Chem history

Question 1

what did John Dalton do and what are his 5 postulates

Answer 1

1803, came up with Dalton’s atomic theory
- all matter is made up of tiny particles called atoms
- atoms cannot be subdivided, created or destroyed
- atoms of the same element all have the same mass, size and other properties
- able to combine with atoms of other elements in variable integer ratios to form chemical compounds
- in chemical reactions, atoms are rearranged, combined and separated

Question 2

why was Dalton wrong

Answer 2

• protons, neutrons, electrons
• isotopes

Question 3

what is a cathode ray tube?

Answer 3

• sealed glass tubes from which most of the air has been evacuated
• high voltage applied across 2 electrodes at 1 end of the tube
• beam of particles (aka cathode ray) flows from cathode to anode

Question 4

what did JJ Thomson discover?

Answer 4

1897, atoms consist of negatively charged and positively charged particles through cathode ray experiment, plum pudding model

Question 5

what are JJ Thomsons 3 ideas?

Answer 5

• cathode ray consist of negatively charged particles
• negatively charged particles have a mass of about 1/2000 of hydrogen atom, therefore must exist within the atom
• these subatomic particles are found within all atoms

Question 6

how did JJ Thomson prove that - particles exist?

Answer 6

• put 2 oppositely charged electric plates around the cathode ray
• cathode ray deflected from - to + electric plate
• cathode ray composed of - charged particles

Question 7

how did JJ Thomson prove that they exist as part of the atom?

Answer 7

• 2 magnets on either side of cathode ray, deflected again
• determined mass to charge ratio of these particles, discovered mass smaller than any known atom, mass 1/2000 hydrogen atom
• must exist as subatomic particles

Question 8

how did JJ Thomson prove that these particles exist for all atoms?

Answer 8

• repeated experiment with different electrode metals
• properties of cathode ray remained constant no matter what cathode material they originated from

Question 9

how was plum pudding model created?

Answer 9

• since overall charge of atom is neutral, positively charged particles also exist
• numerous electrons embedded within a much larger, uniform, positively charged sphere, accounts for most of its mass

Question 10

what did Earnest Rutherford discover?

Answer 10

1911, postively charged nucleus, gold foil experiment

Question 11

Describe gold foil experiment

Answer 11

• fired a beam of alpha particles through a thin sheet of gold foil
• expected alpha particles to pass through the gold foil with little to no deflection
• small number of alpha particles were deflected very significantly, with some even turning back on its original direction

Question 12

why were alpha particles expected to pass through gold foil?

Answer 12

• even though alpha particles had a positive charge, positive charge in PPM assumed to be spread throughout the entire volume of atom
• electric field from the positive charge of atom too weak to deflect the relatively larger and fast moving alpha particles

Question 13

What was nuclear model?

Answer 13

• atoms consisted of mostly empty space occupied by only the low mass electrons
• all of the atoms mass and postive charge is located in a tiny volume of the atom, the nucleus
• positive charge being carried by protons

Question 14

Limitations of Rutherfords

Answer 14

• protons and electrons alone could not account for all the atoms mass
• did not explain emission and absorption spectra

Question 15

What did Niels Bohr discover

Answer 15

1913, existence of shells, electrons occupy specific energy levels in the atom, proved by investigating hydrogen emission and absorption spectra

Question 16

describe AS relative to Niels Bohr

Answer 16

continuous emission spectrum from a black body is passed through a cool sample of gas, some spectral lines were subtracted from continuous ES

Question 17

define random error

Answer 17

an error caused when 1 measurement differs slightly from the next, comes from unpredictable changes that happen during the experiment

Question 18

define systematic error

Answer 18

always cause measured error to be above or below the true value, consistent and will occur again if the investigation is repeated in the same way

Question 19

define accuracy

Answer 19

ability to obtain the correct measurement

Question 20

define precision

Answer 20

ability to consistently obtain the same measurement

Question 21

how to improve flame test (validity reliability etc)

Answer 21

low precision: different perceptions of colour

• perform flame test in dark room, use non-luminous flame, use AES

low accuracy: some metal ions produce similar colours

Question 22

Limitations of Bohr model

Answer 22

Failed to explain the Zeeman effect, effect of magnetic field on the spectra of atoms
Failed to explain the stark effect, electric

Question 23

What did James Chadwick discover?

Answer 23

1932, Existence of neutrons in the nucleus, when he bombarded Beryllium with alpha particles, an unknown radiation was produced
Chadwick interpreted the radiation being composed of particles with a neutral charge and the approximate mass of a proton

Question 24

Why did Chadwick take so long to discover neutrons?

Answer 24

prior experiments utilised the fact that charged particles reacted with an electric or magnetic field, facilitated their earlier discovery
neutrons uncharged, forced a later discovery

Question 25

define Ar, how is it different to atomic mass

Answer 25

Ar is the weighted average of the relative isotopic masses of an element relative to C-12 is taken as 12 units exactly
Atomic mass is the sum of the neutrons and protons in the atom

Question 26

define Ar, how is it different to atomic mass

Answer 26

Ar is the weighted average of the relative isotopic masses of an element relative to C-12 is taken as 12 units exactly
Atomic mass is the sum of the neutrons and protons in the atom