Chemistry - EEE Flashcards
What is the electron configuration of Chromium?
[Ar] 3d5, 4s1
What is the electron configuration of Copper?
[Ar] 3d10, 4s1
What is a transition element?
An element that forms one or more stable ions with a partially-filled d-subshell
Why is scandium not a transition element?
Its only stable ion is Sc3+, which has a configuration of [Ar] 3d0
Why is Zinc not a transition element?
Its only stable ion is Zn2+, which has a configuration of [Ar] 3d10
what are the 4 typical characteristics of transition metals?
1) Variable oxidation state in compounds
2) The metals and their compounds are often catalytically
3) Able to form a wide range of complex ions
4) form coloured compounds
Why do transition metals have variable oxidation states?
The energies of the 4s and 3d electrons are very similar
-> the elements can lose (or share) various number of electrons
when they form stable compounds
Why are transition metals good catalysts?
They are good at absorbing reactants on the metal suface
thereby weaking the bonding in the molecule and hence lowering the activation energy
Give examples of transition metals being used as catalysts
1) Catalytic Converters - Platinum, Rhodium, and Palladium
2) Haber Process - Iron metal
Why are transition metal compounds often catalytically active?
They can easily and reversibly change oxidation states
- allows them to provide alternative reaction pathways in redox reactions
Give an example of a transition metal compound being used as as catalyst
Vanadium Oxide - Catalyses oxidation of SO2 to SO3 in the manufacture of sulphuric acid
What is a complex ion?
A metal ion bonded to a number of ligands by coordinate bonds (dative covalent bonds)
what is a ligand?
An ion or molecule with a lone pair of electrons which it donates to a central metal ion by forming a coordinate bond with it
In what 2 cases does the colour of a transition metal change?
1) When the oxidation state changes
2) When a complex ion with different ligands are formed
What is the colour of iron in:
1) [Fe(H2O)6]2+
2) [Fe(H2O)6]3+
1) 2+: Pale green
2) 3+: Yellow
What are the colours of the following ligands of cobalt:
1) [Co(H2O)6]2+
2) [CoCl4]2+
1) [Co(H2O)6]2+: Pink
2) [CoCl4]2+ : Blue
What colour are the following compounds of Copper?
1) [Cu(H2O)6]2+
2) [Cu(NH3)4(H2O)2]2+
3) [CuCl4]2-
1) [Cu(H2O)6]2+ : Blue
2) [Cu(NH3)4(H2O)2]2+ : Deep blue
3) [CuCl4]2- : Yellow
Write the equation of the reaction which is used as a test for the presence of Fe2+ ions, and the observation to confirm the reaction has occured
Fe2+ (aq) + 2OH- (aq) -> Fe(OH)2 (s)
Green Precipitate
Write the equation of the reaction which is used as a test for the presence of Fe3+, and give the observation used as a positive result
Fe3+ (aq) + 3 OH- (aq) -> Fe(OH)3
Red/Brown Precipitate
Write the equation of the reaction which is used as a test for the presence of Co2+, and give the observation used as a positive result
Co2+ (aq) + 2OH- -> Co(OH)2 (s) Blue Precipitate
Write the equation of the reaction which is used as a test for the presence of Cu2+ , and give the observation used as a positive result
Cu2+ (aq) + 2OH- (aq) -> Cu(OH)2 (s)
Blue precipitate
What are unidentate ligands?
Simple ligands which have a single point of attachment to the ion
What are Bidentate Ligands?
a Lignad with 2 lone pairs, each capable of forming a coordinate bond to a metal ion
what is the coordination number of an ion in a complex?
The number of ligand lone pairs that are bound to the ion - I.e. the number of co-ordinate bonds formed to the metal
What factor affects the co-ordination number of a compound?
Size of the ligands
What factor affects the shape of complex ions?
1) Size - large ligands and small ions favour tetrahedral geometry - less crowded
What is the bond angle in a tetrahedral complex ion?
109.5
what is the bond angle in a square planar complex ion?
90* or 180*
What are the bond angles in octahedral complex ions?
90* or 180*
Why may ligand substitution reaction occur?
