physical chem Flashcards
why do elements in groups have similar chemical properties
similar outer shell electron configuration
periodicity definition
repeating pattern across different periods
order of blocks in the periodic table
s,d,p,f
what happens to atomic radius across a period
- atomic radius decreases
- as there is an increased number of protons, which creates more nuclear attraction
- similar shielding
first ionisation energy definition
the energy needed to remove an electron from each atom in one mole of gaseous atoms
example equation for first ionisation energy
H(g) → H+(g) + e-
which factors affect ionisation energy
- nuclear attraction
- atomic radius
- shielding
why are successive ionisation energies always larger
- the ion formed is smaller than the atom (smaller atomic radius)
- the proton to electron ratio in the 2+ ion is greater than in the 1+ ion
- nuclear attraction is therefore stronger
if there is a big jump in ionisation energy between IE 2 and 3, what does this mean
- the element must be in group 2
- as the 3rd electron is being removed from an electron shell closer to the nucleus and therefore experiences more nuclear attraction
- there must also be less shielding
why do 1st ionisation energies decrease down a group
the outer electrons are found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller
Why is there a general increase in first ionisation energy across a period?
the electrons are being added to the same
shell which has the same distance from the nucleus and same shielding
effect. The number of protons increases, however, making the effective attraction of the nucleus greater.
why is there a small drop in IE from group 2 to 3
- group 2 has its outer electrons in the 3s sub shell, whereas group 3 is starting to fill a 3p sub shell
- The electrons in the 3p subshell are slightly easier to remove because
the 3p electrons are higher in energy and are also slightly shielded by the 3s electrons
why is there a small drop in IE from group 5 to 6
in group 6 r there are 4 electrons in the 3p sub shell and the 4th is starting to doubly
fill the first 3p orbital.
When the second electron is added to a 3p orbital there is a slight repulsion between
the two negatively charged electrons which makes the second electron easier to
remove.
metallic bonding definition
the electrostatic force of attraction between
the positive metal ions and the delocalised electrons
what are the three main factors that affect the strength of metallic bonding
- Number of protons/ Strength of nuclear attraction.
(The more protons the stronger the bond) - Number of delocalised electrons per atom (the outer shell electrons are delocalised)
The more delocalised electrons the stronger the bond - Size of ion.
The smaller the ion, the stronger the bond
why has Mg got stronger metallic bonding than Na
Mg has stronger metallic bonding than Na and hence a higher melting point.
The Metallic bonding gets stronger because in Mg
there are more electrons in the outer shell that are released to the sea of electrons.
The Mg ion is also smaller and has one more proton. There is therefore a stronger electrostatic attraction
between the positive metal ions and the delocalised electrons and higher energy is needed to break bonds
explain the structure of diamond
MACROMOLECULAR
- Tetrahedral arrangement of
carbon atoms.
- 4 covalent bonds per atom
- high melting points because of strong covalent forces in the giant structure. It takes a lot of energy to break the many strong covalent bonds
explain grpahite sutreucue
MACROMOLECULAR
- Planar arrangement of carbon atoms in layers.
- 3 covalent bonds
per atom in each layer. 4th outer electron per atom is delocalised.
- Delocalised electrons between layers.
- high melting points because of strong covalent forces in the giant structure. It takes a lot of energy to break the many strong covalent bonds
which structures have covalent bonding
MACROMOLECULAR
which structures have metallic bonding
GIANT METALLIC LATTICES
properties of macromolecular substances
bp and mp: high due to many covalent bonds which take a lot of energy to break
solubility: insoluble
conductivity when solid: diamond and sand are poor because electrons cant move, whereas graphite is good as free electrons between layers
conductivity when molten: poor
properties of giant metallic substances
mp and bp points: high due to strong electrostatic forces between positive ions and sea of deloc electrons
solubility: insoluble
conductivity: good - deloc electrons can move through structure
explain the general trend of mp and bp across period one
For Na, Mg, Al- Metallic bonding : strong bonding – gets stronger the more electrons there are in the outer shell that
are released to the sea of electrons. A smaller positive
centre also makes the bonding stronger. High energy is needed to break bonds.
