1
Q
how to find empirical formula
A
- moles of each element in compound
- ratio
2
Q
two features of dynamic equilibrium
A
- forward and reverse reactions in same rates simultaneously
- concs of reactants and products constant
3
Q
whats hess’ law
A
Enthalpy change at a constant pressure in a reaction is independent of route taken
- depends only on initial and final states
- first law of thermodynamics stating energy is always conserved
4
Q
Why do isotopes have similar chemical properties and slightly varying physical properties?
A
β’ similar chemical
> have same electronic structure
β’ varying physical
> have diff masses due to diff neutron numbers
5
Q
Whats mass spectrometer used for
A
- used to determine all isotopes present in sample of an element
- so to identify elements
6
Q
Why do mass spectrometers need to be under a vacuum
A
- otherwise air particles would ionise
- and register on the detector
7
Q
Electron impact
A
- vaporised sample injected into mass spectrometer at low pressure
- gun fires high energy electrons at sample
- knocks out outer electron
- forming positive ions
> > used for elements/substances w low formula mass
causes larger organic molecules to fragment
8
Q
Electrospray
A
- sample dissolved in volatile polar solvent
- injected through fine needle (fine mist)
- tip has high voltage (sample molecule gains proton from solvent)
- solvent evaporates; ions move to negative plate
> > used for larger organic molecules
fragmentation doesnt occur
9
Q
Acceleration in mass spectrometry
A
- positive ions accelerated by electric field to contant KE
- all particles have same KE ; velocity of each particle depends on the mass
Lighter molecules have faster velocity
10
Q
Flight tube or ion drift (mass spectrometry)
A
Heavier molecules take longer to move through drift area
> ions distinguished by flight times
11
Q
Detection (mass spectrometry)
A
- ions reaching detector generate small current
- fed to computer for analysis
- current produced by electrons transfer from detector to positive ions
- size of current is proportional to species abundance
Remember: m/z is a ratio (value is calculated !!!)
12
Q
Relative atomic mass
A
Weighted average of all the isotopes of an element
13
Q
Relative atomic mass eqπ‘
A
(SUM OF (isotopic mass x % or relative abundance)) / 100 or tot relative abundance
14
Q
Why does one peak in mass spectra of diatomic molecules have double abundance
A
- has double abundance of other two peaks bc there is double probability of
- 160 m/z Br79-Br81
- as can be Br81-Br79 too
15
Q
Why are mass spectrometers in planetary space probes
A
So elements in other planets can be identified
> other planets have different composition of isotopes in elements
16
Q
If a molecule has an electron impact ionisation stage?
A
- molecule breaks up giving series of peaks
- peak with highest m/z is equal to relative molecular mass - Mr(complete molecule)
» peak is called molecular ion
17
Q
If a molecule goes through electrospray ionisation
A
- no fragmentation
> one peak equals mass of MH+ (M is molecule, H+ is proton from spray)
-> subtract 1 to get Mr of molecule
18
Q
What are the shapes of orbitals
A
- they represent mathematical probabilities of finding an electron at any point
- within certain spatial distributions around nucleus
19
Q
Electronic structure of Cr and Cu
A
They have half filled 4s sub shells (these fill before 3d)
> when d block elements lose electrons, 4s electrons lost first
- Cr»_space;> Cr3+
[Ar] 4s1 3d5»_space;> [Ar] 4s0 3d3 - Cu»_space;> Cu 2+
[Ar] 4s1 3d10»_space;> [Ar] 4s0 3d9
20
Q
Whats first ionisation energy
A
Enthalpy change when one mole of gaseous atoms
- form one mole of gaseous ions with 1+ charge
21
Q
Whats second ionisation energy
A
Enthalpy change when one mole of gaseous ions with 1+ charge forms
- one mole of gaseous ions with 2+ (double) charge
22
Q
Whats shielding
A
- electron in outer shell is repelled by electrons in complete inner shells
- weakening attraction of the nucleus
23
Q
Why are successive ionisation energies always larger (second always larger than first)
A
- ion increases attraction on remaining electrons and so energy required
- to remove next electron is larger
24
Q
Why has He got largest first ionisation energy
A
- electron is closest to nucleus with no shielding
- bigger first ionisation energy than H due to one more proton
25
Whys there general increase in first ionisation energy across period
- electrons are added to same shell across period
- so same distance and shielding
- proton number increases
- making effective attraction of nucleus greater
26
Whys there a small drop in first ionisation from Mg to Al in period 3 (g2-3)
- Al is starting to fill 3p
- Mg has outer electrons in 3s
- 3p slight easier to remove as higher in energy
- and slightly shielded by 3s electrons
27
Why small drop in first ionisation energy from P to S (g5-6)
- 4 electrons in 3p in S; fourth starts to doubly fill first 3p orbital
- second electron added to a 3p orbital makes slight repulsion
- between two negative electrons - second electron easier to remove
28
Second ionisation energy pattern
- graph is plotted similar to first ionisation but all elements have shifted one to left
- group1 element is now peak of graph
..