1) One ligand can form stronger coordinate bonds to the metal ion than another
2) The substituting ligand is present in a higher concentration than the other
Give the equation and observation of the substitution reaction that occurs when Concentrated HCL is added to aqueous Cu2+
[Cu(H2O)6]2+ (aq) + 4Cl- [CuCl4]2- (aq) + 6H2O (l) Colour change from Blue to Yellow
Give the equation and observation of the substitution reaction that occurs when aqueous ammonia is added to aqueous Cu2+
1) when a low conc. of ammonia is added:
a) NH3 + H2O (l) -> (NH4)+ + (OH)- (aq)
b) (Cu)2+ + 2(OH)- -> Cu(OH)2 (s) observation: Blue ppt
2) High concentration NH3:
a) [Cu(H2O)6]2+ + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 4H2O
observation: blue solution to deep blue solution
Give the equation and observation of the substitution reaction that occurs when concentrated HCl is added to aqueous Co2+
[Co(H2O)6]2+ + 4Cl- [CoCl4]2- + 6H2O
Pink solution turns blue
In Haemoglobin, what is the dentate of the Haem group ligand to the (Fe)2+ ion
Tetradentate (4)
What is the co-ordination number of Haemoglobin, and state the types of lone pairs in a haemoglobin molecule
1) 6 - octahedral
2) tetradentate haem group, protein, water
Give the ligand substitution of haemoglobin
let Hb = haemoglobin
Hb-H2O + O2 -> Hb-O2 + H2O
Describe the process of the substitution reaction of haemoglobin in the body
- process allows Hb to transport O2 round the body
1) Forward reaction occurs in the lungs - High O2 concentration
2) In low O2 conc. (tissues) , backward reaction occurs - O2 released
Why does CO bind preferentially to oxygen in Hb?
- CO is a better ligand than oxygen - Binding affinity of CO 200X greater than O2
- > low levels of CO dramatically reduce ability of Hb to carry O2
What are stereoisomers?
Same structural formula, different arrangement in space
Which types of ligand complexes does cis-trans isomerism occur in?
Square planar and octahedral
What orientation are a pair of ligands in if the bond angle between them is 90*?
Cis
What orientation are a pair of ligands in if the bond angle between them is 180*?
Trans
What is Cis - platin, and how does it work?
- Pt with 2 Cls and 2 NH3s [Pt(Cl)2(NH3)2]
- Used in treatment of cancer - stops tumour growth by binding to DNA, stopping cell division
Which ion complexes can exist as optical isomers, and under what circumstances?
- Octahedral - usually 2 or 3 bidentate ligands
- Tetrahedral - only if 4 different ligands
What does the bond angle in Ammonia change to when it acts as a ligand in a complex, and why does it change?
- 109.5 (from 107)
- NH3’s lone pair has been converted to a coordinate bond - so no lone pairs in N’s outer shell
What is the stability constant of a complex ion? give the symbol used also
equilibrium constant for the formation of the complex ion from its constituent ions. (water is discounted)
Kstab
What is an Acid?
A proton donor
What is a base?
A proton acceptor?
What name is given to the other part of an acid or base that remains unchanged in a reaction?
Spectator Ion
Why is the reaction of a metal element and an acid not an acid-base reaction?
H+ IS REDUCED
(no change of oxidation state in acid-base)
What is a strong acid? give an equation to demonstrate this
An acid which is totally ionised in aqueous solution
HCl (aq) -> H+ (aq) + Cl- (aq)
What is a strong base?
A base which is totally ionised in aqueous solution
NaOH (aq) -> Na+(aq) + OH- (aq)
What is a Weak Acid? give an equation to demonstrate this
an acid which is only partially ionised in aqueous solution
CH3COOH (aq) CH3COO- (aq) + H+ (aq)
What is a weak base? give an equation to demonstrate this:
A base which is partially ionised in aqueous solution
Write a Brønstead-Lowry equation for the ionisation of Hydrochloric acid:
HCl (aq) + H2O (l) -> H3O+ + Cl-
What is the name of H3O+ (aq)?
HYDROXONIUM ION
What are Conjugate Pairs?
Acid-base pairs, differing only in the extra H+ possessed by the acid
What correlation is there between the strength of a conjugate acid and the strength of its conjugate base?
