CGP SUMMARY QUESTIONS: EL

Question 1

Draw a diagram showing the structure of an atom, labelling each part.

Answer 1

Question 2

Where is the mass concentrated in an atom, and what makes up most of the volume of an atom?

Answer 2

• most of the mass is concentrated in the nucleus
  
  • proton mass = 1
  • neutron mass = 1
  • whereas electron mass = 0.000549 (negligible in comparison to nucleus)
• electrons make up most of the volume of an atom
  
  • diameter of the nucleus is very small in comparison to the whole atom

Question 3

Draw a table showing the relative charge and relative mass of the three subatomic particles found in atoms

Answer 3

Question 4

Using an example, explain the terms ‘atomic number’ and ‘mass number’.

Answer 4

• Chlorine-35
• mass number = number of protons and neutrons
  
  • for chlorine = 35
• atomic (proton) number = number of protons in the nucleus of an atom
  
  • for chlorine = 17
• for neutral atoms number of protons also equals the number of electrons
  
  • for chlorine = 17
• number of neutrons = mass number - atomic number
  
  • for chlorine = 18

Question 5

Define the term ‘isotopes’ and give an example.

Answer 5

• Isotopes of an element are atoms - with the same atomic number but different mass numbers due to a different number of neutrons
• number and arrangement of electrons decide chemical properties
• isotopes have the same configuration of electrons
  
  • therefore have similar chemical properties
  • but different physical properties = densities, rate of diffusion etc.
  • this is because physical properties depend on the mass of the atom
• example of isotoes = ³⁵Cl and ³⁷Cl

Question 6

What particle did JJ Thomson discover?

Answer 6

• 1897
• discovered the electron
• discovered that an atom must contain small, negatively charged particles, that he called - corpuscles

Question 7

Describe the model of the atom that was adopted because of Thomspn’s work

Answer 7

• 1897
• atoms weren’t solid and idivisible
• JJ Thomson proposed the ‘plum pudding model’
• contained even smaller, negatively charge particles, electrons
• positively charged sphere with negative electrons embedded in it

Question 8

Who develpoed the ‘nuclear’ model of the atom? What evidence did they have for it?

Answer 8

• Ernest Rutherford and his students Hans Geiger and Ernest Marsden conducted the famous Geiger-Marsden experiment
  
  • fired alpha particles (positively charged) at an extremely thin sheet of gold
  • if plum pudding model was correct - most alpha particles should have been deflected very slightly by the positive pudding that made up most of the atom
  • but most alpha particles passed straight through gold atoms, very small number were deflected backward. Disproved plum pudding model
• Rutherford came up with ‘nuclear model’
  
  • tiny positively charged nucleus at the centre of atom, where most of the mass is concentrated
  • nucleus is surrounded by a ‘cloud’ of freely orbiting negative electrons
  • most of the atom is empty space
• Rutherford later discovered positively charged particles called protons and predicted that there were other particles in the nucleus with mass but no charge (neutrons then discovered by James Chadwick)

Question 9

Explain what relative atomic mass (A_r) and relative isotopic mass mean

Answer 9

• relative atomic mass (A_r) - the average mass of an atom of the elements naturally occuring isotopes
  
  • (A_r) is an average and therefore may not always be a whole number
• relative isotopic mass - mass of an atom of an isotope of an element
  
  • always a whole number, because both protons and neutrons have a relative mass of 1, and relative isotopic mass = relative mass of protons + relative mass of neutrons

Question 10

Explain the difference between relative molecular mass and relative formula mass

Answer 10

• relative molecular mass (or relative formula mass) M_r is the average mass of a molecule or formula unit
  
  • to find the relative molecular mass - add up the relative atomic mass values of all atoms in the molecule
• relative formula mass is used for compounds that are ionic (or giant covalent, such as SiO₂)
  
  • to find the relative formula mass - add up the relative atomic masses of all the ions in the formula unit

Question 11

Describe how a mass spectrometer works

Answer 11

Question 12

What is the significance of Avogadro’s constant?

Answer 12

• The amount of particles in one mole
• roughly 6.02 x 10²³ particles in each mole

Question 13

What is the formula used to calculate the concentration of a solution

Answer 13

• number of moles = (concentration x volume (in cm³))/1000

Question 14

What is the state symobl for a solution of hydrochloric acid in water?

Answer 14

• (aq)
• aqueous
  
  • solution in water

Question 15

What is the difference between a molecular formula and an empirical formula?

