Chapter 4: Acids and redox

A species that releases H+ ions into (aqueous) solution.

Hydrochloric acid - HCl Nitric acid - HNO3 Sulphuric Acid - H2SO4 Phosphoric acid - H3PO4 Ethanoic acid (vinegar) - CH3COOH

Strong acid: completely dissociates in aqueous solution (releases all hydrogen atoms as H+ ions) Weak acid: partially dissociates in aqueous solution, and reaction is reversible (only a small proportion of its available hydrogen atoms are released as H+ ions)

Both bases and alkalis neutralise acids to form salts. BUT an alkali dissolves in water releasing OH- ions (hydroxide ions) into solution.

(hydroxides of alkali metals or alkaline earth metals) Sodium hydroxide NaOH potassium hydroxide KOH calcium hydroxide Ca(OH)2 ammonium hydroxide NH4OH Ammonia NH3

metal oxides e.g. - magnesium oxide - calcium oxide - copper oxide metal carbonates e.g. - sodium carbonate - calcium carbonate - copper carbonate

A chemical reaction in which an acid and a base react together to produce a salt. The reaction of H+ and OH- to form H2O.

metals, metal oxides, metal hydroxides, metal carbonates, metal hydrogencarbonates, ammonia metal + acid –> salt + hydrogen metal oxide + acid –> salt + water metal hydroxide + acid –> salt + water metal carbonate + acid –> salt + water + carbon dioxide Ionic eq: 2H+ (aq) + CO3 2- (aq) –> H2O (l) + CO2 (g) metal hydrogencarbonate + acid –> salt + carbon dioxide + water HCO3- (aq) + H+ (aq) –> H2O (l) + CO2 (g)

acid + alkali –> salt + water H+ (aq) + OH- (aq) –> H2O (l)

Water present as individual molecules in the regular structure in the crystalline state. ie. Water molecules that are bonded into a crystalline structure of a compound.

Gradual loss of water of crystallisation on exposure to the atmosphere.

Substances which absorb water from the atmosphere

Absorption of water from the atmosphere and then the further dissolving of the substance forming a solution. All deliquescent substances are hygroscopic, but not vice versa (i.e. not all hygroscopic substances are deliquescent)

Sulphuric acid is a strong acid, but this is only true for one of the 2 hydrogen atoms. When sulphuric acid is mixed with water, each H2SO4 molecule dissociates, releasing just one of its two hydrogen atoms as an H+ ion. The resulting HSO4- ions formed behave as a weak acid. Other strong acids containing more than one hydrogen atom behave similarly.

Technique used to accurately measure the volume of one solution that reacts exactly with another solution.

A solution of known concentration.

• finding the concentration of a solution • identification of unknown chemicals • finding the purity of a substance

A volumetric flask is used to make up a standard solution very accurately. Typical tolerances include: - 100cm3 flask +/- 0.20 cm3 - 250cm3 flask +/- 0.30 cm3 For pipettes: - 10cm3 pipette +/- 0.04cm3 - 25cm3 pipette +/- 0.06cm3 - 50cm3 burette +/- 0.1cm3

• the solid must be sufficiently pure • the solid must be weighed accurately by difference • Accurately control the volume, using standardised volumetric glassware

The difference between a measurement and the true or accepted value.

percentage uncertainty = uncertainty / quantity measured * 100%

When quantities are measured by difference, there will be an uncertainty in each measurement, which must be combined to give the uncertainty in the final value e.g. resultant mass by difference. percentage uncertainty = 2 x uncertainty in each measurement / quantity measured x 100%

• When using apparatus with an analogue graduated scale, the uncertainty is assumed to be A/- half the smallest graduation e.g. for a burette graduated in division of 0.1cm3, the uncertainty in each measurement is +/- 0.05cm3 • When using digital apparatus, the uncertainty is presumed to be +/- the resolution of the apparatus in each measurement.

• Solid is weighed accurately • Solid is dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark. • The solution is transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water. • The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the BOTTOM of the meniscus lines up exactly with the mark. • Finally the volumetric flask is slowly inverted several times to mix the solution thoroughly.