The stronger the conjugate acid, the weaker the conjugate base
(strong acid = equilibrium position of ionisation far to the right)
If two acids are involved in an acid-base reaction, how are their roles decided?
The stronger acid acts as the acid
What is the definition of pH?
pH= -Log[H+]
How do you find the concentration of H+, given pH?
10(-pH)
How do you calculate pH for a strong acid?
Strong acids are COMPLETELY IONISED, so
[Acid] = [H+] pH= -log[acid]
Give the equation for the equilibrium constant for the ionisation of water?
Kc = [H+][OH-]/[H2O]
What is Kw?
the IONIC PRODUCT OF WATER
Kw = [H+][OH-] Kw = Kc[H2O]
What is the value of Kw at 25ºc?
1x10-14 mol2dm-6 (on data sheet)
How the the ratio of H+ ions to OH- ions compare in a neutral solution?
SAME CONC OF H+ AND OH-
How the the ratio of H+ ions to OH- ions compare in an acidic solution?
HIGHER [H+] than [OH-]
How the the ratio of H+ ions to OH- ions compare in an alkali solution?
HIGHER [OH-] than [H+]
How would you calculate the pH of pure water at rtp?
Kw = [H+][OH-],
in pure water [H+] = [OH-]
Kw = [H+]^2
[H+] =√Kw
pH = -log[H+] = 7.00
What formula is used to calculate the pH of strong alkalis?
Kw = [OH-][H+]
[H+] =Kw/[OH-]
Why does the concentration of a weak acid not immediately indicate the concentration of H+?
Weak acids are NOT FULLY IONISED in aqueous solution
What constant is used to calculate [H+] of a weak acid?
the acid DISSOCIATION CONSTANT, Ka
How is Ka defined?
Ka = ([H+][A-])/[HA]
What are the units of Ka?
Moldm-3
What factor has an effect on the value of Ka?
TEMPERATURE
What does the value of Ka indicate about an acid?
LARGER Ka = STRONGER ACID (Higher H+ conc.)
What is pKa?
pKa = -logKa
why is pKa often used instead of Ka?
Ka is often a very small number
What effect does the strength of an acid have on pKa?
STRONGER ACID = SMALLER pKa
What is the general formula for calculating [H+] of a weak acid solution?
[H+] = √(Ka[HA])
What is assumed when calculating the pH of an acid, and in what case is this assumption not true?
[H+]=[A-]
-> Only true for PURE ACIDS not true if extra acid or extra A- is added
What colour is MnO4-?
Purple
What is the colour of the end point of a potassium dichromate titration?
Pink
What is the colour change in iodine titrations?
Brown to colourless
What substance is often used to intensify the colour of iodine, and what colour does it turn in the presence of iodine?
Starch indicator Black
What are the characteristics of a dynamic equilibrium?
1) Rate of forward reaction = rate of backward reaction
2) Closed system - substances are not being added or removed
3) macroscopic properties remain constant
What is meant by Kc?
1) EQUILIBRIUM CONSTANT
Under what conditions is the value of Kc constant?
given TEMPERATURE
How does the value of Kc correlate with the position of equilibrium? for what conditions is this true?
higher Kc = position further to right
similar equilibria - Kc has same unit
What effect does an increase in temperature for an exothermic reaction have on the equilibrium constant, and why?
1) decrease in value of Kc
2) Position of equilibrium has shifted in the endothermic direction - left
What effect does an increase in temperature for an endothermic reaction have on the equilibrium constant, and why?
1) Kc increases
2) equilibrium shifts to endothermic direction to oppose change - right
What effect does an increase in pressure have on the value of Kc?
No effect
only effects position of equilibrium - will cause the equation to no longer equal Kc
How can we tell if a system has not reached equilibrium?
CONCENTRATIONS WILL NOT BE CONSISTENT WITH Kc
If the value of a concentration equation is higher than Kc, which way will the reaction move as it approaches equilibrium? why?
Backward
if calculated value is higher than Kc, concentration on top line must be too high compared to bottom line
concentration reaction will move backward (left) to increase bottom line concentration, and therefore come into line with Kc
What is the movement of electrons in a redox reaction?
electrons are transferred from the REDUCING AGENT to the OXIDISING AGENT
What name is given to the system where electrons flow indirectly from the reducing agent to the oxidising agent, causing an electric current?
