Chemistry Exam mistakes

Question 1

What is the formula for entropy change?

Answer 1

S of products - S of reactants = change in entropy

Question 2

What must you always keep in mind in entropy and Gibbs free energy calculations?

Answer 2

The units of entropy are J k^-1 mol^-1 where’s the units of enthalpy change are kJ mol^-1 so you need to change the units of entropy into kJ K^-1 mol^-1 in order to substitute entropy into the Gibbs free energy equation.

Question 3

3 stages of how a catalyst functions?

Answer 3

Reactants are adsorbed onto the active sites of the Platinum

Reaction or bond breaking? weakening occurs

Products are desorbed or desorption occurs.

Question 4

Which s-block metal has the highest first ionisation energy?

Answer 4

Beryllium, radius and shielding have larger effect than charge.

Question 5

Why do isotopes have the same chemical properties?

Answer 5

They have the same electronic configuration/same number of electrons.

Question 6

Definition of strong acid?

Answer 6

An acid which completely dissociates in aqueous solution.

Question 7

Definition of a weak acid?

Answer 7

An acid which partially dissociates in aqueous solution.

Question 8

When you are doing calculations determining which reactant in a reaction of a strong and weak acid is in excess what are you actually calculating the moles of?

Answer 8

The moles of the H+ and OH- ions being released into solution so if the acid is diuretic you need to multiple the moles you calculate by two. Sam goes for dibasic, 2xmoles to = moles of OH- ions in solution.

Question 9

Why might one of the electrodes in a cell not have a potential of (x) even if that’s the calculated value?

Answer 9

The concentrations might not be 1.0 moldm-3

Question 10

Why is does the compound NaBr have a higher melting point than NaI?

Answer 10

The Br- ion is smaller than the I- ion so there are stronger electrostatic forces of attraction between the positive and negative ions in NaBr than in NaI.

Question 11

Describe repulsion between lone pairs and bond pairs?

Answer 11

4 electron pairs which repel each other to be as far apart as possible. M1

The lone pair-lone pair repulsion is greater than the lone pair-bond pair repulsion.

Question 12

Describe electrospray ionisation?

Answer 12

The sample is dissolved in a volatile polar solvent.
It is then infected through a hypodermic needle that has a potential difference applied across it.
The sample then gain a proton. (CANNOT SAY ATOMS GAIN A PROTON)

Question 13

What Is periodicity?

Answer 13

Repeating patterns of physical or chemical properties across a period.

(MUST SAY REPEATING)

Question 14

With Copper and Chromium what is interesting about them?

Answer 14

Copper has an electron configuration (Ar) 4s2 3d9 however the 4s electtron is more stable in the 3d orbital so is actually (Ar) 4s1 3d10 so Cu+ ion has (Ar) 3d10 configuration.

Chromium has an electron configuration of (Ar) 4s2 3d4 where’s more stable in (Ar) 4s1 3d5 configuration. so for Cr+ ion electron is lost from 4s1 shel to give configuration of (Ar) 3d5

Question 15

What does it actually mean for a substance to be hydrated e.g. hydrated copper sulfate?

Answer 15

It Is soluble and dissolves in water

Question 16

If a question asks for a method for enthalpy calculation what method is it actually asking for?

Answer 16

Calorimetry method. Adding two solutions together or one solid to water. Enthalpy of solution, enthalpy of combustion and probably other enthalpies can be measured in this way.

Question 17

Why are compounds formed via secondary carbocations more stable than compounds formed via primary carbocations?

Answer 17

Due to the electron-releasing/inductive effect of the alkyl groups.

Question 18

In a reaction why is one of the compounds added in excess?

Answer 18

So that the other reagent completely reacts/ is completely hydrolysed.

Question 19

Why would reflux be used in a reaction?

Answer 19

Reflux would be used because it allows the reacting vapours to be returned to the reaction mixture/does not allow the escape of any reactants.

Question 20

If a question asks you for the process of purifying an organic compound, what are they asking?

Answer 20

They can only be asking about the purifying practical of dissolve in minimum volume of hot water etc.

Question 21

In the purifying practical, how do you describe suction filtration?

Answer 21

You say “filter under reduced pressure”.

Question 22

In a reaction why might the yield not be 100%?

Answer 22

There are impurities present.

Question 23

Why does enthalpy of hydration become les exothermic from Li+ to K+?

Answer 23

Ionic radius increases from Li+ to K+

So weaker electrostatic forces of attraction from positive metal ion to delta negative oxygen on water molecule.

Question 24

Definition of relative atomic mass?

Answer 24

The mean mass of one atom of an element relative to 1/12 the mass of an atom of Carbon 12.

Question 25

Why do atoms need to be ionised in Time of Flight Mass Spectrometry?

Answer 25

So the ions can be accelerated by an electric field towards a negatively charged plate. So the ions can produce a current when they hit the detector and gain an electron.

Question 26

In Time of Flight mass Spectrometry questions, if it says a 53 Cr+ ion was accelerated, what is the mass of the one ion?

Answer 26

The 53 is the molar mass, the mass of one mole of these ions/atoms. You need to divide 53 by Avogadro's Constant to get the mass of a single ion.

Question 27

Period 3 element with highest second ionisation energy?

Answer 27

Na NUMBER OF ELECTRON SHELLS HAS BUGGEST EFFECT ON IONISATION ENERGY VALUE.

Question 28

Why does an Aluminium complex ion (Al(H2O)6) 3+ cause the pH of the solution to be less than 7?

Answer 28

The Al 3+ ion has a high charge density so weakens the OH- and in the H2O molecules which releases H+ ions into the water. Equation showing this is just the aluminium complex ion with 6H2O molecules forming aluminium complex with 5H20 and one OH- and one H+ released

Question 29

What is the only variable that affects Kp, Kc and Ka?

Answer 29

Temperature.

Question 30

Why does a catalyst not affect Kp?

Answer 30

It increases the rate of forwards and backwards reactions by the same amount so it does not affect the position of equilibrium.

Question 31

What is the trend with silver nitrate and halides?

Answer 31

As silver nitrate rects with the different halides going down the group it gets less soluble in water. AgF is highly electronegative so is soluble in water so forms a colourless solution. AgCl is not soluble so form a white precipitate and so on.

Question 32

Role of Chloride ions in reaction of sodium chloride and sulphuric acid?

Answer 32

Base or proton acceptor

Question 33

Is the dissociation of water exo or endo?

Answer 33

Endothermic.

Question 34

Define enthalpy change

Answer 34

Heat change at consent pressure.

Question 35

What is the pH of a solution which contains H3PO4?

Answer 35

pH=1

Question 36

Why is the Fe 3+ ion more acidic than the Fe 2+ ion?

Answer 36

The Fe 3+ ion has a higher charge density so is more polarising to the water molecules. This weakens the O-H bonds in the water ligands.

Question 37

Why is the reaction slow between S2O8 2- ions and I- ions?

Answer 37

Both ions are negatively charged so they repel each other. So the Activation energy is high.

Question 38

Why is the pH probe washed before it is used again between readings?

Answer 38

To remove any residual solution.

Question 39

Why are many reading taken close to the end-point of the reaction in a titration?

Answer 39

To avoid missing the end-point.

Question 40

What is an electrochemical series?

Answer 40

Electrode potentials in order of electrode potential values.