Precipitation Reactions
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why do we usually use precipitation reactions in the first place
• to use in chemical tests - and in working out chemical equations
what is the test for CO2
• bubble the sample gas through limewater - if it turns cloudy or looks like a milky emulsion, CO2 is present
what is the limewater actually called
calcium hydroxide solution
why does the limewater test work
• when carbon dioxide is bubbled through calcium hydroxide • they react together to form calcium carbonate • this is insoluble so it forms a white precipitate • known to be the cloudyness
what is the equation for the reaction between calcium hydroxide and carbon dioxide
Ca(OH)2(aq) + CO2(g) = CaCO3(s) + H2O(l)
what is used to test for the presence of sulfate ions
• the addition of barium ions - usually from barium chloride or barium nitrate
what happens when sulfate ions react with barium ions
• it forms barium sulfate - which shows up as a white precipitate
what is the equation for the reaction between barium chloride and sodium sulfate
BaCl2 + Na2SO4 = BaSO4 + 2NaCl
what can the addition of silver ions from silver nitrate be used to testing
the presence of halide ions
what kind of precipitates form between silver ions and halide ions
silver halide precipitates
what is the equation for the reaction between silver nitrate solution and sodium chloride solution
NaCl(aq) + AgNO3(aq) = AgCl(S) + NaNO3(aq)
what is the word equation for the reaction between lead nitrate and potassium iodide
lead nitrate + potassium iodide = lead iodide + potassium nitrate
what do the two reactants and lead iodide look like
• the two reactants are colourless solutions - but lead iodide is a yellow precipitate
what is the step by step process of the experiment of reacting lead nitrate with potassium iodide
• place the same volume of potassium iodide solution in a series of test tubes • add different volumes of lead nitrate solution to the tubes • place each one in a centrifuge and spin the tubes for the same length of time • measure the depth of precipitate in each tube
what would the concentrations of both solutions usually be
1 moldm-3
what does the depth of the precipitate indicate
the mass of the precipitate in grams
the increase in the amount of precipitate stops increasing after the fifth tube out of 7. what does this mean about the reactions going on in all the other tubes
• the reactions in tube 1 to 4 are incomplete • but the reactions in 5 to 7 are complete • 6 and 7 have excess solutions
in the fifth tube, the volume of potassium iodide solution is 5cm^3 and for lead nitrate it is 2.5cm^3. how would you use this to work out the moles of each substance that have reacted together and their ratio
• you were given the conc of 1 moldm-3 • and mol = conc x vol • so 1 x 0.005 = 0.005 moles for potassium iodide • 1 x 0.0025 = 0.0025 moles for lead nitrate • giving you a ratio 2 : 1
what does that ratio mean
that two moles of potassium iodide react with one mole of lead nitrate
given that the equation for lead nitrate is Pb(NO3)2, what is the equation for this reaction
Pb(NO3)2(aq) + 2KI(aq) = 2KNO3(aq) + PbI2(s)