Precipitation Reactions

Question 1

why do we usually use precipitation reactions in the first place

Answer 1

• to use in chemical tests

- and in working out chemical equations

Question 2

what is the test for CO2

Answer 2

• bubble the sample gas through limewater

- if it turns cloudy or looks like a milky emulsion, CO2 is present

Question 3

what is the limewater actually called

Answer 3

calcium hydroxide solution

Question 4

why does the limewater test work

Answer 4

• 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

Question 5

what is the equation for the reaction between calcium hydroxide and carbon dioxide

Answer 5

Ca(OH)2(aq) + CO2(g) = CaCO3(s) + H2O(l)

Question 6

what is used to test for the presence of sulfate ions

Answer 6

• the addition of barium ions

- usually from barium chloride or barium nitrate

Question 7

what happens when sulfate ions react with barium ions

Answer 7

• it forms barium sulfate

- which shows up as a white precipitate

Question 8

what is the equation for the reaction between barium chloride and sodium sulfate

Answer 8

BaCl2 + Na2SO4 = BaSO4 + 2NaCl

Question 9

what can the addition of silver ions from silver nitrate be used to testing

Answer 9

the presence of halide ions

Question 10

what kind of precipitates form between silver ions and halide ions

Answer 10

silver halide precipitates

Question 11

what is the equation for the reaction between silver nitrate solution and sodium chloride solution

Answer 11

NaCl(aq) + AgNO3(aq) = AgCl(S) + NaNO3(aq)

Question 12

what is the word equation for the reaction between lead nitrate and potassium iodide

Answer 12

lead nitrate + potassium iodide = lead iodide + potassium nitrate

Question 13

what do the two reactants and lead iodide look like

Answer 13

• the two reactants are colourless solutions

- but lead iodide is a yellow precipitate

Question 14

what is the step by step process of the experiment of reacting lead nitrate with potassium iodide

Answer 14

• 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

Question 15

what would the concentrations of both solutions usually be

Answer 15

1 moldm-3

Question 16

what does the depth of the precipitate indicate

Answer 16

the mass of the precipitate in grams

Question 17

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

Answer 17

• the reactions in tube 1 to 4 are incomplete
• but the reactions in 5 to 7 are complete
• 6 and 7 have excess solutions

Question 18

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

Answer 18

• 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

Question 19

what does that ratio mean

Answer 19

that two moles of potassium iodide react with one mole of lead nitrate

Question 20

given that the equation for lead nitrate is Pb(NO3)2, what is the equation for this reaction

Answer 20

Pb(NO3)2(aq) + 2KI(aq) = 2KNO3(aq) + PbI2(s)