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what is thermal decomposition?
a reaction which a compound decomposes on heating
Soluble and insoluble in water rules
Explain how the trend in the reactivity of the Group 2 elements is determined by their electronic configurations
atomic radii increases as outer electron is further from nucleus
there is more shielding
so the first ionisation energy increases/outer electrons removed more easily and reactivity increases
what does group 1 metal reacting with water form?
what does group 1 metal reacting with chlorine form?
hydroxides
colourless, ionic chlorides, soluble in water
why are carbonates alkaline?
the carbonate ions remove H+ ions from water molecules to form hydrogen carbonate ions and hydroxide ions
what does nitrate decompose into?
metal nitrite MNO2 + oxygen, lithium forms oxide + nitrogen dioxide + oxygen like group 2
Why are compounds of sodium and potassium widely used as chemical agent?
ions of alkali metals are unreactive
most of these compounds are soluble in water; most other metal hydroxides and carbonates are insoluble so not available in aqueous solution
ions of alkali are colourless in aqueous solution so they do not interfere with colour changes
describe the flame test procedure
dip a nichrome wire into conc. HCl to clean; dip into crystals to be tested; put on blue flame on bunsen burner, observe colour change
why can you observe an colour in a flame test?
electrons of the ionic compound get promoted as they get heated up, the electrons gain enough kinetic energy to be raised to a higher energy level. The electrons then emit photons as energy as the electron drop back to lower energy level, and also emit electromagnetic radiation in the visible spectrum. Different energy lost/emitted so gives off different flame colours for different elements
what does group 1 metal reacting with water form?
what does group 1 metal reacting with chlorine form?
metal hydroxides
white chlorides
what are basic oxides?
Metal oxides which react with acids to form salts and water; the oxide ions which act as base by taking a hydrogen ion from the acid; basic oxides which dissolve in water are alkalis
group 2 metals are basic oxides except beryllium oxide; they react with acids to form salts
the hydroxides of the elements Mg to Ba are:
-similar in that they all have the formula M(OH)2, and are soluble in water forming alkaline solutions
different in that their solubility increases down the group - if the negative ion is relatively large, the metal compounds of the metal with the largest ions are least soluble; if the negative ion is small, the compounds of metal with the smallest ions are least soluble
what is the product of group 2 metal nitrates decomposition?
oxides + nitrogen dioxide + oxygen
what is thermal stability?
an indication of the ease with which compounds decompose on heating. Compounds are stable if they do not tend to decompose into their elements or into other compounds.
what factor affect thermal stabilities?
charge on the metal ions - the larger the charge, the less stable the compound
the size of the metal ions - the smaller the metal ion, the less stable the compound
group 1 carbonates do not decompose except for lithium because they dont have a big enough charge density to polarise the carbonate ion as they only form 1+ ions; but lithium is small enough to have a polarising effect.
are halogens more or less soluble in water than organic solvents?
less soluble, more in organic solvents
iodine does not dissolve in water
reactions of halogens with metals
chlorine and bromine react with s-block metals to form ionic halides
iodine reacts with small cations or highly charged cations to form iodides because of the polarisability of large iodide ion
also react with most metals in d-block
reactions of halogens with non-metals
chlorine reacts with most non-metals to form molecular chlorides, not with carbon, oxygen or nitrogen
hydrogen burns in chlorine to produce the colourless acidic gas hydrogen chloride
bromine also oxidises non-metals forms molecular bromides
iodine oxidises hydrogen to form hydrogen iodide, reversible reaction
reactions of halides with concentrated sulfuric acid
hydrogen chloride gas + hydrogen sulfide
hydrogen halides properties:
colourless gases at room temperature which fume in moist air
very soluble in water, forming acidic solutions(strong) which ionise completely in water
boiling point increases down the group because of increase in electron number and so London forces increase, more energy require to weaken the London forces
Explain whether magneisum carbonate is more or less thermally stable than barium carbonate
how does this affect the enthalpy change for thermal decomposition
the larger the charge and smaller the ionic radius, the greater its charge density. The greater the charge density the greater the polarising power of the ion. Magnsium has a greater charge and smaller ionic radius.
