Further Organic & Inorganic Chemistry Practicals (OCR A Level Chemistry A): Flashcards

False.

Benzoic acid dissolves in hot water but is relatively insoluble in cold water, which is precisely what makes it suitable for purification by recrystallisation.

The mixture can be stirred or the inner surface of the glass vessel scratched with a glass rod to provide nucleation sites for crystallisation.

Benzoic acid does not decompose on heating, melts sharply within a useful temperature range, and produces a precise, reproducible melting point.

The nitrating mixture is a combination of concentrated nitric acid and concentrated sulfuric acid used to introduce a nitro group (NO₂) into an aromatic compound via electrophilic substitution.

During the nitration of methyl benzoate, the temperature must be kept below 6 °C by placing the flask in an ice water bath to prevent di nitration.

True.

Pouring the mixture over crushed ice causes the product to solidify, allowing it to be recovered by Buchner filtration.

The mixing of concentrated acids is highly exothermic, so cooling prevents the temperature from rising above the safe limit and causing unwanted di-nitration.

An electrophilic substitution is a reaction in which an electrophile replaces a hydrogen atom on an aromatic ring, preserving the ring's delocalised electron system.

Ethanol is flammable, so a naked flame presents a fire hazard; a hot plate provides a safer heat source.

True.

A sample that melts at this temperature with a sharp, narrow range confirms a pure product; a lower or broadened melting point indicates impurities.

Tollens' reagent is an ammoniacal silver nitrate solution used to distinguish aldehydes from ketones. It oxidises aldehydes to form a silver mirror but does not react with ketones.

Bromine water is used to test for unsaturation (alkenes). A positive result is shown by the decolourisation of the orange solution, indicating that an addition reaction has taken place.

True.

Silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide is insoluble in both dilute and concentrated ammonia.

2,4-dinitrophenylhydrazine (2,4-DNPH) is added; a deep-orange precipitate confirms the presence of an aldehyde or ketone.

The orange precipitate is purified by recrystallisation and its melting point is measured and compared to literature values.

The silver mirror test is a test using ammoniacal silver nitrate in which an aldehyde reduces Ag⁺ ions to silver metal, producing a mirror-like coating on the inner surface of the test tube.

True.

The carboxylic acid reacts with sodium carbonate to produce carbon dioxide gas, observed as bubbling or effervescence.

A salt bridge is a strip of filter paper soaked in a saturated ionic solution that connects two half-cells, allowing ion flow to complete the circuit while preventing mixing of the two solutions.

Before setting up an electrochemical cell, metal strips should be cleaned with sandpaper to remove any oxide coating that would affect the EMF reading.

False.

Standard conditions are difficult to achieve in a school lab, so measured values may differ from theoretical values, although the relative trend between cells should still hold.

A negative reading means the terminals are connected the wrong way round; swapping the terminal connections will give a positive reading.

Reusing a salt bridge risks cross-contamination between solutions, which would alter the ionic composition of the half-cells and affect the EMF reading.

An EMF (electromotive force) is the potential difference between two half-cells under open-circuit conditions, measured in volts. It reflects the difference in electrode potential between the two half-cells.

True.

A larger EMF reflects a greater difference in electron-pushing power between the two metals, corresponding to a larger gap in the electrochemical series.