OCR B A Level Chemistry Specimen Papers Questions

A flame test should be carried out using a [blank_start]nichrome[blank_end] wire and [blank_start]concentrated[blank_end] HCl. If it was calcium that was being tested the flame would be [blank_start]brick-red[blank_end].

To test for the presence of iron(III) chloride in a sample of limestone you would add a solution of [blank_start]silver[blank_end] nitrate to the sample and a [blank_start]white[blank_end] precipitate will be formed for the chloride ions. You would also do a test for the Fe(III) by adding [blank_start]sodium hydroxide[blank_end] giving a [blank_start]brown[blank_end] precipirate.

The reagent used with sodium iodide to produce purer hydrogen iodide is [blank_start]phosphoric acid[blank_end].

The absorption of UV radiation by ozone is important for human life since UV radiation causes damage to [blank_start]skin[blank_end].

The shape of an ozone molecule is [blank_start]bent[blank_end] because there are [blank_start]3[blank_end] groups of electrons around a central O atom. The bond angle could be [blank_start]120°[blank_end].

First ionisation enthalpy [blank_start]increases[blank_end] across a period since the atoms [blank_start]increase[blank_end] in size meaning there are [blank_start]more protons & neutrons[blank_end] in the nucleus. The outer shell electrons are roughly the same distance from the nucleus since they are in the same shell so the attraction to the electrons is [blank_start]greater[blank_end] to they are [blank_start]more difficult[blank_end] to remove.

Rate of reaction decreases as temperature decreases since the molecules have [blank_start]less[blank_end] kinetic energy so there will be fewer collisions with sufficient acitvation [blank_start]enthalpy[blank_end] and also fewer collisions per second.

An equilibrium reaction is exothermic, increasing the temperature will shift the position of equilibrium in the [blank_start]endothermic[blank_end] direction (to the [blank_start]left[blank_end]) which will mean the concentration of product will [blank_start]decrease[blank_end].

% yield may be low if the reaction [blank_start]hasn't[blank_end] gone to completion or if there was [blank_start]loss[blank_end] of materials when transfering between vessels.

Reactions go [blank_start]faster[blank_end] at higher temperatures since the particles have [blank_start]more[blank_end] kinetic energy and a larger proportion have enough kinetic energy for [blank_start]sucessful[blank_end] collisions to take place therefore the rate of reaction is [blank_start]faster[blank_end].

When doing recrystallisation you must ensure the solute has a [blank_start]high[blank_end] solubility in your chosen solvent at a high temperature & a [blank_start]low[blank_end] solubility in cold. You must dissolve the [blank_start]minimum[blank_end] amount of solute in the hot solvent. [blank_start]Filter[blank_end] the hot solution to remove insoluble impurities and leave to [blank_start]cool[blank_end] so soluble impurities are removed then filter off the crystals and [blank_start]dry[blank_end] them.

Nuclear [blank_start]fission[blank_end] is when to light nuclei form a heavier nucleus. It must be done at a [blank_start]high[blank_end] temperature and pressure to overcome the repulsion between the [blank_start]positive[blank_end] nuclei.

A reason for an enthalpy change of combustion to be much less than the predicted value is [blank_start]incomplete[blank_end] combustion or evapouration of fuel from the [blank_start]wick[blank_end].

Dynamic equilibrium is where the rate of the forward reaction [blank_start]is equal[blank_end] to rate of the backward reaction so that the overall concentrations of substances [blank_start]is constrant[blank_end].