- lithium now has largest second ionisation of all elements
- as second electron removed from first 1s shell closest to nucleus, w no shielding from inner shells
- Li has bigger second than He as more protons
29
Whats mole definition
- the amt of substance in grams that has
- the same number of particles as there are atoms in
- 12g of C-12
30
Relative atomic/molecular mass definition
- average masa of one atom/molecule compared to
- one twelfth of mass of one atom of C-12
31
Whats avogadros constant
One mole of any specified entity contains 6.022 x 10^23 of that entity
32
How to calculate moles in gases π‘
PV = nRT
Pa
m3 (VOLUME)
K
33
How to convert temp
C + 273 > K
34
How to work number of particles from avogadros
No of particles = moles of substance (mol) x avogadros constant
35
Density units
D= m/v
Density is in g cm3
36
Empirical formula definition and how to find
The simplest ratio of atoms of each element in compound
- find mol of each element
- for each answer divide by the smallest one
- multiply up to get whole numbers
37
Molecular formula definition
- actual number of atoms of each element in the compound
38
Heating in a crucible: why should large masses (50g) of hydrated substance not be used
Decomposition is likely to be incomplete
39
Heating in a crucible: how do we use the lid
- improves accuracy of experiment preventing solid loss
- but should be loose fitting so gas can escape
40
Heating in a crucible: why should crucible be dry
- wet crucible would give innacurate result
- causing mass loss to be too large as water would be lost while heating
41
Heating in a crucible; Why shouldnt small amounts of hydrated solid be used (0.1g)
- percentage uncertainties when weighing will be too high
42
Converting mol dm-3 to g dm-3
g dm-3 = mol dm-3 x Mr
43
Making a solution
- weigh sample bottle containing required mass of solid on balance
- transfer to beaker and reweigh sample bottle: record difference in mass
..
- add 100cm3 of distilled water to beaker and stir to help dissolve
> sometimes may not dissolve well in cold water so the beaker contents
> could be heated gently until all dissolved
..
- pour solution to 250cm3 graduated flask via a funnel
- rinse beaker and funnel ; add washings from the beaker to volumetric
- fill up to the mark with distilled water
- invert flask several times to ensure uniform solution
44
Dilutions: Why is a volumetric pipette is more accurate
More than measuring cylinder because it has smaller uncertainty
45
What happens when diluting a solution π‘π΄
- wont change amount of solute moles but
- increases solution volume so conc will lower
So if moles doesnt change..
New diluted conc = orig conc x (orig vol/ new diluted vol)
46
Using gas syringe
- they measure gas volume to work out gas moles or follow reaction rates
- volume of gas depends on temp and pressure so note down temp and pressure of room
..
Some gases like CO2 or SO2 are soluble in water so true amount of gas isnt measured
47
Changing conditions of a gas π‘π΄
Questions may involve same amount of gas under different conditions
> calculating changed pressure of gas mixture in flask
P1 x V1 = P2 x V2
48
How many molecules of a gas are present
- equal vols of any gas measured under same temp and pressure
> have equal numbers of molecules or atoms if monatomic
49
Whats the volume of one mole of any gas at room temp and pressure
Has volume of 24 dm3
50
How to work out limiting reactant
1. Calculate moles of each reactant
2. Use balanced eq to find limiting
> use ratio to work out amts of moles needed for each reactant depending on values calculated
..
