Understanding Physical chemistry: Edexcel IGCSE Chemistry Explained

Physical chemistry is where the subject shifts from "what happens" to "how fast, how much energy, and in which direction." These three topics are among the most commonly examined on the entire specification.

The physical chemistry section of the Edexcel IGCSE Chemistry specification covers energetics, rates of reaction, and reversible reactions and equilibria. Together they account for a significant share of marks on both Paper 1 and Paper 2, and they test a combination of conceptual understanding and calculation skill that separates strong candidates from average ones. These edexcel igcse chemistry revision notes work through each topic with the kind of precision the exam demands.

If you are looking for physical chemistry edexcel igcse material that covers the igcse 4ch1 specification point by point, this is it.

Energetics

Every chemical reaction involves an energy change. Reactions that release heat to the surroundings are exothermic. Reactions that absorb heat from the surroundings are endothermic.

Feature	Exothermic	Endothermic
Temperature of surroundings	Increases	Decreases
Energy transfer	Energy released to surroundings	Energy absorbed from surroundings
Sign of enthalpy change	Negative (-)	Positive (+)
Examples	Combustion, neutralisation, many displacement reactions	Thermal decomposition, dissolving some salts (e.g. ammonium nitrate)

Calorimetry measures the temperature change during a reaction and uses it to calculate the heat energy change. The key equation is:

Q = mcΔT

Where Q is the heat energy change (in joules), m is the mass of the solution being heated (in grams), c is the specific heat capacity of water (4.18 J/g/degrees C), and ΔT is the temperature change (in degrees C).

Worked example: 50.0 cm³ of 1.0 mol/dm³ hydrochloric acid is added to 50.0 cm³ of 1.0 mol/dm³ sodium hydroxide solution. The temperature rises by 6.8 degrees C. Calculate Q.

Total volume of solution = 100.0 cm³, so mass = 100.0 g (assuming density = 1 g/cm³)
Q = 100.0 x 4.18 x 6.8 = 2842 J (or 2.84 kJ)

To calculate the molar enthalpy change (ΔH), divide Q by the number of moles of the limiting reagent:

ΔH = Q / moles

In the example above, moles of HCl = 1.0 x (50.0/1000) = 0.05 mol, so ΔH = 2842 / 0.05 = 56,840 J/mol = 56.8 kJ/mol. Since the temperature rose, the reaction is exothermic, so ΔH = -56.8 kJ/mol.

Energy level diagrams show the relative energy of reactants and products. For an exothermic reaction, the products are lower than the reactants (energy has been released). For an endothermic reaction, the products are higher (energy has been absorbed). The vertical difference between them represents the enthalpy change (ΔH).

Bond energies: Breaking bonds is endothermic (requires energy). Making bonds is exothermic (releases energy). The overall enthalpy change of a reaction can be calculated from bond energies:

ΔH = total energy needed to break bonds - total energy released when bonds form

If more energy is released than absorbed, ΔH is negative (exothermic). If more energy is absorbed than released, ΔH is positive (endothermic).

Worked example: Calculate the enthalpy change for the combustion of hydrogen: 2H₂ + O₂ → 2H₂O
Bond energies: H-H = 436 kJ/mol, O=O = 498 kJ/mol, O-H = 464 kJ/mol

Bonds broken: 2 x H-H + 1 x O=O = (2 x 436) + 498 = 1370 kJ
Bonds formed: 4 x O-H (two water molecules, each with two O-H bonds) = 4 x 464 = 1856 kJ
ΔH = 1370 - 1856 = -486 kJ/mol (exothermic)

Rates of reaction

The rate of a chemical reaction measures how quickly reactants are used up or products are formed. Five factors affect it:

Surface area of a solid: Increasing surface area (e.g. using powder instead of lumps) increases the rate because more particles are exposed to the other reactant, so collisions are more frequent.
Concentration of a solution: Higher concentration means more particles per unit volume, so collisions between reactant particles are more frequent.
Pressure of a gas: Higher pressure pushes gas particles closer together, increasing collision frequency.
Temperature: Higher temperature gives particles more kinetic energy. They move faster, collide more often, and a greater proportion of collisions have energy equal to or greater than the activation energy.
Catalysts: A catalyst increases the rate without being used up. It provides an alternative reaction pathway with a lower activation energy.

All of these explanations rest on particle collision theory: for a reaction to occur, particles must collide with sufficient energy (the activation energy) and in the correct orientation. Anything that increases the frequency of successful collisions increases the rate.

Reaction profile diagrams show how the energy of the system changes during the course of a reaction. The peak represents the activation energy (E_a). For an exothermic reaction, the products sit lower than the reactants; for an endothermic reaction, the products sit higher. A catalyst lowers the peak, providing a pathway with lower E_a, but does not change the overall ΔH.