ELECTROCHEMICAL CELL
Which electrode in a half cell contains the oxidising agent?
POSITIVE ELECTRODE
What is the purpose of a Salt Bridge?
to complete the circuit by allowing the controlled movement of IONS between the salt bridge (not electrons)
How can you construct a simple salt bridge?
A piece of filter paper soaked in saturated potassium nitrate solution
What is the correlation between cell potential and current?
Electrons move from a region of NEGATIVE POTENTIAL to an area of POSITIVE POTENTIAL
What is another term for voltage?
POTENTIAL DIFFERENCE
What is meant by cell potential?
The potential difference measured in a cell - measured in volts - always positive
What is the hydrogen electrode made out of?
Platinum - Hydrogen gas is bubbled over it in a H+ solution
What is the equation for the reaction at the hydrogen electrode?
H+ (aq) + e- > 1/2 H2 (g)
What factor effects the direction the hydrogen electrode reaction goes in, and the charge of the hydrogen electrode?
How easily the other half cell is oxidised/reduced
What are standard conditions for a half cell?
1 moldm-3 solution 1 atm pressure 298K/25*c temp
What is meant by Standard Electrode Potential?
The voltage measured in a cell which that half cell is connected to a standard hydrogen electrode, under standard conditions
How does the value of the standard electrode potential of a half cell affect whether it becomes the positive or negative electrode?
If E^ø is positive, when connected to a hydrogen electrode it will become the positive electrode
How does the reactivity of a metal correlate with the Standard Electrode Potential?
More reactive = reduction of ions less favourable = more negative E^ø
How would you make a half cell for a gas?
Bubble gas over Platinum electrode
and immerse in solution of ions
formed by reduction/oxidation of gas
How would you construct a half cell for ions of one element in solution?
Use a platinum electrode with BOTH IONS AT THE SAME CONCENTRATION
What is meant by Standard Cell Potential (E^ø cell)?
the DIFFERENCE between 2 E^ø values - always positive
How would you decide which half cell would be a negative electrode?
More negative E^ø = oxidation more likely = more negative electrode
How could you tell if a reaction is feasible from electrode potentials?
If the one with the more negative electrode potential is being reduced, it is not feasible
What factors may cause a measured E^ø value to be different form predicted E^ø values?
1) non-standard conditions - due to le chateliers principal - position of equilibrium shifts, affecting whether reduction or oxidation is more favourable
2) rates of reaction - high activation energy
- > reaction occurs too slowly to come to equilibrium - changes measured E^ø
What is a storage cell?
An electrochemical cell with the voltmeter replaced by an electrically powered device (e.g. light bulb)
The free energy created by the cell is used to create a voltage
Why may an electrochemical cell with sufficient cell potential not be able to be used in practice,?
1) Salt bridge has HIGH RESISTANCE
What is an example of a storage cell?
NICKEL-CADMIUM BATTERIES
Why are nickel-cadmium batteries not widely used anymore?
1) HIGHLY TOXIC compounds - difficult to dispose of
2) EXPENSIVE - NON RENEWABLE
What type of storage cell are nickel-cadmium batteries?
RECHARGEABLE CELLS
What factor is necessary for a cell to be rechargable?
1) electrodes must remain physically intact as the reaction proceeds
- i.e cant change state -
do not lose physicall integrity
How would you recharge a cell?
1) connect positive electrode to the positive terminal of a power supply (and negative to negative)
2) Electrons are driven back into the cell
What is the difference between storage cells and fuel cells?
1) Storage cells are sealed systems - the free energy is stored in the cell
2) Fuel cells are not sealed - chemicals constantly flow into the cell
What 2 things do fuel cells generally require?
1) FUEL (usually HYDROGEN)
2) OXIDANT (usually OXYGEN from the air)
What are the electrodes in a Hydrogen fuel cell made out of?
Platinum catalyst coated metal
What is the reaction at the negative electrode in a hydrogen fuel cell?
1) 1/2 H2 -> H+ + e-
Hydrogen is OXIDISED
What is the reaction at the positive electrode of a hydrogen fuel cell?
1) 2H+ + 1/2O2 + 2e- -> H2O
What is the overall cell reaction for a hydrogen fuel cell?
