reactant changes the final volume.
Extended. Particles react only when they collide with at least the activation energy. Higher concentration, higher pressure and larger surface area put more particles in contact, so the frequency of collisions increases. Higher temperature makes particles move faster: they collide more often and, more importantly, more collisions have energy at least equal to the activation energy. A catalyst provides a pathway with a lower activation energy, so more collisions are successful.
"More collisions" is not enough for the marks: say more frequent collisions (more collisions per second). For temperature, you must also say that more particles have energy greater than or equal to the activation energy; that is the bigger effect.
A reversible reaction can go in either direction, shown by the symbol ⇌. Heating hydrated copper(II) sulfate (blue) drives off water to leave anhydrous copper(II) sulfate (white); adding water reverses it. Extended: in a closed system a reversible reaction reaches dynamic equilibrium, where the forward and reverse reactions happen at the same rate and the concentrations of reactants and products stay constant. Changing the conditions moves the position of equilibrium to oppose the change.
At equilibrium the concentrations are constant, not equal. There may be far more product than reactant, or the other way round. And a catalyst never moves the position of equilibrium: it speeds up the forward and reverse reactions equally.
Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g). Nitrogen comes from the air and hydrogen from methane. Conditions: 450 °C, 20 000 kPa (200 atm), iron catalyst. Contact process: 2SO2(g) + O2(g) ⇌ 2SO3(g). Sulfur dioxide comes from burning sulfur or roasting sulfide ores; oxygen from the air. Conditions: 450 °C, 200 kPa (2 atm), vanadium(V) oxide catalyst. Extended: both forward reactions are exothermic and reduce the number of gas molecules, so the conditions are a compromise.
The catalyst does not increase the yield. And 450 °C is not chosen for the best yield: the best yield would be at a low temperature. It is a compromise between yield and rate. Name both in your answer.
Oxidation is gain of oxygen and reduction is loss of oxygen; a reaction in which both happen is a redox reaction. A Roman numeral in a name gives the oxidation number of the element, as in iron(III) oxide. Extended: oxidation is also loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease in oxidation number. An oxidising agent oxidises another substance and is itself reduced; a reducing agent reduces another substance and is itself oxidised.
The oxidising agent is the substance that is reduced, not the one that is oxidised. It sounds backwards, so reason it out: it causes oxidation in something else by taking that thing's electrons, and taking electrons is reduction.