Exam questions

Question 1

state the meaning of the term bronzed-lowry acid

Answer 1

acid is a proton donator

Question 2

write a expression for the acid dissociation constant Ka for ethnic acid

Answer 2

Ka = [H+][CH3COO-] / [CH3COOH]

Question 3

write an equation for the dissociation of chloroethanoic acid in aqueous solution

Answer 3

ClCH2COOH ⇌ H+ + ClCH2COO-

Question 4

suggest why chloroethanoic acid is stronger acid than ethnic acid

Answer 4

chlorine is electronegative so withdraws electrons from the O-H bond, so weakens it

Question 5

write an expression for the acid dissociation constant (Ka) for the weak acid HX

Answer 5

Ka = [H+][X-] / [HX]

Question 6

state which one is a strong base

ammonia or ethylamine

Answer 6

ethylamine is a stronger base

positive inductive effect

Question 7

write an expression of pH

Answer 7

pH = -log[H+]

Question 8

hydrogen carbonate ions act as a weak cid in aqueous solution. write an equation for this equilibrium

Answer 8

HCO3- ⇌ H+ + CO3^2-

Question 9

use an equation to explain how the solution containing sodium hydrogencarbonate and sodium carbonate Cana its a buffer when small amount of acid or alkali are added

Answer 9

small amount of acid= shifts equilibrium to the left (HCO3- + H+)
small amount of alkali = shirt equilibrium to the right ( H+ + OH-)

overall [H+] stays the same

Question 10

state the meaning of the term weak as applied to carboxylic acids

Answer 10

partially dissociated

Question 11

two solutions, one with a pH of 4 and another one of 9, were left open to the air. the pH of 9 solution changed the most. suggest what substance might be present in the air to cause the pH to change

Answer 11

CO2

pH has decrees as it has reacted with the H+ ions in the solution

Question 12

suggest why chloroethanoic acid is stronger acid than ethanoic acid

Answer 12

chloroethanic acid has a chlorine which is more electronegative than hydrogen, so withdraws electrons which weaken the O-H bond

Question 13

the dissociation of methanoic acid in aqueous solution is endothermic. deduce whether the pH of a solution of methanoic acid will increase, decades or stay the same if the solution is heated.

Answer 13

decreases
if the forwards reaction is endothermic then more product is made to oppose the change of the increase in temperature. the equilibrium shifts to the left

Question 14

the product formed exists as racemic mixture. state the meaning of the term racemic mixture and explain why such a mixture is formed in this reaction

Answer 14

50:50 ratio of an optical isomer

reagent can go either side of the C=O bond

Question 15

state the meaning of the term stereoisomerism

Answer 15

two or compounds have the same structural formula, different arrangement of bonds in space

Question 16

what type of isomerism is shown by but-2-ene

Answer 16

E-Z isomerism

Question 17

define the term optically active

Answer 17

rotate plane of polarised light

Question 18

why is a racemic mixture not optically active?

Answer 18

each isomer polars light in a direction and they cancel each other out

Question 19

explain why a molecule can be optically active

Answer 19

the carbon is bond to four different chains

Question 20

an ester was prepared by reacting (CH3)2CHOH with (CH3CO)2O

write an equation for this reaction and give the IUPAC name of the ester formed

Answer 20

(CH3)2CHOH + (CH3CO)2O —– CH3COOH + CH3COOCH(CH3)CH3

1-methyl,ethyl ethanoate

Question 21

give the IUPAC name of glycerol

Answer 21

propane-1,2,3-triol

Question 22

Ethnic acid, propyl ethanoate and propane-1-ol are all colourless liquids. esters do not give a positive result with any of the usual tests for functional groups. state how you could use chemical test to show the presence of ethnic acid and propane-1-ol in a mixture of the acid.

Answer 22

ethanoic acid
add NaCO3
bubbles observed as carbon dioxide given off

propane-1-ol
acidified K2Cr2O7
orange colour turns green with primary alcohols

Question 23

a student prepared a sample of aspirin in the lab and attempted to purify it by recrystallisation. to check the purity of aspirin the student determined its melting point.
state two observations during the melting point process that would indicate that the sample is not pure

Answer 23

melt greatly before or after the given melting point

the meting point range would be wider

Question 24

suggest why a pure sample of aspirin may sometimes appear to melt at a different from 135

Answer 24

the temperature on the thermometer is different from that of the sample

Question 25

how many structural isomers (which are esters) have the molecular formula C4H5O2

Answer 25

4

Question 26

state and explain how you could distinguish between the two enantiomers

Answer 26

use polarised light

rotated in opposite directions

Question 27

why in practice KCN rather than HCN is added to the carbonyl compound

Answer 27

contraction is lower
weak acid
:CN- is very low

Question 28

which one of the following statements abbey but-2-enal is not true
A-it has stereoisomers
B-shows a strong absorption in the infra-red at about 1700cm-1
C- turn an acidified solution of potassium dichromate green
D- dehydrated by concentrated sulphuric acid

Answer 28

D- dehydrated by concentrated sulphuric acid

Question 29

state in terms of bonding why benzene is more stable than cyclohexane-1,3,5-triene

Answer 29

benzene is more stable because it has overlapping p-orbitals so the electrons delocalised

Question 30

what is the difference in bonding between cyclohexane-1,4-diene and cyclohexane-1,3-diene

Answer 30

cyclohexa-1,3-dienece is more stable
double bons coller together
more p-orbitals overlapping