OCR A A Level Chemistry Module 2

Relative mass of an electron

1/1836

Relative isotopic mass

Mass of an isotope relative to 1/12of the mass of an atom of carbon-12

Relative atomic mass (Ar)

The weighted mean mass of an atom of an element relative to 1/12 of the mass of an atom of carbon 12

mass spectrometer

- sample placed in

- sample is vaporised and ionised to form positive ions

- ions are accelerated separating ions of each isotope

- ions detected on a mass spectrum as a mass/charge ratio

- greater the abundance, the larger the signal

mass to charge ratio (m/z)

Relative mass of ion/relative charge on ion

Working out relative atomic mass from isotopes

(Abundance x mass number for Y) + (abundance x mass number for X)/100

binary compound

Contains 2 elements only

Polyatomic ion

Ion containing atoms of more than one element

Ions with 1+ charge

NH4+

Ions with 1- charge

OH- (hydroxide)

NO3- (Nitrate)

NO2- (Nitrite)

HCO3- (Bicarbonate)

MnO4- (Permanganate)

Ions with 2- charge

CO3 (carbonate)

SO4 (sulfate)

SO3 (sulfite)

Cr2O7 (dichromate)

Ions with 3- charge

PO43- (phosphate)

Phosphurus

P4

Sulphur

S8

Diatomic molecules

N2 H2 O2 F2 Cl2 Br2 I2

Avogadro's number

6.02 x 10^23

molecular formula

Total number of atoms of each element in a molecule

Working out formula of a hydrated salt

- moles of anhydrous

- moles of water

Ratio

Improving accuracy of water of crystallisation experiments

Heat to constant mass

No further decomposition occurs

Moles of a gas

Volume/molar gas volume (24 or 24000)

Ideal Gas Equation

pV = nRT

p = Pressure in Pa

V = Volume in m3

n = moles

R = Ideal gas constant 8.314

K = Temperature in K

Cm3 to m3

X 10-6

dm3 to m3

X 10-3

kPa to Pa

x1000

Why is theoretical yield hard to acheive

Reaction may not have gone to completion

Other reactions may have taken place alongside main reaction

Purification may result in the loss of some product

Percentage yield

Actual yield/theoretical yield x 100

The element with the greater big number is the

Limiting reagent

Reactions with high atom economies

Produce a large proportion of desired products and few unwanted waste products

Are important for sustainability as they make the best use of natural resources

Atom economy

Sum of molar masses of desired products/sum of molar masses of all products x 100

Why does atom economy not give whole picture on sustainability

Costs for starting materials (low = sustainable)

Efficiency depends on both atom economy and % yield

Strong acid

Completely dissociates in aqueous solution

Weak acid

Partially dissociates in aqueous solution

Base

Neutralises an acid to form a salt

Alkali

A base that dissolves in water releasing hydroxide ions into solution

Importance of purity in pharmaceuticals

Impurity - drug causes harm to patient

100cm3 volumetric flask error

+/- 0.20cm3

250cm3 volumetric flask error

+/- 0.30cm3

10cm3 pipette error

0.04cm3

25cm3 pipette error

0.06cm3

50cm3 pipette error

0.10cm3

Oxidation number of H in metal hydrides e.g. NaH

-1

Oxidation number of O in peroxide's e.g. H2O2

-1

Oxidation number of O bonded to F e.g. F2O

+2

O normally has an oxidation number of

-2

H normally has an oxidation number of

+1

F always has an oxidation number of

-1

Sum of oxidation numbers =

Total charge on ion

Nitrate

NO3-

Sulfate

SO42-

Redox in terms of electrons

Reduction is gain

Oxidation is loss

Redox in terms of oxidation number

Reduction is a decrease in oxidation number

Oxidation is an increase in oxidation number

Principal quantum number

Shell number/energy level number

atomic orbital

Region around the nucleus that can hold up to two electrons with opposite spins

S orbital

Spherical

The greater the shell number ...

the greater the radius of its s orbital

P orbital shape (electron cloud)

Dumb-bell

The greater the shell number n (p orbital)

The further the p orbital from the nucleus

N=4 f orbital number

7

N=3 d orbital number

5

Sub-shells

Orbitals of same type within an orbital

Orbitals fill in order of

Increasing energy

Opposite spins help to counteract

The repulsion between negative charges of the electrons

4s sub shell quirks

Fills before

Empties before

Ionic bonding

Electrostatic attraction between positive and negative ions

Greater ionic charges mean ...

Stronger attraction between ions

Covalent bonding

Strong electrostatic attraction between a shared pair of electrons ans the nuclei of bonded atoms (overlap of atomic orbitals)

Boron 1s22s22p1

Only three outer shell electrons can be paired

Forms covalent compound BF3

Six electrons around boron atom

- shows predicitions for bonding cannot be based solely on noble gas electron structure

Phosphorus

PF3

PF5

Sulfur

SF2

SF4

SF6

Chlorine

ClF

ClF3

ClF5

ClF7

SF8 term

Expansion of the octet

Possible only from n = 3 shell when d shell becomes available for expansion

Multiple covalent bond

When two atoms share more than one pair of electrons

Double bond

Two shared pairs of electrons

Displayed formula of NH4+ ion

Use one double headed arrow to show coordinate/dative bond

Average bond enthalpy

Measure of covalent bond strength

The larger the value the stronger the bond

Isoelectronic

Same number of electrons

Solid wedge

Comes out of plane of paper

Dotted wedge

Goes into plane of paper

A lone pair of electrons ...

Slightly closer to central atom

Occupies more space than a bonded pair

Repels more strongly

Bond angle is reduced by ... for each lone pair

2.5

CH4 shape and bond angle

Tetrahedral 109.5

NH3 shape

Pyramidal 107

H2O shape and bond angle

Non-linear 104.5

Carbon dioxide shape and bond angle

Linear 180

The greater the number of electron pairs...

The smaller the bond angle

What shape and bond angle does 3 electron pairs give e.g. BF3

Trigonal planar 120

What shape and bond angle does six electron pairs give e.g. SF6

Octahedral 90

When bonded atoms are different elements ...

Nuclear charges different

Different sized atoms

Shared pair of electrons closer to one nucleus than other

Electronegativity

The attraction of a bonded atom for the pair of electrons in a covalent bond

Pauling scale measures

Electronegativity

Across the periodic table ... (period)

Nuclear charge increases

Atomic radius decreases

Large Pauling value

Very electronegative

Fluorine Pauling value

4

If the electronegativity difference is large ...

One bonded atom will have a much greater attraction for the shared pair than the other bonded atom

Gained control of electrons

Ionic bond rather than covalent

polar covalent bond

Electronegativity difference 0-1.8

ionic electronegativity difference

Greater than 1.8

Non-polar bond

Electron pair shared equally

Same atoms

Same electronegativity

pure covalent bond

Exists in diatomic molecules, when two atoms of same element bond together

Polar bond

Bonded electron pair shared unequally between bonded atoms

Small partial charge

Dipole

Separation of opposite charges