magnesium ion has a higher polarising power (causes more polarisation of ions) attracts the bonding electrons in neighbouring ions more strongly. This pull on the electrons of an ion distorts the bonding and as C-O bond is weakened so easier to break down.
hence magnsium carbonate is less thermally stable than barium carbonate, so enthalpy is more endothermic
write the ionic equation of hydrochloric acid reacting with sodium carbonate
H+ + CO32- → CO2 + H2O
descirbe how you would compare the thermal stability of two different group 2 nitrates
group 2 metal nitrates form metal oxides + nitrogen dioxide and oxygen, as there are gases form, we can use a gas syringe to indicate the end of a reaction
hence, set up the apparatus and when started heating using same bunsen burner, start the timer
stop the timer when the volume in the gas syringe is constant
then record the time taken for the reaction to complete for different group 2 nitrates, then compare the time.
same amount of each nitrate in separate test tubes safety precaution: fume cupboard
give a reason why magneisum ion does not produce a flame colour
there is no emission of light in the visible region
State one way which you would ensure a fair test in the experiment of comparing thermal decomposition
make sure the distance between the flame and test tube is constant
give a reason why carrying out a flame test on a mixture of two metal cation does not clearly show two different metal ions are present
one of the colour will mask the other
State why hydrochloric acid is used in the second stage of the flame test before adding to cations
to stick to the metal ions
What colour is NO2 gas?
brown
How to test for ammonium ion?
using sodium hydroxide, ammonia gas is produced, so this will turn red litmus paper blue
How to use dilute ammonia/concnetrated ammonia to confirm the identity of halides?
add dilute ammonia, bromide precipitate is insoluble, add concentrated ammonia, bromide precipitate is soluble, only iodide is insoluble
give a reason why hcl acid is needed in test for sulfate ions
to react with/remove any carbonate ions, which would form a white precipitate of barium carbonate
Write the ionic equation when chlorine react with hot aqeous sodium hydroxide
3Cl2 + 6OH- → 5Cl- + ClO3- + 3H2O
Explain which halides is the strongest reducing agent by referring to reacting with sulfuric acid
When chloride reacts with sulfuric acid, there is no redox reaction, so sulfuric acid is not reduced, hence chloride ion is the worst reducing agent.
When bromide reacts with sulfuric acid, sulfuric acid is reduced to sulfur dioxide, sulfide ion is reduced from +6 to +4, total decrease of oxidation number 2
When iodide reacts with sulfuric acid, sulfide reduced from oxidation number of +6 to -2 and 0, hence is reduced much further than in the reaction with bromide, so iodide is the strongest reducing agent
hydrogen chloride does not conduct electricity in gas form but does in acid. Explain
the covalent bond in hydrogen chloride changes to an ionic bond in aqueous solution
write an ionic half-equation for the oxidtion of chlorine molecules to chlorate(I) ions in the presence of cold, aqueous hydroxide ions
Cl2 + 2OH- → H2O + ClO-+ Cl-
outline a procedure that you could use to obtain a sample of dry,solid
Make sure that the solvent used fulfil that the solid is soluble in hot solvent and insoluble in cold solvent
use minimum volume of hot solvent first and filtrate out the insoluble impurties
then use minimum volume of cold (e.g. add ice) solvent and filtrate in a vacuum of the wanted solids
pat dry with filter paper/leave to dry
Explain why when aqueous potassium bromide was added to aqeous iodine, there is no reaction.
iodine is a weaker oxidising agent than bromine
Describe an experiment to compare thermal stability of group 2 nitrates
use gas syringe to measure the volume of gas produced
need to control the temperature, so same heat applied
same amount of nitrate in separate test tubes
experiment conducted in a fume cupboard
Group 2 oxides + water
metal hydroxides
What is the colour change when added cyclohexane to chloride, bromide and iodide
green, orange, purple