Si is Macromolecular: many strong covalent bonds
between atoms high energy needed to break covalent
bonds– very high mp +bp
Cl2 (g), S8 (s), P4 (S)- simple Molecular : weak London forces between molecules, so little energy is needed to break them – low mp+ bp
S8 has a higher mp than P4 because it has more electrons
(S8 =128)(P4=60) so has stronger London forces
Ar is monoatomic weak London forces between atoms
what happens to atomic radius down group 2
- increases
- atoms have more shells of electrons
melting point down group 2
- decreases
- metallic bonding weakens as the atomic size increases
- distance between the + ions and deloc electrons increases
- therefore, electrostatic
attractive forces between the positive ions and the
delocalized electrons weaken
what happens to first and second IE’s down group 2
- decrease
- The outermost electrons are held more weakly because they are successively further from the nucleus in additional
shells - In addition, the outer shell electrons become more shielded from the attraction of the nucleus by the repulsive force of inner shell electron
reactivity down group 2
- increases
- As the atomic radii increase there is more shielding.
- The nuclear attraction decreases and it is easier to remove outer electrons.
- Cations form more
easily
group 2 metals reactions with oxygen
The group 2 metals will burn in oxygen.
Mg burns with a bright white flame
2Mg + O2 → 2MgO
- Mg will also react slowly with oxygen without a flame.
- Mg ribbon will often have a thin layer of magnesium oxide on it formed by reaction with oxygen
- this is cleaned off by emery paper before doing reactions with Mg ribbon as Mg and MgO react at different rates
Mg + steam
produces magnesium oxide and hydrogen
the Mg would burn with a bright white flame
Mg + warm water
produces magnesium hydroxide and hydrogen
this is a slower reaction than with steam and produces no flame
group 2 metals and cold water
- react with increasing vigour down the group to form hydroxides
eg : Ca + 2 H2O (l) → Ca(OH)2
(aq) + H2(g)
what are the observations for group 2 metals reacting with cold water
what are the observations for group 2 metals reacting with cold water
group 2 metals reacting with acid
- react with acids with increasing vigour down the group to form a salt and hydrogen
why does barium react slowly with H2SO4
the insoluble barium
sulfate produced will cover the surface of the metal and act as a barrier to further attack.
group 2 oxides + water
- forms hydroxides
what is magnesium hydroxide used for
s used in medicine (in suspension as milk of magnesia) to neutralise excess acid in the stomach and to treat constipation.
Mg(OH)2 + 2HCl → MgCl2 + 2H2O
It is safe to use as it is weakly alkaline.
what is calcium hydroxide used for
- It is used in agriculture to neutralise acidic soils.
- If too much calcium hydroxide is added to the
soil, excess will result in soils becoming too alkaline to sustain crop growth
appearance of the halogens
Fluorine (F2): very pale yellow gas. It is highly reactive
Chlorine : (Cl2) greenish, reactive gas, poisonous in high concentrations
Bromine (Br2) : red liquid, that gives off dense brown/orange poisonous fumes
Iodine (I2) : shiny grey solid sublimes to purple gas.
mp and bp down the halogens
increases down the group
As the molecules become larger they have more electrons and so have larger induced dipole-dipole forces
(London forces) between the molecules. As the intermolecular forces get larger more energy has to be put into break these intermolecular forces. This increases the melting and boiling points
reactivity of halogens down the group
- decreases
- as the atoms get bigger with more shielding so they less
easily attract and accept electrons. They therefore form -1 ions less easily down the group
reactivity of halogens explanations
- Chlorine is more reactive than bromine because it will
gain an electron and form a negative ion more easily than
bromine. - This is because an atom of chlorine is smaller than bromine and the outermost shell of chlorine is less shielded than bromine so the electron to be gained is
attracted more strongly to the nucleus in chlorine than
bromine.
halogen displacement equations
Cl2(aq) + 2Br – (aq) → 2Cl - (aq) + Br2(aq)
Cl2(aq) + 2I – (aq) → 2Cl – (aq) + I2(aq)
Br2(aq) + 2I – (aq) → 2Br – (aq) + I2(aq)
disproportionation definition
the name for a reaction where
an element simultaneously oxidises and reduces.
chlorine + water equation
how is it disproportionation?
what will happen is UI is added?
