Acids & Bases, Electrochemical Cells, Transition Metals, Aqueous ions Flashcards
How does the body ensure the concentration of the weak acid in blood doesn’t get too high?
- The carbonic acid is broken down, forming CO, which is exhaled via the lungs.
Why is a Burette preferred over a measuring cylinder when doing titrations
A burette is more precise
What is the difference between a Strong Base and a Weak Base?
The difference between a Strong Base and a Weak Base is their concentration of hydroxide ions (OH-) in water. Strong Bases have a high concentration of OH- ions and Weak Bases have a low concentration.
What is the difference between a Strong Base and a Weak Base?
The difference between a Strong Base and a Weak Base is their concentration of hydroxide ions (OH-) in water. Strong Bases have a high concentration of OH- ions and Weak Bases have a low concentration.
State why an indicator may not be required for a titration
the reactants are self-indicating
Would the pH of solution containing Mg(OH)2 have a differing pH to Ca(OH)2 and why?
Smaller/Lower pH because magnesium is less/sparingly soluble.
why is the pH probe washed with distilled water between each of the calibration measurements
to prevent contamination
Why is the volume added of a selected acid or base reduced between each pH measurement during and experiment?
To avoid missing the end point
What is a half-cell?
A half cell is one half of an electrochemical cell. They are constructed of a metal dipped in its ions, or a platinum electrode with two aqueous ions.
How do we make half cells with ions with no solid counterpart, or with two or more ions?
If there is a half cell with 2 aqueous ions we must use an inert but electrically conductive electrode e.g Platinum
What does an electrode potential show?
electrode potentials (V) show how easily the half cell gives up electrons (oxidation)
Define a Transition Metal
- A transition element is a d-block element that can form at least one stable ion with a partially filled d-subshell
When given a single metal rod and solution of ions of the same chemical species, why may it be difficult to measure the potential difference and thus the reducing ability of the metal rod?
- You cannot measure the potential of a single half cell, we can only measure the potential difference between two different half cells
Redox Couple
A redox couple is the combination of two forms of the same chemic species separated by the loss or gain of electrons so that they have two different oxidation states
- the position of equilibrium will vary for different redox couples
Redox Equilbirum
- A dynamic redox equilibrium gets established when the rate of electron loss equals the rate of electron gain
Electrode Potential
- At the equilibrium, the electrons on the metal strip set up a potential between the metal and the ions in solution
- the potential is an indication of how easily the metal loses electrons. The greater the tendency for the metal to lose electrons, the greater the magnitude of potential
- the potential of a single half cell cannot be measured but you can measure the potential difference of two half cells
Types of Half Cells
Metal/metal ion
metal rod dipped in a solution of one of its ions
Non-metal/non-metal ion
- a platinum or graphite (note the graphite gets weaker over time) is dipped into the non-metal ion solution
- the non-metal gaseous element can then be bubbled over the electrode
-the electrode has a dynamic equilibrium established on its surface (due to being in contact with the element and aqueous ions.)
Ion/Ion
- the half cell contains a solution of two different ions if the same element. A platinum or graphite rod is used as the electrode.
Electrochemical Cell Structure
- two half cells
- salt bridge, completed electrical circuit, typically filter paper soaked in a salt solution i.e. potassium nitrate, it allows ions to flow
- salt bridge mitigates the effects of an increasingly positive anode and increasingly negative cathode
- It does this by using it’s inert ions to move and balance the charges to keep the cell working
Why are Scadium and Zinc not transition metals?
- A transition metal is a metal which forms at least one stable ion with partially filled d-orbitals
- zinc only forms +2 ions (electrons from 4s) with no change of the d-orbitals due to high effective nuclear charge holding it together
- scandium only forms +3 ions (electrons from 4s) with 0 electrons in d-orbitals
- (releases 3 electrons easily due to its weaker nuclear charge)
Why must a voltmeter have very high resistance when measuring electrode potentials?
The voltmeter needs to have a very high resistance to prevent a current from flowing so that voltage can be measured
What causes a cell to stop working?
- The ions within the salt bridge have been exhausted
- The element being oxidised has completely thinned out/run out
What is an Electrode Potential?
