Free VCE Chemistry Flashcard Review

A reaction that releases energy to the surroundings because the products store less chemical energy than the reactants. The enthalpy change is negative (ΔH < 0); the activation-energy 'hump' is followed by a net drop to a lower product energy level.

A reaction that absorbs energy from the surroundings because the products store more chemical energy than the reactants. ΔH is positive (ΔH > 0); the product energy level sits above the reactant level on the profile.

States of all species (s, l, g, aq) AND the ΔH value with correct sign and units (kJ mol^-1), written next to the balanced equation. ΔH applies to the molar amounts exactly as the equation is balanced.

q = m c ΔT, where q = energy (J), m = mass of water/substance being heated (g), c = specific heat capacity (4.18 J g^-1 °C^-1 for water), ΔT = temperature change (°C or K). Divide by 1000 for kJ.

Petrodiesel releases slightly more energy per gram than biodiesel. Biodiesel molecules already contain C–O bonds (ester groups) that are partly oxidised, so less energy is released on combustion compared with the fully reduced hydrocarbon chains of petrodiesel.

The CO₂ released when bioethanol burns was recently absorbed from the atmosphere by the crop during photosynthesis, so combustion returns carbon that was recently fixed. Petrol releases carbon locked underground for millions of years, adding net CO₂ to the modern carbon cycle.

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l). Complete combustion (excess oxygen) gives carbon dioxide and water only.

Anode = site of oxidation (electrons released, loss of electrons); it is the negative terminal in a galvanic cell. Cathode = site of reduction (electrons gained); it is the positive terminal. Mnemonic: oxidation is loss, reduction is gain (OIL RIG); AN-OX, RED-CAT.

Completes the internal circuit and maintains electrical neutrality by allowing ion migration: anions move toward the anode half-cell, cations move toward the cathode half-cell. Without it, charge builds up and current stops almost immediately.

E°cell = E°(cathode, reduction) − E°(anode, oxidation). The stronger oxidant (higher on the table, more positive E°) is reduced; the stronger reductant (lower on the table) is oxidised. A positive E°cell indicates a spontaneous reaction.

Electrons flow only through the external wire, from anode to cathode (never through the solution or salt bridge). Ions carry charge internally through the electrolyte and salt bridge. Confusing the two loses marks.

Overall: 2H₂(g) + O₂(g) → 2H₂O(l). Advantage: converts chemical energy directly to electrical energy continuously while fuel is supplied (not limited by the Carnot cycle like combustion), with high efficiency and only water as the product.

It gives values at SLC/standard conditions (1 M, 25 °C, 100 kPa); real conditions differ. It predicts whether a reaction is spontaneous (thermodynamics) but NOT the rate — a predicted reaction may be too slow to observe.

Galvanic: spontaneous reaction, produces electrical energy, anode is negative. Electrolytic: non-spontaneous reaction driven by an external power supply, consumes electrical energy, anode is positive. In both, oxidation still occurs at the anode.

Q = I × t (Q in coulombs, I in amperes, t in seconds). n(e^-) = Q ÷ F, where F = 96 500 C mol^-1. Use the half-equation's electron ratio to find moles, then mass, of product deposited or gas formed.

Water can be oxidised (at the anode) or reduced (at the cathode) instead of the dissolved ions. Compare standard potentials: the species more easily reduced is favoured at the cathode, the one more easily oxidised at the anode (allowing for overpotential effects with gases).

The change in concentration of a reactant or product per unit time. Measured via quantities such as gas volume produced, mass lost, colour/absorbance change, pH change, or precipitate (turbidity) formed over time.

Reactant particles must collide with energy equal to or greater than the activation energy (Ea), AND collide with the correct orientation/geometry. Collisions failing either condition are unsuccessful and do not form products.

Particles move faster, so collisions are more frequent. More importantly, a greater proportion of particles have energy ≥ Ea (the Maxwell–Boltzmann distribution shifts right), so a much larger fraction of collisions are successful. The energy effect dominates.

It provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have sufficient energy to react. It is not consumed and does not change ΔH or the position of equilibrium — only the rate.