Photons, electrons and quantum theory

Question 1

What is Fajan’s method as a titration technique?

Answer 1

Uses DCF (dichlorofluorescein) indicator to determine the endpoint of a reaction by labelling with a green suspension that turns pink.

Precipitation titration.

Question 2

The indicator could be like DCF or it could also be…

Answer 2

the titrant/titrand.

Question 3

How many people operate titration equipment at a time?

Answer 3

Only 1 person.

Question 4

What is the chance of hitting the equivalence point?
What do we do to solve this problem?

Answer 4

Low.
Usually mark just after.

Question 5

List titration equipment.

Answer 5

Boss, clamp, stand, conical/Erlenmeyer flask, analyte/titrand, valve, burette, titant, white tile, indicator (if needed).

Question 6

Where must the stand face?
Why?

Answer 6

The flask, to stabilize the setup and ensure accurate delivery of the titrant into the analyte solution.

Question 7

How might the valve fail?

Answer 7

If too much is in the burette/the funnel drips/the equipment is knocked.

Question 8

Where do the titrant and analyte normally start?
What are they?

Answer 8

Analyte = in conical flask, unknown solution.
Titrant = in burette, known solution.

Question 9

Why is a white tile used?

Answer 9

To help with observing colour changes more easily and determining the endpoint of the titration.

Question 10

Define titre.

Answer 10

The volume of titrant transferred to the conical flask.

Question 11

For assessed pacticals the funnel should be…

Answer 11

removed from the burette before the titration is carried out.

Question 12

For assessed practicals, the conical flask or a…

Answer 12

beaker (backstop) should always be under the burette.

Question 13

Why is a backstop necessary?

Answer 13

Minimizes the risk of chemical spills on the laboratory bench or floor, which can be hazardous.

To avoid inaccurate measurements and potentially affecting the outcome of the experiment because the container collects the titrant and ensures that every drop is accounted for.

Question 14

What is always worn during a titration?

Answer 14

Personal protective equipment.

Lab coat as soon as entering the lab.
Safety specs always in the lab.

Question 15

How must you fill the burette?
Include what not to do.

Answer 15

Lower burette to fill it, having eye level at the meniscus.

Do not fill to zero (optional - time saving tip), do not put apparatus on chairs because they spin and do not sit/kneel on chairs to fill burette because they spin.

Question 16

Keep desk space…

Answer 16

clear.

Question 17

What is argentometry?

Answer 17

A type of titration using a silver (I) ion.

Question 18

The sample (salt) solution (analyte) was titrated against a solution of silver nitrate (titrant) of known concentration.
What indicator is added to the sample? Describe the system of this indicator.

Answer 18

DCF - a conjugated system (system of connected p-orbitals with delocalised electrons in a molecule).

Question 19

What colour is DCF in different solutions?

(refer to summary sheet for a diagram)

Answer 19

Green in aqueous solution but turns pink when connected to positively charged surfaces.

Question 20

What colour is the solution with DCF when there are more Cl- than Ag+?

Answer 20

Green

Question 21

What colour is the solution with DCF when there is more Ag+ than Cl-?

Answer 21

Pink

Question 22

What would happen if

Question 23

Why is this titration described as a precipitation titration?

Answer 23

Ag+ + Cl- → AgCl
AgCl = insoluble solid

Question 24

What would happen if the analyte was AgCl?

Answer 24

There would be no reaction.

Question 25

Around 400BC, what did Democritus do?

Answer 25

Experiment - take a piece of sulphur and divide.

Question 26

What was his hypothesis?

Answer 26

Eventually there are indivisibles ‘atomos’.

Question 27

What did Einstein come up with in 1905?

Answer 27

The photoelectric effect.

Question 28

What is the photoelectric effect?
(look at summary sheet for diagrams and graphs)

Answer 28

Phenomenon where electrons are emitted from a material (typically a metal) when it is exposed to light or electromagnetic radiation of sufficient energy.

Question 29

When UV light hits a metal surface, electrons are emitted.
How are they collected?

Answer 29

By a scintillator - emit light when absorb particles/EM waves (from electrons).

Question 30

What was Einstein’s expectation?

Answer 30

Increasing light intensity increases electron production/energy.

Question 31

What was the result?

Answer 31

Electron emission depends on the frequency of light.
There is a minimum frequency where no electrons are emitted.

Question 32

What equation was formed?

Answer 32

Work function (energy is required to release electrons from metal):

Ee = hv

(h = Planck’s constant)
(v = frequency)

Question 33

Therefore, what was the proposal made?

Answer 33

Light is a particle with discrete energy ‘packets’.

Question 34

What led to the proposal of the Bohr model?

(refer to summary sheet for diagrams)

Answer 34

Pattern observed:

v is proportional to 1/a^2 - 1/b^2
a, b are integers >/= 1 and a>b

hv = Eb - Ea = ∆E
a and b are energy levels

These ‘levels’ are ‘orbits’ which obey equations of electrostatics.

Question 35

What is an observation relating to the Bohr model of the atom?

Answer 35

Light is refracted in a glass prism but closer inspection reveals discrete peaks/troughs.

Refracted refers to the change in direction of a wave, such as light, as it passes from one medium into another medium with a different density.

Question 36

What was the rebuttal (to challenge a claim)?

Answer 36

What’s stopping the electrons from crashing into the nucleus?

Question 37

What’s stopping the electrons from crashing into the nucleus?

Answer 37

They obey the rules of quantum mechanics according to which they can only have very specific energies.

Question 38

What was Bohr’s final proposal?

Answer 38

Bohr proposed that electrons orbit the nucleus in specific, discrete energy levels (or shells) rather than in any arbitrary path.