- a measure of the voltage generated by a half-cell in an electrochemical cell
- it is a measure of the energy that is generated by the movement of ions between the half-cell and the solution it is immersed in.
Explain how a Potential Difference is produced between to half cells
- The “left-side” electrode where oxidation of a metal occurs will (due to the position of equilibrium being to the right) release electrons as the metal is oxidised to it’s ions
- the influx of ions in the solution will create a potential difference with the other electrode, allowing the electrons to cross between them to the other electrode
- the metal ions of the “right-side” electrode will be reduced, forming more of its metal constituent
- this continues until the lefthand electrode is highly positive and right hand electrode highly negative which stops the flow of these electrons
- to reverse this effect the charged ions of the salt bridge will cancel out these charges.
What does the Standard Electrode potential value of a half cell tell you about its redox ability?
- The more positive the value of the standard electrode potential, the greater the tendency for reduction, they are strong oxidising agents
-The more negative the value of the standard electrode potential, the greater the ability of the elements to be oxidised, they are strong reducing agents
e.g.
Factors Affecting Electrode Potentials
- changes in temperature, pressure or concentration will effectively change the position of the redox equilibrium and thus the electrode potential of the half-cell
- If the position of equilibrium is shifted in favour of the forwards reaction (the reduction),the electrode potential becomes more positive
- if instead the position of equilibrium is shifted in favour of the backwards reaction (the oxidation), the electrode potential becomes more negative
- therefore the cell that is most likely to be oxidised is the one with the most negative standard electrode potential.
Double vertical solid line - Salt Bridge
Vertical Solid Line - phase boundary, e.g between an aqueous solution and a solid
- Species with eg HIGHEST oxidation state is written closest to the salt bridge
- The half cell with the more negative potential goes to the left
- platinum electrode is used when no solid electrodes are in the half cell
Calculating Standard Electrode Potential
- electrons flow from the more negative electrode to the more positive electrode
- causing an EMF
EMF = E⦵ (reduced) – E⦵ (oxidised)
or
EMF = E⦵ (right electrode) – E⦵ (left electrode)
What directions to electrons flow in an electrochemical cell
- The electrons flow from the more reactive element (most negative electrode potential) to the less reactive element (metal with the most positive electrode potential)
How can Standard Electrode Potentials be used to predict Reaction Feasibility?
- A positive overall EMF indicates that a reaction is feasible
- however this prediction is only correct under standard conditions
- to determine whether a reaction is likely to occur reaction conditions must also be considered
- i.e a reaction may be feasible in terms of EMF but have a very high activation energy.
Advantages and Disadvantages of Non-Rechargeable Batteries
ADVANTAGES
Cheaper
Easy to use and has a wide range of Applications
Portable and easy to replace
DISADVANTAGES
once the reactants have been used up, the battery must be disposed of
toxic chemicals can leach from landfill sites into water sources
the casing of the battery can be corroded by the electrolyte, cussing leakages
Non-sustainable - the materials to make batteries are finite
ADVANTAGES AND DISADVANTAGES IF RECHARGEABLE BATTERIES
ADVANTAGES
- have a longer life-span than non-rechargeable batteries
- Can be recharged multiple times
- more energy efficient as chanting takes less energy than making new batteries
DISADVANTAGES
Cost more than non-rechargeable batteries
Need to be regularly recharged
Not all appliances are suitable to be used with rechargeable batteries
Advantages and Disadvantages of Hydrogen-Oxygen Fuel Cells
ADVANTAGES
only reaction product is water
all bond energy is converted in to electrical energy (more efficient than conventional engines)
no pollutants
can produce enough electricity to run vehicles which are much lighter and smaller than conventional engines and batteries
DISADVANTAGES
hard to store Hydrogen gas (needs to be under high pressure)
highly flammable/explosive
hydrogen is obtained via electrolysis (needs fossil fuels)
hydrogen is also obtained directly from fossil fuels
low energy density (for the same volume of other fuels hydrogen produced less)
Reaction occurring within Lithium Cells
- Rechargeable Batteries
How do Rechargeable Batteries work?
