topic 5 - amounts of substance

Question 1

theoretical yield definition

Answer 1

the maximum possible mass of a product, assuming there was a complete reaction and no transfer losses

Question 2

what are the 3 reasons why the actual yield is less than the theoretical yield

Answer 2

-incomplete reaction, either because not all the reactants reacted or the reaction is reversible
-side reactions lead to unwanted products
-impurities in the product so it may need to be purified, leading to a loss of product
-practical losses when transfering the reactants/products between different equipment

Question 3

percentage yield equation

Answer 3

actual yield/theoretical yield x 100

Question 4

actual yield definition

Answer 4

the actual mass of products obtained from a reaction by weighing

Question 5

what 5 factors do industries need to take into account when deciding the suitability of an industrial process

Answer 5

-atom economy
-percentage yield
-availability of raw materials
-cost of raw materials
-amount of energy needed

Question 6

equation for atom economy

Answer 6

molar mass of desired product/molar mass of all products x 100
molar mass means the relative molecular mass times the molar ration (so you include the big number when working out the RMM of the compounds)

Question 6

what is the atom economy of addition reactions, why

Answer 6

100% because there is only 1 product so all the atoms in the reactants are used to make the desired product

Question 6

Definition of a mole

Answer 6

The amount of substance that contains the same number of particles as the number of carbon atoms in exactly 12g of the carbon12 isotope

Question 7

what can we say about the atom economy of elimination, substitution and multistep reactions

Answer 7

they have lower atom economies

Question 7

Avogadro’s constant definition

Answer 7

The number of carbon12 atoms in exactly 12g of carbon12

Question 7

what is meant by a standard solution

Answer 7

a solution where its the concentration is accurately known

Question 7

describe how to make a standardised solution of sulfamic acid, when using 2.4g of solid sulfamic acid

Answer 7

-measure 2.4g of solid sulfamic acid into a weighing boat, and note the mass of the acid+weighing boat
-transfer the acid from the weighing boat to a beaker, reweigh the weighing boat
-add about 100ml of deionised water into the beaker and stir till all sulfamic acid dissolves
-remove the stirring rod (washing any solution away with deionised water)
-place a funnel into the neck of the volumetric flask and pour the solution from the beaker into the flask
-rinse the inside of the beaker with deionised water and add this to the flask
-fill the flask with deionised water till the graduation mark
-add a stopper and invert it several times to make a uniform solution

Question 7

what is the equivalence point

Answer 7

the point at which there are exactly the right volume of substances to complete a reaction

Question 7

describe how to conduct a titration, to find out the concentration of the sodium hydroxide solution
(sulfamic acid and methyl orange is used)

Answer 7

-rinse the conical flask, burette and pipette with deionised water
-rinse the pipette again but with NaOH, use a pipette to measure 25cm^3 of NaOH and transfer to the conical flask
-add 3 drops of methyl orange
-rinse the burette again but with sulfamic acid, fill burette with sulfamic acid
-runs some sulfamic acid through the tap till the tip is full with acid, record the reading at the miniscus
-first add the acid at a steady pace, then slower once its getting near to the end point, then drop by drop very near the end point, swirling all the time
-once the solution turns yellow close the tap and record the burette reading
-calculate the titre
-repeat the titration untill you get concordant results

Question 8

what is the reason for using a white tile in a titration

Answer 8

provides a constant white background so the indicator colour change can be seen clearly

Question 9

why is it important to use the same number of drops of indicator for each titration

Answer 9

if more indicator is used it will dilute or change the PH of the solution in the conical flask so more of the solution in the burette is needed
it can also make the solution a different strength of colour so you might judge the end point differently depending on the colour strength

Question 10

why do you need to run solution through the tap of the burette before the titration

Answer 10

to make sure the tap is full
if this is not done when the level the of the burette goes down this solution will fill the tap instead of going into the conical flask

Question 11

why is a burette set up so its tip is inside the neck of the conical flask

Answer 11

to minimise the risk of some solution from the burette ending up outside the conical flask

Question 12

why is the conical flask rinsed with deionised water instead of NaOH

Answer 12

because if NaOH was used to rinse the conical flask and extra unknown volume of NaOH would be titrated, which would make the end titre a greater value than it should be

Question 12

why is the solution from the burette added drop by drop near the end point, and why is it swirled constantly

Answer 12

to decrease the chance of overshooting the end point
it is swirled to ensure solutions are constantly mixed

Question 12

what colour methyl orange is alkali, neutral and acid

Answer 12

alkali-yellow
neutral-yellow
acid-red

Question 13

what colour is phenolphthalein in alkali, neutral and acid

Answer 13

alkali-pink
neutral-colourless
acid-colourless