Cl2(g) + H2O(l) →HClO(aq) + HCl (aq)
Chlorine is both simultaneously reducing and oxidising. It changes from 0 in Cl2 to -1 in HCl and +1 in HClO
If some universal indicator is added to the solution it will first turn red due to the acidity of both reaction products. It will then turn
colourless as the HClO bleaches the colour.
what are the benefits and negatives of chlorine being used in water treatment?
- kills bacteria
- toxic and irritates the respiratory system
chlorine + cold dilute NaOH
Cl2(aq) + 2NaOH(aq) → NaCl (aq) + NaClO (aq) + H2O(l)
- the colour of the halogen solution will fade to colourless
- The mixture of NaCl and NaClO (sodium chlorate (I)) is used as Bleach and to disinfect/ kill bacteria
chlorine + hot NaOH
3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O
- sodium chlorate (V) is formed
reaction of halide ions with silver nitrate
- used for?
- why is nitric acid added?
- colours?
- how can ammonia be used to help differentiate
used as a test to identify which halide ion is present. The test solution is made acidic with nitric acid, and then Silver nitrate solution is added drop wise
The role of nitric acid is to react with any carbonates
present to prevent formation of the precipitate
Ag2CO3 - This would mask the desired observations
- Fluorides produce no precipitate
- Chlorides produce a white precipitate
Ag+(aq) + Cl- (aq) →AgCl(s) - Bromides produce a cream precipitate
Ag+ (aq) + Br- (aq) → AgBr(s) - Iodides produce a pale yellow precipitate
Ag+ (aq) + I- (aq) →AgI(s)
The silver halide precipitates can be treated with ammonia
solution to help differentiate between them if the colours look similar:
Silver chloride dissolves in dilute ammonia to form a
complex ion
AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Cl- (aq)
Silver bromide dissolves in concentrated ammonia to form a complex ion
AgBr(s) + 2NH3(aq) →[Ag(NH3)2]+(aq) + Br - (aq)
Silver iodide does not react with ammonia – it is too insoluble
is △H positive or negative in exothermic and endothermic reactions?
In an exothermic reaction the
∆H is negative
In an endothermic reaction
the ∆H is positive
activation energy definition
the minimum energy which particles need to collide to start a reaction
standard conditions
- 100 kPa pressure
- 298 K (room temperature or 25oC)
- Solutions at 1mol dm-3
- all substances should have their normal state at 298K
enthalpy change of reaction definition
the enthalpy change when the number of moles of reactants as specified in the
balanced equation react together
standard enthalpy change of formation definition
the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all
reactants and products being in their standard states
standard enthalpy change of combustion definition
the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kPa), all reactants and products being in their
standard states
enthalpy change of neutralisation definition
the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.
errors in calorimetry experiments
Energy losses from calorimeter
* Incomplete combustion of fuel
* Incomplete transfer of energy
* Evaporation of fuel after weighing
* Heat capacity of calorimeter not included
* Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment
mean bond enthalpy
the enthalpy change when
one mole of bonds of (gaseous covalent) bonds is broken (averaged over different molecules)
hess’ law definition
total enthalpy change for a reaction is independent of the route by which the chemical changes takes place
effect of increasing conc and pressure on rate of reaction
At higher concentrations(and pressures) there are more
particles per unit volume and so the particles collide
with a greater frequency and there will be a higher
frequency of effective collisions.
rate of reaction definition
the change in concentration of a substance in unit time
Its usual unit is mol dm-3s-1
catalysts definition
Catalysts increase reaction rates without getting used up.