- when the battery is being used it operates like your standard non-rechargeable battery where a reactive electrode releases electrons which reduce ions at another electrode
- when being recharged, the reverse reactions occur and electrons flow in the opposite direction from the electricity supply to the e.g. lithium ions.
Fuel Cells
- a fuel cell produces electricity by using a fuel on the positive electrode and an oxidant on the negative electrode. They react in the presence of an electrolyte
- as long as there is a continuous supply of fuel, the cells can operate continuously
- most common in automotive industry
e.g Hydrogen-Oxygen fuel cell
pic
- Hydrogen enters at the negative electrode and releases electrons (oxidised by hydroxide ions to water)
- The electrons flow through the external circuit
(electrons flow to positive electrode) - The electrons are accepted and released hydroxide ions
(electrons are accepted by the oxygen entering at the positive electrode) - The hydroxide ions travel to the negative electrode
(hydroxide ions travel through the semi permeable membrane to negative electrode) - cycle repeats
What’s the Overall Equation for a Hydrogen-Oxygen Fuel Cell
What is an electrolyte
- an ionic compound that is melted or dissolved in water
Acid-Base Indicators
- acid-base indicators act as weak acids where the unionised acid and its conjugate base have different colours
- the end point of a titration is the point at which an indicator changes colour
- though due to indicators working of small ranges of pH, it may not be exactly at the equivalence point.
Factors Influencing Choice of Indicators
The colour change occurs at the equivalent point
- the colour change must occur at the equivalence point
The indicator has a narrow pH range
- the indicator chosen needs to cover a narrow pH range
The indicator shows a sudden distinct colour change
- only a few drops needed for a distinct colour change at neutralisation point
What are some Suitable Indicators for each Acid-Base Strength pairing?
(you do not need to know the indicators to detail)
Acid: STRONG Base: STRONG
pH range at equivalence point - 3-11
suitable indicators - ANY
Acid: WEAK Base: STRONG
pH range at equivalence point - 7-11
suitable indicators - phenol red(6-8), thymol blue(9-10), phenolphthalein (8-10)
Acid:STRONG Base:WEAK
pH range at equivalence point - 3-7
suitable indicators - methyl orange (3-4), methyl red (4-6)
Acid:WEAK Base:WEAK
- no clear equivalence point
- no suitable indicator
Why do transition elements have different oxidation states?
- oxidation state: maximum no. of electrons that can be lost without energy needed to remove electrons> energy recovered in bonding
- due to the large no. of valence electrons of d-block elements it would require a lot of energy to remove them all
- thus they can only give up some of their d-electrons (after removing their 4s ones)
What is a Complex Ion?
- A Complex Ion is a metal cation surrounded by ligands dative-covalently bonded to it
What is a Ligand?
- A Ligand is a species which can use its lone pair of electrons to form a dative covalent bond with a transition metal
What are the conditions for which Cations can form complex ions?
- they must have high charge density, and thus be able to attract electrons from ligands
- they must have empty orbitals of low energy, so that they can accept the lone pair of electrons from the ligands
What is the Coordination Number
- The number of lone pairs of electrons which a cation can accept is known as the coordination number of the metal cation
What are Mono/Bi/Multi-Dentate Ligands?
Monodentate - Can form only ONE dative covalent bond per ligand e.g H2O, Cl-
Bidentate - Can form only TWO dative covalent bonds per ligand e.g 1,2-diaminoethane, ethandionate
Multidentate - Can form multiple dative covalent bonds per ligand e.g EDTA^4-, Haem
Complex Ion Isomerism
- Square planar complexes show cis-trans isomerism only
- ligands can either be adjacent or opposite to one another if there are two different ligands attached
Octahedral
- Octhaedral complexes also show cis-trans isomerism but also Optical Isomerism
octahedral complexes show cis-trans isomerism when:
- they contain for ligands (or two bidentate ligands) of one type and two ligands of another type
octahedral complexes show optical isomerism when:
- there are three bidentate ligands
- two bidentate ligands and two monodentate ligands
- one hexadentate ligands
Structures of Transition Metal Complexes of differing Coordination numbers
- 6-coordinate complexes are all octahedral
- most complexed involving multidentate ligands are octahedral
- 4-coordinate complexes are generally tetrahedral, and are formed with larger ligands such as Cl-
- Larger Ligands cannot fit around the transition metal so easily so form smaller complexes
- some 4-coordinate complexes in the case of Pt and Ni form square planar structures
- 2-coordinate complexes are generally linear (and typically form with Ag^+ ions)
Who does size and mass of a ligand affect the shape of a complex ion)
- Larger ligands cannot fit around the transition metal so easily and hence form smaller complexes
- this will typically be tetrahedral, linear or square planar (if Pt or Ni)
Why are transition metal ions coloured?