3. Check if there is more mols for one reactant than in ratio - in excess
4. Other reactant is therefore limiting
51
Percentage yield π‘
Actual yield/ theoretical yield
Multiply by 100 for percentage
52
Percentage atom economy
Mass of useful products/ mass of all reactants
Muktiply all by 100 for percentage
53
How to know when titrations are concordant
- repeat titration until two condcordant results
> two readings within 0.1 of each other
..
- we can confirm results are accurate and repeatable and titration technique is good/consistent
54
In titrations what happens if jet space isnt filled properly
Leads to errors as will fill during titration
> so will produce larger than expected titre reading
55
Why is a concial flask used in titration
- easier to swirl without spilling contents
56
Why do we only add a few drops of indicator
- are weak acids
- if too much added will affect titration result
57
Uncertainty readings and measurements
β’ readings
Values found from a single judgement when using a piece of equity
β’ measurements
Values taken as the difference between judgements of two values
> using burette in titration
58
Ionic bonding
- electrostatic force of attraction between oppositely charged ions
- formed from electron transfer
59
Uncertainty and judgements
Uncertainty of a reading (one judgement) is at least +- 0.5 of smallest scale reading
..
Uncertainty of a measurement (two judgements ) is at least +-1 of smallest scale reading
60
Calculating apparatus uncertainties π‘π‘
Uncertainties are +- half the smallest division
..
Calculating percentage error for each piece of equipment
%uncertainty = +- uncertainty/measurement made on apparatus
Multiply by 100
61
Dative covalent bonding
The shared pair of electrons in covalent bond
- come from only one of bonding atoms
62
Metallic bonding
Electrostatic force of attraction between positive metal ions
And delocalised electrons
63
The factors affecting metallic bonding strength
1. Number of protons- strength of nuclear attraction
2. Number of delocalised electrons per atom
3. Size of ion (smaller ion, stronger bond)
64
Structure of covalent bonded substances
Simple molecular
65
General description of ionic vs metallically bonded substances
- ionic (crystalline solids)
- metallic
- (shiny metal,
- malleable as positive ions in lattice are all identical so planes of ions can slide easily
- attractive forces in lattice are same whichever ions are adjacent)
66
How to explain shape of molecule
1. State number of bonding pairs and lone pairs
2. Say electron pairs repel so try get far apart as possible
3. If no lone pairs electron pairs repel equally
4. If lone pairs say lone pairs repel more than bonding pairs
5. State actual shape and bond angle
67
Bond angles for
1- 1LP 5BP
2- 2LP 3BP
1. 89
2.89
68
Electronegativity and factors affecting (period and group)
- relative tendency of atom in a covalent bond in a molecule to attract electrons in covalent bond
..
Factors:
- increasing across period as proton numbers increase so atomic radius decreases
- bc electrons in same shell are pulled in motr
-
- decreases down group as distance between nucleus and outer electrons increases so shielding increases
69
Formation of permanent dipole or polar cov bond
Polar covalent bond forms when elements in bond have diff electonegativities
> when bond is polar covalent it has unequal distribution of electrons in bond
> produces charge separation (dipole) delta+/- ends
70
Hydrogen bonding
Occurs in compounds that have H atoms attached to F, O or N (most electronegative atoms)
> these must have a lone pair
71
Enthalpy change definition
Amt of heat energy taken in or given out during any change in a system
> provided pressure is constant
72
What are some common exothermic processes
Are combustion of fuels and oxidation of carbohydrates (like glucose in respiration)
73
Standard enthalpy change of formation
enthalpy change when one mole of compound
> is formed from its elements under standard conditions
> all reactants and products in standard states
74
Standard enthalpy change of combustion
enthalpy change when one mole of compound
> is combusted completely in oxygen under standard conditions
> all reactants and products in standard states
75
Eq for reaction with a solution : energy change (Q)π‘
Q= mCΞT
J
g
J g-1 K-1
K
>> this only gives energy for actual quantities used (not per mole which it is usually converted to)
76
In calorimetry : why is exact temp rise hard to obtain when reaction is slow
- cooling occurs simultaneously with
Reaction
- to counteract we take readings at regular time intervals and
- extrapolate temp curve / line back to when reactants were added together
77
In calorimetry : Why do we take temp for reactants few mins before added together
To get better more accurate average temp
- if both reactants are solutions then temp of both solutions need to be measured this way for avg temp
78
Errors on calorimetry
- energy loss from surroundings
- approximation in SHC of solution, assumed to be water
- neglecting SHC of calorimeter, ignoring any energy absorbed by apparatus
- reaction or dissolving may be incomplete or slow
- density taken as same as water
79
Calculating enthalpy change of reaction - ΞH from experimental data π‘
1. Q=mCΞT
2. Work out moles of reactants used
3. Divide q by moles not in excess
4. Units are Jmol-1
80
What is hess's law used for
- measure enthalpy change for a reaction that cannot be measured directly by experiments
-
- instead alternative reactions are carried out that can be measured experimentally
81
Mean bond energy definition
- enthalpy needed to break covalent bond into gaseous atoms averaged over different molecules
- definition only applies when substances start and end in gaseous state
82
How to work ΞH from mean bond energies π‘
Sum of bond energies broken β sum of mean bond energies made
..
