2019 chem mistakes

Question 1

sodium reacts with chlorine

Answer 1

flame
* (white) solid forms
* colour of gas / chlorine
disappears / fades

Question 2

Mendeleev arranged the elements in order of their atomic weight.
What is the modern name for atomic weight

Answer 2

relative atomic mass

Question 3

how are elements ordered

Answer 3

increasing atomic number

Question 4

titration

Answer 4

Key steps
* measure the volume of acid
* add indicator to the acid
* add sodium hydroxide solution
* until the colour changes
* record volume of sodium hydroxide solution added
* repeat procedure with the other acid
Use of results
* compare the two volumes of sodium hydroxide solution to find
which sample P or Q is more concentrated
Other points
* pipette to measure volume of acid
* use a few drops of indicator
* swirl
* use a white tile
* rough titration to find approximate end point
* add dropwise near the endpoint
* read volume from bottom of meniscus
* repeat and take a mea

Question 5

how results of alpha particle experiment led to develepment of neutron

Answer 5

most (alpha) particles passed
(straight) through (the gold foil)
(so) the mass of the atom is
concentrated in the nucleus /
centre
or
(so) most of the atom is empty
space
some (alpha) particles were
deflected / reflected
(so) the atom has a (positively)
charged nucleus / centre

Question 6

Hydrogen fuel cells can be used to power different forms of transport.
Some diesel trains are being converted to run on hydrogen fuel cells.
A newspaper article referred to the converted trains as the new ‘steam trains’.
Suggest why

Answer 6

(in a fuel cell) hydrogen is
oxidised (to produce water)
water is produced / released as
gas / vapour / steam

Question 7

Explain why the positive electrode must be continually replaced

Answer 7

the electrode reacts with oxygen
the electrode is carbon /
graphite
(so) carbon dioxide is produced

Question 8

balanced half equation at the anode

Answer 8

2O2-→ O2 + 4e-