- Transition metal ions are coloured because d-electrons can absorb light and get excited into higher energy d-orbitals
- the resultant light is thus missing certain frequencies and is hence coloured
- the d-orbitals must be partially filled. If the d-orbitals are empty then there are no electrons which can be excited and if it is full then there are no empty orbitals which electrons can be excited to (making them colourless).
What are the colours of complex ions dependent on?
- The ligand
- The coordination number
- the oxidation state of the metal
- the identity of the metal*
Substitution of Ligands (Each Scenario)
- Substitution by similar size ligands
-> no change in coordination number - Substitution by larger/smaller ligands
-> may cause change in coordination number
e.g H2O (octahedral) vs Cl- (tetrahedral) - Substitution by bidentate/multidentate ligands
- multidentate ligands will almost always replace monodenate ligands at a metal centre - CHELATE EFFECT
- because same number of co-ordinate bonds broken and made therefore ΔH is negligible
- Large increase in entropy (greater no. of atoms as products)
What is the Chelate Effect?
- There are more species in the products than the reactants when multidentate ligands substitute monodentate ligands
- The entropy of the system thus increases, and multidentate complexes are therefore more stable then complexes involving misidentified ligands
Factors affecting the oxidation state of transition metals
- The pH
- oxidation is favoured by alkaline conditions and reduction is favoured by acidic conditions
- The choice of Ligand
Colours of Vanadium at different oxidation states
What is Ligand Exchange/Substitution?
- Ligand exchange is when one ligand in a complex is replaced by another
- Ligand exchange forms a new complex that is more stable than the original one
- The ligands in the original complex can be partially or entirely substituted by others
- all 5 of the d-orbitals due to being incomplete can exist at a different energy levels
- hence why electrons can be excited and excited between them
- if the gaps between them correspond to visible light, they will absorb that wavelength of light, which is the colour we see of the complex
Haemoglobin and why CO is a poisonous gas
- Haem consists of a Fe2+ ion and a tetradentate ligand
- the complex is found with a protein - globin (which forms a 5th coordinate bond)
- and oxygen (6th coordinate bond)
- it is responsible for carrying oxygen in the blood throughout the human body
- Carbon monoxide is a similar size and shape to oxygen AND forms a much stronger bond with iron
- thus it displaces oxygen from the complex
- this reduces the bloods ability to carry oxygen around the body
- making it poisonous
What are the Characteristics of Transition metal ions?
- form complex ions
- form coloured ions
- have variable oxidation states
- have catalytic activity/act as catalysts
What can a Colorimeter tell us about complex ions?
- A simple Colorimeter can be used to determine the concentration of coloured ions
- because complex ions form coloured ions due to the absorption of light waves causing electron excitation
- the greater the absorption of light of a substance containing complex ions, the greater the concentration of those ions
Homogenous Catalysts
- In the same ‘Phase’ as reactants
e.g 2I- + S2O82- —> I2 + 2SO42-
S2O82- + 2Fe2+ —> 2SO42- + 2Fe3+
2Fe3+ + 2I- —> I2 + 2Fe2+
both negatively charged so require a lot of energy to overcome that electrostatic repulsion
by adding an Fe2+ catalyst this provides an alternative energy pathway for the reaction to occur
Contact Process
- reactants diffuse onto surface of catalyst
- reactants are ‘absorbed’ onto catalyst
- reaction starts and intermediate gets formed
- reaction completes and product gets formed