Answer 15

• empirical formula - gives just the smallest whole number ratio of atoms in a compound
• molecular formula - gives the actual numbers of atoms in a molecule
  
  • molecular formula is made up of a whole number of empirical units

Question 16

What is the difference between a hydrated and an anyhydrous compound?

Answer 16

• some compounds have water molecules incorporated into them - this water is called water of crystallisation
• hydrated compounds contain water of crystallisation while anhydrous compounds do not
• One mole of a particular hydrated compound always has the same number of moles of crystallisation (and its always a whole number)
  
  • for example, hydrated iron(III) chloride has six moles of water for every mole of the iron(III) chloride. So its formula is FeCl₃•6H₂O
• Many hydrated compounds lose their water of crystallisation when heated, to become anhydrous

Question 17

Explain what is meant by percentage yield

Answer 17

• percentage yield is the actual amount of product you collect, as a percentage of the theoretical yield you could have gotten
  
  • actual yield is always less than the theoretical yield
  • sometimes not all the reactants react fully. And some chemicals are always lost, some solution gets let on filter paper (through filtration), lost during transfers between containers etc.

Question 18

Describe the steps needed to make a standard solution from a solid

Answer 18

• standard solution = any solution that you know the concentration of
• Example: To make 250cm³ of a 6.00 mol dm^-3 solution of sodium chloride
• 1) Work out how many moles of solute you need to be using using the formula moles, = (concentration x volume (in cm³))/1000
  
  • in this example = (6.00 x 250)/1000 = 1.50 mol
• 2) Now work out how many grams of solute is needed using the formula, mass = moles x molar mass
  
  • = 1.50 mol x 58.5 = 87.75 g
• 3) Carefully weigh out this mass of solute - first weigh the beaker, note the weight then add correct mass
• 4) Add a small amont of distilled water to the beaker and stir with a glass rod until all the solute has dissolved
• 5) Tip the solution into a volumetric flask - make sure - it’s the right size for the volume you’re making. Use a funnel to make sure it all goes in.
• 6) Rinse the beaker and stirring rod with distilled water and add that to the flask too. This makes sure there is no solute clinging to the beaker or rod
• 7) Now top the flask up to the correct volume (250cm³) with more distilled water. Make sure the bottom of the meniscus reaches the line - when you get close to the line use a pipette to add water drop by drop. If you go over the line you’ll have to start all over again
• 8) Stopper the bottle and turn it upside down a few times to make sure it’s all mixed
  9) Check the meniscus again and a drop or two of water if needed

Question 19

Describe the procedure for doing a titration

Answer 19

• Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali
• You measure out some alkali using a pipette and put it into a flask, along with somr indicator, e.g. phenolphthalein
• First do a rough titration to get an idea where the endpoint is (the point where the alkali exactly neutralised and the indicator changes colour)
  
  • to do this, take an initial reading to see how much acid is in the burette to start off with. Then add the acid to the alkali - giving the flask a regular swirl, stop when your indicator shows a permanent colour change (the end point). Record the final reading from your burette
• Now do an accurate titration. Run the acid in to within 2cm³ of the end point, then add the acid dropwise. If you don’t notice exactly when the solution changed colour, you’ve gone too far, and your result won’t be accurate
• Work out the amount of acid used to neutralise the alkali. This is just the final reading minus the initial reading. This volume is known as the titre.
• It’s best to repeat the titration a few times, making sure you get a similar answer each time - readings should be within 0.1 cm³ of each other. Then calculate the mean, ignoring any anomalous results. Wash out the conical flask between each titration to remove any acid or alkali left in it

Question 20

What equation links the number of moles, concentration (in mol dm^-3) and volume?

Answer 20

• number of moles = (concentration x volume (in cm³))/1000

Question 21

How many electrons do full s, p, and d sub-shells contain?

Answer 21

Question 22

Draw diagrams to show the shapes of an s and a p orbital

Answer 22

• Orbital - is the bit of space that an electron moves in
  
  • orbitals in the same sub-shell have the same energy
• electrons in each orbital have to ‘spin’ in opposite directions - this is called spin-pairing
• s orbitals are spherical
• p orbitals have dumbbell shapes
  
  • there are 3 p orbitals and they are all at right angles to each other

Question 23

What does the term spin-pairing mean?

Answer 23

• Orbital - is the bit of space that an electron moves in
  
  • orbitals in the same sub-shell have the same energy
• electrons in each orbital have to ‘spin’ in opposite directions - this is called spin-pairing

Question 24

Write down the sub-shells in order of increasing energy up to 4p

Answer 24

• 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p
  
  • 4s sub-shell has a lower energy level than the 3d sub-shell, this means 4s fills up first

Question 25

What is a compound?