Physical chemistry flashcards

Topic 3: Physical chemistry

Exothermic and endothermic: heat, surroundings and sign of ΔH

Exothermic: heat given out, surroundings get hotter, ΔH negative. Endothermic: heat taken in, surroundings get colder, ΔH positive.

Calculating ΔH from a calorimetry experiment

Q = m × c × ΔT (m = mass of water or solution, not the fuel). Convert to kJ (÷ 1000). ΔH = Q ÷ moles of the reactant not in excess. Temperature rose → negative sign.

200 g of water warms by 15.0 °C. Calculate Q. (c = 4.2 J/g/°C)

Q = 200 × 4.2 × 15.0 = 12 600 J (12.6 kJ)

Bond breaking and bond making: which is endothermic, and how is ΔH found from bond energies? (Paper 2 only)

Breaking is endothermic; making is exothermic. ΔH = total energy of bonds broken − total energy of bonds made.

Activation energy vs ΔH on a reaction profile

Activation energy: from the reactants line up to the peak. ΔH: from the reactants line to the products line.

Why does raising the temperature increase the rate? (two reasons)

Particles move faster, so they collide more often, and a greater proportion of collisions have energy equal to or greater than the activation energy.

More collisions vs more successful collisions

Only temperature and catalysts change the proportion of collisions that succeed. Concentration, pressure and surface area just make collisions more frequent.

Catalyst

A substance that increases the rate of a reaction and is chemically unchanged at the end. It provides an alternative pathway with a lower activation energy; ΔH is unchanged.

Marble chips + dilute HCl rate practical: what to measure and what to keep the same

Measure mass loss (cotton wool plug) or gas volume at regular times. Change chip size or acid concentration only; keep the mass of chips, volume of acid and temperature the same.

Heating hydrated copper(II) sulfate, then adding water to the product

Heating: blue → white (endothermic). Adding water: white → blue, and it gets hot (exothermic).

Equal rates vs equal concentrations at equilibrium (Paper 2 only)

At dynamic equilibrium the forward and reverse rates are equal; the concentrations stay constant, but are not necessarily equal. It needs a sealed container.

Effect of temperature, pressure and a catalyst on the position of equilibrium (Paper 2 only)

Temperature up: shifts in the endothermic direction. Pressure up: shifts to the side with fewer moles of gas. Catalyst: no change in position — equilibrium is reached sooner.