H2 + 1/2 O2 -> H2O
What is the reaction for the negative electrode of a hydrogen fuel cell under alkaline conditions?
1/2H2 + OH- -> H2O + e-
What is the reaction for the positive electrode of a hydrogen fuel cell under alkaline conditions?
1/2 O2 + H2O + 2e- -> 2OH-
What methods can be used for storing hydrogen in Fuel Cell Vehicles, and what are their limitations?
1) High pressure - risk of explosion, specialised tanks needed
2) storage as liquid - low temps required, expensive
3) Adsorption on a solid surface - expensive materials
4) Absorption in a solid metal - (release H2 as pressure eases)
5) Hydrogen rich fuels - H2 released by reforming - releases CO2
What are the advantages of FCV’s?
1) (hydrogen FCVS) only produce water as waste - less polluting
2) much more efficient than traditional combustion engines - even reforming hydrogen rich fuels will produce less CO2
What are the limitations of FCV’s?
1) Hydrogen supply - H2 doesn’t occur on earth naturally, must be manufactured - costs energy and often produces CO2
2) Hydrogen storage - Compression and liquefaction have high energy costs and risks of explosion - Absorption and Adsorption systems for H2 have a limited lifetime
3) Fuel cells - need regular replacement - expensive to manufacture and involves toxic chemicals
What is meant by the Hydrogen Economy?
An economy in which our energy needs are met to a substantial extent by hydrogen in the place of fossil fuels
What obstacles need to be overcome in order to reach a hydrogen economy?
1) Public and political acceptance of hydrogen as a fuel - fear over explosion due to Hindenburg
2) Establishment of infrastructure to ensure hydrogen is widely available - storage and distribution facilities
3) generation of large quantities of hydrogen, which requires energy - will only significantly reduce CO2 emissions is fossil fuels are not used to generate H2 - electrolysis of water using sustainable energy sources
What is an Acid?
A substance that can donate a H+ ion
What is a base?
A substance that can accept a H+ ion
What name is given to ions in acid-base reactions that are not actively involved in the process?
SPECTATOR IONS
What is a strong acid?
An acid that is TOTALLY IONISED IN AQUEOUS SOLUTION HX -> H+ + X-
What is a strong base?
A base that is TOTALLY IONISED IN AQUEOUS SOLUTION XOH -> X+ + OH-
What is a weak acid?
An acid that is only PARTIALLY IONISED IN AQUEOUS SOLUTION HX H+ + X-
What is a weak base?
A base that only PARTIALLY IONISES IN AQUEOUS SOLUTION XNH2 + H2O XNH3+ + OH-
Write the Brønstead-Lowry equation for the ionisation of HCl:
HCl (aq) + H2O (aq) Cl- (aq) + H3O+ (aq)
What is the name of H3O+?
HYDROXONIUM ION
What is a Conjugate acid?
A Base which has accepted a H+
What is a conjugate base?
An acid which has donated a H+
What name is given to acid-base pairs that only differer by a H+ ion?
CONJUGATE PAIRS
How does the strength of a conjugate acid affect the strength of a conjugate base?
THE STRONGER THE CONJUGATE ACID, THE WEAKER THE CONJUGATE BASE
What is rate of reaction?
the rate of change of concentration of a reactant or product with time
What units are used to measure rate of reaction?
Moldm-3s-1
How can you measure rate of reaction on a graph plotting concentration against time?
Draw a tangent to curve at given time and measure gradient
What is meant by the Rate Equation of a reaction?
An equation showing how the rate depends on the concentrations of reactants
How do you write a rate equation?
Rate=K[A]m[B]n
How do you calculate the overall order of a reaction?
Sum all the separate orders in the rate equation
What name is given to the K in a rate equation?
Rate Constant
What is K a measure of?
The probability that a reaction will happen when the particles collide
What effect does temperature have on K, and why?
Increase in temp = increase in K Due to increase in proportion of reactant molecules or ions with energy greater than the activation energy
What effect do other variables - Concentration, pressure, etc. - have on the value of K?
NO EFFECT
What effect does the addition of a catalyst have on the value of K?
May increase K - but often the entire rate equation changes due to a new pathway with lower activation energy
What are the units of K?
