Chemistry Exam Question Answers

Where do you take electrons from when forming a positive ion?

Take from the 4s before 3d

How do ligands cause colour?

Type of ligand

Arrangement of ligands

Number of each ligands

Affects splitting

What are some examples of ligands?

Water

Ammonia

Chloride ion

Ethane dioate

Why do dyes have colour?

Absorb light of the opposite colour

Delocalised electrons excited to a higher state

Colour is transmitted

Why is the ring structure of benzene a better match for the structure of benzene?

Benzene is hexagonal with equal bond lengths.

The ring structure is hexagonal with equal bond lengths while the Kekulé model is a distorted hexagon because C=C bonds are shorter than C-C bonds. (Three short, three long bonds)

With the Kekulé structure, you would expect two 1,2 isomers but there's only one.

One electron from each carbon delocalises for form two rings, one above one below.

Electron density contour maps show that benzene is a hexagon with equal bond lengths.

Why does the ring model of benzene fir the actual reactions of benzene better?

Benzene doesn't decolorise bromine water but should do if it had C=C bonds.

The ring model undergoes electrophilic substitution becase it keeps the more stable delocalised structure.

Enthalpies of hydrogenation for benzene shows that it is more stable than what the Kekulé model suggests (it is lower than three times the enthalpy for cyclohexene.)

Benzene needs ‘forcing' conditions to undergo addition.

Electrophilic substitution reactions like with Br+ or SO3

What property of a dye is caused by sulfonation?

Increased solubility in water

In electrophilic substitution, what is the role of iron in forming Br+?

Fe reacts with Br2 to form FeBr3

FeBr3 + Br2 → FeBr4- + Br+

(Acts as a halogen carrier.)

How does a graph of titre against time show that a reactant is zero order?

Titre is proportional to concentration

Rate is proportional to the slope

The gradient does not change as concentration decreases

When investigating the effect of a reactant on the rate of reaction, why should total volume be kept constant?

Keeping the total volume the same means changes of the concentration of one reactant are the independent variable

How do you form a primary amine from a haloalkane?

React with concentrated ammonia solution

What is the procedure for comparing rates of reactions of haloalkanes?

• Use the same volumes of the three haloalkanes in test tubes

• Add silver nitrate from tubes to each of the haloalkane solutions quickly

• Shake each tube to mix contents

• Time until precipitate forms

What is the fine detail for the procedure to compare rate of reaction of haloalkanes?

• Use ethanol as a solvent

• Leave in a warm water bath

• Place three tubes of equal volumes of silver nitrate solution in water bath

• Wait until the same temperature has been reached by all test tubes

Why does the procedure for comparing rates of reaction of haloalkanes work?

• As hydrolysis occurs, C-X bond breaks (RX + H2O → ROH + H+ + X-

• A silver halide precipitate is formed (Ag+ + X- → AgX(s))

How can the test for rates of reaction of haloalkanes be interpreted?

• The quicker the precipitate forms, the faster the rate of hydrolysis

• Precipitate should form in the order iodo, bromo, chloro

• Therefore chloro has the slowest rate of reaction, followed by bromo then iodo

• Reactivity appears to depend on bond strength

• C-Cl bond is strongest followed by C-Br then C-I

What is a limitation of the enthalpy change procedure, what effect does this have, and how can it be solved?

Using SHC of water

Gives a more exothermic value

Use the SHC of solution

What does the salt bridge do?

Allows ions to flow between half cells and completes the circuit

Using electrode potentials, how can it be shown that a reaction is not feasible?

When a species will only lose electrons to a more positive electrode potential (would only be oxidised)

What must you use when typing into a calculator?

BRACKETS

In ferroxyl indicator, what does blue and pink show?

Blue shows rusting

Pink shows hydroxide ions

What does comment mean?

State, not necessarily explain

Why are metal complexes coloured?

• D-orbital energy levels are split by ligands

• Light absorbed excites electrons to a higher energy level, with energy equivalent to energy gap (E=hv)

• Colour results from wavelengths not absorbed

Why do different ligand change the colour of a complex?

Different ligands split the d-orbital to different energy gaps