general exam tips Flashcards
(20 cards)
name the property of an element or compound that can enable it to react in two ways
or reacts with acids and bases
Amphoteric
when can u distill a product from a reaction
-if it has a lower boiling point to the reactants
why are some organic reactions heated under reflux
-can apply constant heat to a mixture without losing any of the liquid to evaporation - way of getting reaction to go to completion
-many organic reactions need to be heated as this supplies the Ea to break bonds and allow the reaction to occur
using a condsenser will allow this to happen whilst preventing the reagants from leaving the reaction vessel
what do anti-bumping granules prevent
-bumping - obvs
-produce many bubbles instead of large ones - prevents uneven boiling
what are some of the potential errors when using a gas syringe
-gas escapes before bung is inserted
-syringe sticks so cant move
-some gases (CO2 and SO2 etc) are soluble in water so the true amount of gas is not measured
-temp measured is less than temp in the syringe
-mass of liquid measured that is transferred to the syringe is less than actual mass
what to do if reaction is slow and need to get accurate results
-take readings at regular intervals + extrapolate the temp line back to when reactants were added together becasue if reaction is slow the temp rise is hard to obtain as reaction is cooling quickly
-take readings for few mins before adding second reagant
enthalpy practicle errors
- energy transfer to/from surroundings (usually loss)
- approximation in specific heat capacity of solution
- the method assumes all solutions have the heat capacity of water
- density of solution is taken to be the same as water
- some of the water could have evaporated
- reaction or dissolving may be incomplete or slow.
how is bioethanol produced and what are the environmental impacts
-photosynthesis - natural process in plants that takes CO2 from the air
-6CO2 + 6H2O -> C6H12O6 + 6O2
-fermentation - the process used to make bioethanol that releases some CO2
C6H12O6 -> 2C2H5OH + 2CO2
-combustion is the process where bioethanol is burned and releases CO2
2C2H5OH + 6O2 -> 6H2O + 4CO2
-deforestation is used to provide raw materials and land is sacrificed that could be used for food
-there is a loss of biodiversity and habitat when trees are cut down and land used to build factories
-6CO2 in and 6CO2 out but it isnt actually carbon neutral as the fuel is used in production, distrubution etc
what to remember when finishing a q=mcdeltaT question
final answer needs to be -ve - so just whack onto what ever answer u have - if combustion as its exothermic
why can some molecules not be soluble
-not polar enough
-cannot form hydrogen bonds
how to find % yield of an organic substance
-find max mass possible - times mr by the limiting reagent mol
-then divide mass found by the max mass and times 100
why is the % yield not 100%
-reaction is reversible
-losses due to distillation/purification/transfer/sode reactions
-incomplete reaction
how to work out the enthalpy of vapourisation
- kW = kJ s-1
- Multiply the kW by the number of seconds
eg 2.4 x 100s
-then work put mol - mass water loss/ Mr
-then divde the kJs-1/ mol - to find enthalpy
Which change causes the pH of 10 cm3 of 1.0 mol dm–3 NaOH to be halved at 298 K?
Kw = 1.0 × 10–14 at 298 K
adding 10 cm3 of 1.0 mol dm–3 HCl
-bc the same value
-so will be nuetralised as this is pH 7
what to do if question says leave to an appropriate number of significant figures
-nearest whole number
-or if decimal with zeros nearest whole after these
what to do if the question says leave to an appropriate number of decimal places
-look at closet decimal and round it
when reacting with excess NaOH or another strong sol and something what is often produced
-salt
-so O- and Na+ or other eg K+
isomer of alkene
-always cycloalkane
which part of the molecule is planar when answering questions abt racemix/stereoisomerism
-carbonyl
- C=O
which compounds will react with postassium dichromate and turn it green
-alcohols
-aldehyde
-not ketone