Mol(1-x)dm(3x-3)S-1 Where x is the overall order of reaction
What is the half life of a reaction?
The time taken for the concentration of a reactant to fall to half of its original value
What is the trend in Half Lives for a zero order reaction?
SUCCESSIVE HALF LIVES DECREASE
What are the units of K for a zero order reaction?
Moldm-3s-1 (same as rate)
What does a rate v concentration graph look like for a zero order reaction?
Horizontal Line
What is the trend in half lives for 1st order reactions?
SUCCESSIVE HALF LIVES ARE CONSTANT
What are the units of K for first order reactions?
S-1
What does a Rate v concentration graph look like for first order reactions?
Positive gradient linear
What is the trend of half lives for second order reactions?
SUCCESSIVE HALF LIVES INCREASE
What are the units of K for second order reactions?
Mol-1dm-3s-1
What does a rate v concentration graph look like for a second order reaction?
Quadratic with positive gradient past origin
What are Enthalpy Profile Diagrams?
Diagrams which show the difference between the enthalpy of reactants and products
What are standard conditions for enthalpy change calculations?
100 kPa PRESSURE, SPECIFIC TEMP - 25ºc (unless stated otherwise)
What is the standard enthalpy change of a reaction?
The enthalpy change when the number of moles of reactants shown in an equation react together under standard conditions
What is the standard enthalpy change of formation? (/\Hf)
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
For what group of compounds is /\Hf always negative?
ALL COMMON IONIC COMPOUNDS
What is bond enthalpy?
The enthalpy change required to BREAK ONE MOLE OF BONDS between TWO ATOMS in a GASEOUS STATE
What is the first ionisation energy of an element?
The energy needed to REMOVE ONE ELECTRON FROM EVERY ATOM IN EVERY ATOM IN ONE MOLE OF GASEOUS ATOMS in an element
What is the second ionisation energy of an element?
The energy required to remove ONE ELECTRON IN EVERY ION from ONE MOLE OF GASEOUS 1+ IONS OF AN ELEMENT
Why are all ionisations Endothermic? (+/\H)
Energy is needed to OVERCOME THE ATTRACTION BETWEEN OUTER ELECTRONS AND NUCLEUS
Why are second ionisation energies more Endothermic (more +ve) than first ionisation energies?
1) Same no of protons as 1st I.E., but 1 LESS ELECTRON
2) LESS REPULSION between remaining electrons
3) STRONGER ATTRACTION TO NUCLEUS
What is Hess’ Law?
The enthalpy change in a chemical reaction is INDEPENDENT OF THE ROUTE taken by the reaction, provided the initial and final conditions are the same
What is the standard enthalpy change of atomisation?
The enthalpy change when ONE MOLE of gaseous atoms of an element are formed, FROM THE ELEMENT IN ITS STANDARD STATE
How does the trend in /\Hatomisation change as you go along a period, and why?
INCREASES ACROSS A PERIOD No. of outer shell electrons -> MORE ELECTRONS CONTRIBUTING TO METALLIC BONDING
When is /\H atomisation not positive?
GROUP 8 ELEMENTS - exist as individual atoms
What is the first electron affinity of an element?
The enthalpy change when ONE ELECTRON is added to EACH ATOM in ONE MOLE OF GASEOUS ATOMS OF AN ELEMENT
What is the second electron affinity?
The enthalpy change when ONE ELECTRON is added to EACH ION IN A MOLE OF GASEOUS 1- IONS of an element
Why are all first electron affinities of elements that commonly form 1- ions negative?
The added electron is MORE STRONGLY ATTRACTED TO THE NUCLEUS than it is repelled by other electrons
Why are all second electron affinities positive?
ENERGY MUST BE PUT IN TO OVERCOME THE REPULSION between the negatively charged X- ion and e- that is added
What is lattice enthalpy?
the enthalpy change when ONE MOLE of the SOLID COMPOUND is FORMED form its GASEOUS IONS
What is lattice enthalpy a measure of?
The strength of ionic bonding in a compound
Are lattice enthalpies exo or endothermic?
Exothermic - when ions form ionic bonds, heat energy is released
What is the correlation between lattice enthalpy and strength of ionic bonds?
The more exothermic the lattice enthalpy, the STRONGER the ionic bonds
What is the main factor that affects lattice enthalpy?