- these values are less accurate than using formation or combustion data as mean bond energies arent exact
83
Trend in enthalpies of combustion for successive members of a homologous series
- constant rise in size of enthalpies of combustion as number of C atoms increase
- constant amt of extra bonds being broken and made so enthalpy of combustion increases by constant amt
84
Whats activation energy (Ea)
Minimum energy particles need to collide to start a reaction
> energy is used to break bonds in one or either of reactant molecules
85
Whats the maxwell boltzmann distribution
Shows spread of energies that molecules of a gas or liquid have at a particular temperature
> y is fraction of molecules w energy
> x is collision energy ->
86
Whats Emp in mb curve
Most probable energy (not same as mean)
Its the highest peak
87
Why do few molecules have low energy, and why should curve go through origin of mb curve
- collisions cause some particles to slow down
- there are no molecules with no energy so emergy distribution goes through origin
88
Where is mean energy of mb curve
- not at the peak
- just after the peak
89
Why does mb curve never meet x axis
- are no max energy for molecules,
- but only few particles have energy above Ea so stays low but constant after
90
Where do most molecules have energy mb curve
Between two extremes but distribution isnt normal/symmetrical
91
How can a reaction go to completion if few particles have energy greater than Ea in mb curve
Particles can gain energy by collisions
92
Units and definition of rate of reaction
The change in conc of substance in unit time
> mol dm-3 s-1
93
How to find reaction rate with curve (reactant conc vs time)
- gradient of curve
94
Whats initial rate of reaction
Rate at start of reaction where its fastest
95
Sodium thiosulfate and HCl experiment: reaction rate eq a d why, and chemical eq π‘π΅
- measure reaction rate 1/time
> an approximation for rate of reaction as doesnt include conc;
>we use bc assume amt of sulfur produced is fixed and constant
..
- Na2S2O3 + 2HCl > 2NaCl + SO2 + S + H2O
96
Effect of increasing conc or pressure in mb curves
- more particles per unit volume so particles collide with greater freq
- so a higher freq of effective collisions
..
- so shape of curve doesnt change but curves will be higher as will be more particles per unit volume
97
What happens when conc or rate doubles
Double numbers if particles per unit volume and double freq of effective collisions
98
Comparing rate curves (amount of product eg gas vol, vs time)
> different volumes of same initial concs
- have same initial rate if other conditions are same
- will end at diff amounts
99
Comparing rate curves (amount of product eg gas vol, vs time)
> what is amt of product proportional to
Moles is proportional to amt of product
- initial moles of reactants distinguish between diff finishing volumes
100
Increasing temp mb curve
- energy of particles increase so collide more frequently and often with energy greater than activation energy
> freq of successful collisions increases
101
Increasing surface area on mb curve
- causes successful collisions to occur more frequently between reactant particles
> increases rate of reaction
102
Effect of (what they are) catalyst in mb curve
Catalysts increase reaction rates wo getting used up
> provide alternative route or mechanism w lower activation energy
> so higher freq of effective collisions so reaction faster
103
Whats le chateliers principle
- we use it to work out how changing external conditions affect position of equilibrium (temp,pressure,conc)
-
- it states if an external condition is changed the eq will shift to oppose the change
104
Structure for answering dynamic equilibrium change questions
Eg, temperature increase
1. Equilibrium shifts to oppose the change Temp inc
2. Will move in eg endothermic backwards direction to reactants
3. To try decrease temperature
5. giving eg lower yield of ammonia
105
What happens if pressure is changed and moles of gas are same on both sides of equation
No effect on eq position
106
Effect of catalysts on equilibrium