The classic experiments the specification names:

Marble chips and hydrochloric acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Vary the surface area (large chips vs small chips) or the concentration of acid. Measure the volume of CO₂ produced over time.
Catalytic decomposition of hydrogen peroxide: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). Test different catalysts (manganese(IV) oxide is the most effective). Measure the volume of oxygen produced over time.

Reversible reactions and equilibria

Some reactions are reversible, meaning they can proceed in both the forward and reverse direction. This is shown by the equilibrium symbol: ⇌

Examples from the specification:

Hydrated copper(II) sulfate (blue) ⇌ anhydrous copper(II) sulfate (white) + water. Heating drives the forward reaction; adding water reverses it.
Ammonium chloride ⇌ ammonia + hydrogen chloride. Heating causes decomposition; cooling causes the gases to recombine.

A dynamic equilibrium is established in a sealed container when the rate of the forward reaction equals the rate of the reverse reaction. At equilibrium, the concentrations of reactants and products remain constant (though not necessarily equal). Both reactions are still happening, just at the same rate.

Changing the position of equilibrium:

Change	Effect on position of equilibrium
Increase temperature	Shifts in the direction of the endothermic reaction
Decrease temperature	Shifts in the direction of the exothermic reaction
Increase pressure	Shifts towards the side with fewer moles of gas
Decrease pressure	Shifts towards the side with more moles of gas
Add a catalyst	No effect on position. Speeds up both forward and reverse reactions equally, so equilibrium is reached faster.

Worked example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ/mol (exothermic forward)

Increasing pressure shifts the equilibrium to the right (towards NH₃) because the right side has 2 moles of gas compared with 4 moles on the left. Fewer moles = less volume = favoured at high pressure.

Increasing temperature shifts the equilibrium to the left because the forward reaction is exothermic. The system opposes the increase in temperature by favouring the endothermic (reverse) direction.

Common mistakes

Saying a catalyst "gives the particles more energy." It does not. A catalyst lowers the activation energy, providing an alternative pathway. The particles' energy does not change.
Confusing rate with yield. Temperature affects both, but they are different concepts. Increasing temperature always increases rate, but the effect on equilibrium position (and therefore yield) depends on whether the reaction is exothermic or endothermic.
Forgetting that bond-breaking is endothermic. Students often state it the wrong way round. Think of it this way: pulling atoms apart takes effort (energy in), pushing them together releases energy.
Ignoring units in calorimetry. If you calculate Q in joules but give the answer in kJ without converting (divide by 1000), you will lose the mark.
Stating that a catalyst changes the position of equilibrium. It does not. A catalyst speeds up both forward and reverse reactions equally. The position stays the same; equilibrium is just reached sooner.

How physical chemistry connects to the rest of the specification

Edexcel igcse chemistry physical chemistry is not a standalone topic. Energetics connects to every reaction in the inorganic and organic sections. Rates of reaction connects to collision theory, which in turn links to particle behaviour from the principles section. Equilibria connects to industrial chemistry and the conditions chosen for manufacturing processes like the Haber process (which, while not named on this specification, illustrates the same principles).

The edexcel igcse chemistry explained in this section is designed to help you see those links. When you answer a question about why a catalyst speeds up a reaction, you are drawing on both rates of reaction (alternative pathway, lower activation energy) and energetics (the reaction profile diagram). When you explain why increasing temperature shifts an equilibrium, you are connecting equilibrium theory with the concept of endothermic and exothermic processes. These cross-topic connections are precisely what the edexcel igcse chemistry physical chemistry material is built on.

Self-check questions

25.0 cm³ of 2.0 mol/dm³ sulfuric acid is neutralised by sodium hydroxide solution. The temperature rises by 12.5 degrees C. Calculate Q. (Use c = 4.18 J/g/degrees C and assume the total mass of solution is 50.0 g.)
Draw a reaction profile diagram for an exothermic reaction, labelling the activation energy and ΔH. Then add a second curve showing the effect of a catalyst.
Explain why increasing the concentration of hydrochloric acid increases the rate of its reaction with magnesium ribbon.
For the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the forward reaction is exothermic. Predict the effect on the equilibrium position of (a) increasing the temperature and (b) increasing the pressure.
A student says that adding a catalyst to a reversible reaction at equilibrium will produce more product. Is the student correct? Explain your answer.

Physical chemistry rewards the student who can handle both concepts and calculations with equal confidence. These edexcel igcse chemistry practice questions cover the range of what the exam asks. For more edexcel igcse chemistry notes on energetics, rates, and equilibria, explore the topic resources on the Green Bridge CBT platform, where every question maps to the igcse 4ch1 physical chemistry specification.