CHARGE DENSITY
How does charge density affect the attraction of oppositely charged ions?
HIGHER CHARGE DENSITY = STRONGER ATTRACTION
How does the size of ions affect lattice enthalpy?
SMALLER IONIC RADIUS = GREATER CHARGE DENSITY -> STRONGER ATTRACTION between ions = MORE EXOTHERMIC lattice enthalpy
How does charge affect lattice enthalpy?
GREATER CHARGE = GREATER CHARGE DENSITY -> STRONGER ATTRACTION -> MORE EXOTHERMIC
What is the correlation between lattice enthalpies and melting points?
HIGHER L.E. = HIGHER MPT as both depend on how much energy is needed to disrupt ionic lattice
What is the Enthalpy Change of Solution of an ionic compound?
The enthalpy change when 1 MOLE of a COMPOUND DISSOLVES IN WATER
What are the 2 steps that form the overall enthalpy change of solution?
1) BREAKING OF LATTICE - ENDOTHERMIC - overcoming attraction between oppositely charged ions - = LATTICE ENTHALPY WITH REVERSED CHARGE (+ve)
2) HYDRATION - EXOTHERMIC - ion-dipole forces form - addition of individual ions hydration enthalpies
What is meant by Enthalpy Change of Hydration?
Enthalpy change when ONE MOLE of gaseous IONS DISSOLVE to form ONE MOLE OF HYDRATED IONS
What factor affects the Enthalpy Change of Hydration of an Ion?
CHARGE DENSITY GREATER CHARGE DENSITY = STRONGER ION - DIPOLE FORCES = MORE EXOTHERMIC
What is entropy?
A measure of how disordered the particles in a system are
What happens to a system as entropy increases?
A SYSTEM BECOMES MORE ENERGETICALLY STABLE AS IT BECOMES MORE DISORDERED (higher entropy = more feasible reaction
What are the units for entropy?
JK-1Mol-1
What happens to the favourability of a system as the value of /\S increases?
Higher /\s = MORE FAVOURABLE
Why is a solid a low entropy state?
Particles are locked together in in a regular LATTICE -> little freedom of movement = HIGHLY ORDERED = LOW ENTROPY
Why is a gas a high entropy state?
particles free to move = HIGHLY DISORDERED
When is the entropy change for the dissolution of a substance negative?
for some ions with very high charge density, as water molecules become more ordered
what is the formula for calculating /\s?
/\s = entropy of products - entropy of reactants (must times by no. of moles of substance in reaction)
What factors are favourable for a reaction to occur?
1) negative /\H
2) positive /\s
what is Free energy?
The balance between enthalpy and energy changes, which allow us to predict whether a reaction is feasible or not
What symbol is given to free energy change?
/\G
what is the formula for calculating /\G?
/\G = /\H - T/\s
how do you calculate T/\s?
translate /\s units to kjK-1mol-1
(usually given in JK-1mol-1 - /1000)
convert temp from *c to K (+273) =T
Tx/\s = T/\s
In order for a reaction to be energetically feasible, what value must /\G be?
/\G<0
Why may a reaction still not happen, given that /\G is negative?
high activation energy
What conditions are necessary for an endothermic reaction to be feasible?
ENTROPY INCREASES IN FORWARD REACTION - +/\S
What is the definition of pH?
-log10[H+]
How would you find the pH of a strong acid?
[acid] = [H+] pH = log10[H+]
What is Kw?
The IONIC PRODUCT OF WATER,
Kw = [H+][OH-] ( constant at given temp - value on data sheet)
What is the ratio of H+ to OH- in a neutral aqueous solution?
Equal concentrations
What is the ratio of H+ to OH- in an acid aqueous solution?
Higher conc of H+ than OH-
What is the ratio of H+ to OH- in an alkali aqueous solution?
Higher conc. OH- than H+
How would you calculate the pH of a strong alkali solution?
[H+] =Kw/[OH-]
What do you need to be able to work out the pH of a weak acidic solution?
DISSOCIATION CONSTANT (Ka)
What is Ka defined as?
Ka = [H+][A-]/ [HA]
What are the units of Ka?
Moldm-3
What is the relationship between the value of Ka and the strength of an acid?
