CHEM mistakes log

desribe the test tube tube reaction practical ( generic ) for ammonium ____, ____sulfate , _____ carbonate and

• ( can score 6/6 by stating 3 reactions + stating that the final one then must be___ )

STAGE 1 ( tests) :

• add NaOH , warm the test tube , then hold damp red litmus paper at mouth of test tubes

• add dilute HCL

• add BaCl₂ ( acidifes from HCl added previously )

STAGE 2 ( observations ) :

• the test tube where the litmus paper turns from red to blue, is where ammonium ions present

• effervessence will be given off at one of the test tubes ( CO2) → this means that carbonate ions are present here → test with limewater → colourless to cloudy

• a white precipitate will be formed in one of the test tubes → means that sulfate ions are present

• the final test tube will therefore be ____

STAGE 3 ( list all equations: ) :

• NH₄⁺ + OH^- → NH₃ + H₂O

• CO₃^2- + 2H⁺ → CO₂ + H₂O

• SO₄^2- + Ba²⁺ → BaSO₄
  

Explain why electronegativity of halogens decreases as you go down

• more shielding

• weaker attraction between nucleus and bonding pair of electrons

what are similarities and differences between the reactions concentrated sulfuric acid with chlorine and bromine

similarity: one from form hydrogen halides / undergo acid-base reaction / misty white fumes are observed / form sodium sulfate / form sodium hydrogensulfate / exothermic / effervescence

Difference: one from bromide undergoes a redox reaction / bromide ions are oxidised / bromide ions reduce sulfur in sulfuric acid / red-brown-orange fumes are observed with bromide / Br2 produced with bromide / (choking gas) SO2 produced with bromide / different hydrogen halides

q5 halogens

when iodide ions are oxidised using concentrated sulfuric acid, sulfur dioxide, a yellow solid and a foul-smelling gas are all formed. Give an equation to show the reaction between iodide ions and concentrated sulfuric acid to form the yellow solid. Identify the foul-smelling gas. Equation ___________________________________________________________ Identity of foul-smelling gas _

• ‘ solid’ - so must show S as product

• if ‘chocking gas’ → SO2 product

• if ‘ foul smelling gas’ → H2S product

equation: 6 I– + 6 H+ + H2SO4 → S + 3 I2 + 4 H2O Allow 6HI Allow 6I− + 8H+ + SO42− Foul smelling gas – H2S / hydrogen sulphide

a student does an additional experiment to show that solution D contains a mixture of halide ions. One of the halide ions is chloride. Method: Step 1 Add an excess of AgNO3(aq) to 10.0 cm3 of solution D. Step 2 Filter, wash, dry and weigh the precipitate. Step 3 Add an excess of dilute ammonia to the dry precipitate. Step 4 Filter, wash, dry and weigh the solid that remains. Explain how the masses recorded during this experiment can be used to show that solution D contains a mixture of halide ions.

• the mass is step 4 will be less than mass in step 2

• because the AgCl will dissolve in dilute ammonia

give an equation for the reaction of chlorine with water to form a solution containing two acids. Explain, with reference to electrons, why this is a redox reaction.

chlorine/Cl/Cl2 gains electron(s) (to form Cl−) and loses electron(s) (to form ClO−)

explain why the third ionization energy of beryllium is much higher than the second ionization energy of beryllium

• the electron is removed from the 1s subshell rather than 2s subshell

• the 1s subshell is closer to nucleus / lower in energy

• the electrostatic forces of attraction between nucleus and OUTER electron is stronger so more energy required

suggest why the solution is kept until no more solid needs to be added ( titration)

• to ensure the solution is saturated ( maximum possible amount dissolved)

• ignore concentrated, ‘ so it is all reacted ‘

suggest why it is important to remove the undissolved strontium hydroxide before the titration.

• the titre volume would be larger

• / more acid ( HCl ) required

• / because the solid will react with the acid

suggest a reason for stoppering the flask?

to prevent reaction with carbon dioxide (in the air)

Allow so flask can inverted/shaken (to ensure homogeneous mixture)

OR To prevent evaporation (of water/from solution)

c

Describe the structure and bonding in magnesium

• a giant lattice of cations/ Mg 2+ ions

• strong electrostatic forces of attraction between cations/ Mg2+ ions and negatively charged delocalized electrons

observations seen when magnesium oxide reacts with steam?

• bright white light

• white ash/powder

give the medical use of magnesium hydroxide

• laxatives / to neutralise excess stomach acid / indigestion releif

explain why the melting point of magniesium is higher than sodium

• Mg2+ ions have a higher charge than Na+ / higher charge density

• so stronger electrostatic forces of attraction

State what is observed when dilute aqueous sodium hydroxide is added to

separate solutions of magnesium chloride and barium chloride.

Observation with magnesium chloride _____________________________

___________________________________________________________

Observation with barium chloride _

name the S block metal with the highest first ionisation energy

• says METAL

• berylium → not lithoum as Be has simialr shieliding but also has higher nuclear charge

State the ASSUMPTION

• all the SrCO3 reacted, as it was heated to constant mass

give an equation for the reaction between phosphorous and an excess of oxygen

• P4 + 5O2 → P4O10

draw the dispalyed formula of the anion formed when sulfur trioxide reacts with water

draw the dispalyed formula of the molecule formed when phosphorus reacts with water

• dip below Al,

• WHY?
  cus the trend is now ‘ shifted’ left 1 , so the same dip between Mg and Al now happens between Al and Si

which element is period 3 has highest second ionisation energy?

Na → removing from the 2p subshell which is much closer to the nucleus → stronger attravtive forces

which period 3 element from Na toCl has highest electronegtaivty

chlorine

Explain the increase in melting point from sodium oxide to magnesium

oxide.

Greater/increased charge/charge density on magnesium ion/Mg2+ (specific

mention of ion(s) can be scored from M2)

Allow magnesium ion is smaller (than sodium ion);

Ignore atomic radius

If mention of molecules, intermolecular forces,

metallic bonding then CE=0

1

Stronger attraction for anions/oxide ion / stronger attraction between

oppositely charged ions/ stronger attraction between Mg2+ and O2– /

stronger ionic bonding

A sample of the highest oxide of phosphorus was prepared in a laboratory.

Describe a method for determining the melting point of the sample.

State how the result obtained could be used to evaluate its purity.

• Put sample in a melting point apparatus / capillary tube in oil bath

• Heat slowly and record the melting point/range

• Pure sample melts sharply at data book value; impurities lower and broaden melting point range

Describe a test you could carry out in a test tube to distinguish between

sodium oxide and the product of the reaction in part (a) in general, just distinguisng an alkaline oxide from acdic oxide

• React both oxides with water ( other wise no marks )

• use glass rod to add a drop of soltuion to litmus paper ]

• acididc → remain red

• alkaline → red to blue

explain why the metling point of sulfur is greater than phosphorus ( P4 )

• S8 is bigger

• so stronger VDW forces

explain why sodium oxide from an alkaline solution when it reacts with water?

• sodium oxide contains O2^- ions

• these ions react with water to form OH^- ions

(iv) Write two equations to show the amphoteric nature of the oxide of aluminium oxide

DONT FORGET MINUS ON THE IONS FOR BASE REACTION!!!!!!!!!!!!!!!!!!!!!!