- no effect on equilibrium position but will speed up rate which equilibrium is achieved
> speeding up back and forward reactions by same amt
107
Applying equilibrium to industrial processes : temperature
- because eq is exothermic for example, low temp gives good yield
- but slow reaction rates meaning compromise temp used so
- reasonable yield and rate achieved
108
Applying equilibrium to industrial processes: pressures
- high pressure only gives slight better yield and high rate
- too high pressure leads to too high energy cost for pumps to produce pressure
- equipment is expensive to contain high pressure
109
Production of methanol from CO π΅
CO + 2H2 <> CH3OH (exo)
400C
50 atm
Chromium and zinc oxides catalyst
110
Hydration of ethene to produce ethanol π΅
CH2CH2 + H2O <> CH3CH2OH (-ve)
300C
70 atm
Conc H3PO4 catalyst
> high pressure also gives unwanted polymerisation of ethene to (poly)ethene
111
Whats one way of improving overall yields of industrial processes
Recycling unreacted reactants
112
Carbon neutrality with methanol production
Carbon neutral refers to an activity that has no net annual carbon emissions to atm
- if CO used to make methanol was extracted from atm then could be classed as carbon neutral
-
- only can be classed if energy required to carry out reaction was not made by combustion of fossil fuels
113
In Kc calcs what is different if there are no units
- are equal numbers of reactants and products
- do not have to do mol/vol to find conc
- equilibrium moles can be put straight into kc eq
114
Effect of changing conditions on kc
Larger Kc, the greater the amt of products.
if Kc is small we say equilibrium favours reactants
- value of Kc get smaller when less products produced in equilibrium
115
What affects kc value π΄
Only temperature
- catalysts pressure and conc have no effect on kc value
116
Practical working out Kc: esterification
- ethanol and ethanoic acid are mixed with sulfuric acid catalyst
> NaOH from burette will react with sulfuric acid and any unreacted carboxylic acid
> in equilibrium mixture
117
Why does indicatior colour fade after end point in reversible reactions (esterification eg)
- addition of base may make equilibrium shift toward reactants
118
how are vdw imf of attraction formed
- random electron movement in one molecule forms temporary dipole
- this is an imbalance of electron density in one molecule
- induces tempory dipole in neighbouring molecule
- temporary attraction between delta+ and delta-
119
What are oxidising and reducing agents
1. Electron donors
2. Electron acceptors
120
How do you name agents
- refer to full name of substance not just element or ion
> like bromine water
121
How to find percentage uncertainty π‘
(Uncertainty / value) x100
>> if for balance (how many times balance used x uncertainty)/mass change
122
With percentage uncertainties how to find range of values π‘
Value +/- (value x percentage uncertainty total in decimal)
123
In disappearing cross prac, why does the cross on boiling tube go invisible
- insoluble precipitate forms
124
In disappearing cross prac what are some controls
- same person observing cross
- view cross from same distance
A Level Chemistry π«§π§
flashcards
Decks in class (33)
# Cards
-
1.1 Atomic Structure31
-
1.3 Bonding27
-
equations!6
-
1.2 Amount Of Substance41
-
3.1 Intro To Organic Chemistry13
-
3.2 Alkanes22
-
3.3 Haloalkanes17
-
Oxidation, Reduction And Redox Equations14
-
Periodicity, Group 2 And 738
-
Alkenes22
-
Energetics19
-
Alcohols19
-
Kinetics9
-
Organic Analysis12
-
Chemical Equilibria and Le Chatelier's principle29
-
Kp5
-
Period 3 Oxides (A Level)21
-
Organic Naming And Isomerism (A Level)15
-
Rate Equations (Kinetics A Level)19
-
Carbonyls: Aldehydes And Ketones // Carboxylic Acids And Derivatives (A Level)33
-
Thermodynamics31
-
Physical Exam Questions58
-
Electrode Potentials26
-
A2 - Physical148
-
AS - Physical124
-
AS - Inorganic39
-
A2 - Inorganic58
-
AS - Organic100
-
A2 - Organic151
-
AS And A2 Mechanisms π’1
-
AS and A2 Practicals81
-
Organic Exam Questions34
-
Inorganic Exam Questions11