Explanation: They do this by providing an alternative route or mechanism with a lower activation energy so
more molecules have energy above activation energy.
heterogeneous catalysts vs homogenous catalysts
A heterogeneous catalyst is in a different phase from the reactants
- Heterogeneous catalysts are usually solids whereas the reactants are gaseous or in solution.
- The reaction occurs at the surface of the catalyst.
A homogeneous catalyst is in the same phase as the reactants
- When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species.
benefits of catalysts
- lower temps and pressures can be used
- This can save energy costs as there is reduced energy demand for providing high temperature and less electrical pumping costs for producing pressure.
- This can mean fewer CO2 emissions from burning
of fossil fuels. - Catalysts can enable different reactions to be
used, with better atom economy and with
reduced waste, or fewer undesired products or
less use of hazardous solvents and reactants. - Catalysts are often enzymes, generating very specific products, and operating effectively close to room temperatures
and pressures.
effect of increasing temp of rate of reaction
At higher temperatures the energy of the particles
increases. They collide more frequently and more
often with energy greater than the activation energy.
More collisions result in a reaction
effect of increasing surface area on rate of reaction
causes collisions to occur more frequently between the reactant particles and this increases the rate of the reaction.
effect of catalysts on rate of reaction
If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster
what are the conditions of dynamic equilibrium
- forward and backward reactions are occurring at equal rates.
- The concentrations of reactants and products stays
constant and the reaction is continuous.
effect of temp on equilibrium
If temperature is increased the equilibrium will
shift to oppose this and move in the endothermic direction to try to reduce the
temperature by absorbing heat.
what affect does temp have on yield
Low temperatures may give a higher yield of product but will also result in slow rates of
reaction. Often a compromise temperature is used that gives a reasonable yield and rate
effect of pressure on equilibrium
Increasing pressure will cause the equilibrium to shift
towards the side with fewer moles of gas to oppose
the change and thereby reduce the pressure.
If the number of moles of gas is the same on both
sides of the equation then changing pressure will have
no effect on the position of equilibrium
what affect does pressure have on yield
Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high
pressures are expensive to produce (high electrical energy costs for pumping the gases to make a high
pressure) and the equipment is expensive (to contain the high pressures)
effect of conc on equilibrium
- equilibrium shifts to opposite direction to oppose this
effect of catalysts on equilibrium
A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved.
It does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
haber process
- equation
- effect of temp and pressure
N2 + 3H2 →2NH3 △H = -ve exo
Low temp gives good yield but slow rate: compromise temp used
High pressure gives good yield and high rate:
too high a pressure would lead to too high energy costs for pumps to produce the pressure
contact process
- equations
- temp and pressure effect
Stage 1 S (s) + O2 (g) → SO2(g)
Stage 2 SO2 (g) + ½ O2 (g) → SO3 (g)
△H = -98 kJ mol-1
Low temp gives good yield but slow rate: compromise
moderate temp used
High pressure gives slightly better yield and high rate: too
high a pressure would lead to too high energy costs for
pumps to produce the pressure
production of methanol from CO
- equation
- effects of temp and pressure
CO (g) + 2H2(g) →CH3OH (g) △H = -ve exo
Low temp gives good yield but slow rate:
compromise temp used
High pressure gives good yield and high rate: too
high a pressure would lead to too high energy
costs for pumps to produce the pressure
hydration of ethene to produce ethanol
- equation
- effects of temp and pressure
CH2=CH2 (g) + H2O (g) →CH3CH2OH(l) △H = -ve
Low temp gives good yield but slow rate: compromise
temp used.
High pressure gives good yield and high rate: too high a
pressure would lead to too high energy costs for pumps
to produce the pressure.
High pressure also leads to unwanted polymerisation of
ethene to poly(ethene).
what affects Kc or Kp
ONLY TEMP