LARGER Ka = higher [H+] = STRONGER ACID
What is pKa?
-Log10[Ka]
What is the correlation between the strength of an acid and the value of pKa?
STRONGER ACID = bigger Ka = SMALLER pKa
What assumption must be made to calculate the pH of a weak acid?
[H+] = [A-], will not apply if extra acid or salt is added
What is the formula for calculating the [H+] of a weak acid?
[H+] = √(Ka[HA])
What is meant by the Equivalence Point of a titration curve?
The point at which just enough acid has been added to neutralise the alkali solution the steep, almost vertical point of a pH curve
What shape will the acidic portion of a curve have if the acid is strong?
exponential curve up to equivalence, vertical part reaches to a quite low pH
What shape will the acidic portion of a curve have if the acid is weak?
Gradual, almost linear gradient of increase in pH up to equivalence point, reaches vertical point around pH 7
What shape will the alkali portion of a pH curve have if the alkali is strong?
logarithmic shape to the curve, vertical point finishes around pH 10
What shape will the alkali portion of a pH curve have if the alkali is weak?
linear curve, starts around pH 8 will level out at a lower pH, but will not fully reach the pH of the pure alkali added
How would you choose a suitable indicator for a titration curve?
The pH RANGE OF THE INDICATOR SHOULD BE WITHIN THE VERTICAL SECTION OF THE CURVE
What would be observed if an unsuitable indicator was chosen, and why?
GRADUAL COLOUR CHANGE will not reach/already past EQUIVALENCE POINT - CURVE IS NOT VERTICAL IN INDICATORS pH RANGE
What is the standard enthalpy change of neutralisation?
THE CHANGE IN ENTHALPY WHEN AN ACID AND BASE UNDERGO A NEUTRALISATION REACTION TO FORM *ONE MOLE OF WATER* UNDER STANDARD CONDITIONS *any ∆H neutralisation reaction equations must be scaled to form one mole of water
What equation is used to calculate enthalpy change of neutralisation?
mc∆T (Results in J - may need to convert to kJmol-1
Why cant mc∆T be used to calculate the enthalpy change of neutralisation of weak acids and bases?
The ionisation of the acid/base will also contribute something to the overall enthalpy change
What is a buffer solution?
A system that MINIMISES pH changes on addition of an acid or base
What is an acidic buffer solution formed out of?
A WEAK acid and its CONJUGATE BASE
What happens if acid is added to an acidic buffer solution?
BEFORE EXTRA ACID IS ADDED: XH X- + H+ WHEN ACID IS ADDED: If a small amount of another acid is added to the buffer solution, most of the H+ ions are removed by the reaction with the CONJUGATE BASE X- ( i.e. by the BACK REACTION OF THE DISSOCIATION EQUILIBRIUM) X- + H+ -> HX So [H+] doesnt change much = pH is only slightly changed
Describe the action of a buffer solution in terms of Le Chatelier’s Principle?
When acid is added to the Buffer, the increase in H+ is opposed by the equilibrium shifting to the left, and forming HX
Give the overall equation for the reaction that takes place when more alkali is added to an alkali buffer solution:
YOH + OH- YO- + H2O
How would you calculate the pH of a buffer based on a weak acid, HA and its salt, A-?
[H+] = Ka x ([HA]/[A])
Under what conditions is a buffer solution most effective?
1) if it contains substantial amounts of acid and conjugate base 2) when pH is close to pKa (if pH = pKa, concs are equal) 3) One pH unit either side of pKa
Give an equation for a basic buffer:
NH3 + H2O NH4+ + OH-
What is the equation for the equilibrium of the dissociation of CO2 in water (e.g. in blood)?
H2O + CO2 H2CO3 (carbonic acid)
What is the equation for the reaction that forms a buffer solution in blood?
H2CO3 H+ HCO3-
How is blood CO2 concentration maintained, and how is H2CO3 produced?
1) CO2 is produced in respiration and dissolves in blood, forming H2CO3, and is breathed out in the lungs at the same rate
How is a roughly constant HCO3- concentration maintained in the body?
excess ions are reabsorbed in the kidney
Why is maintenance of the bloods pH important?
Enzymes that control the bodys biochemistry only work over a narrow pH range
