chap 9- developing metals

Question 1

what has to be the same in half equations before you can write the overall equation

Answer 1

number of electrons lost / gained must balance

Question 2

what 3 things can you add to balance a half equation

Answer 2

e-, H2O and H+

Question 3

what do redox titrations show

Answer 3

how much oxidising agent is needed to exactly react with a quantity of reducing agent but you need to know the conc of either the oxidising or reducing agent

Question 4

outline the process of a redox titration where manganate (VII) ions are the oxidising agent

Answer 4

1. measure out quantity of reducing agent eg Fe2+ using pipette and put in conical flask
2. add some dilute sulfuric acid to flask as an excess
3. add aqueous MnO4- (oxidising agent) using a burette and swirl conical flask
4. stop when mixture in flask becomes tainted with colour of MnO4- and record vol of oxidising agent added
5. repeat until you have concordant results

Question 5

what is the colour change that occurs when manganate ions are added to the reducing agent

Answer 5

manganate ions are purple then when they start reacting with the reducing agent they’re reduced to colourless Mn2+. this continues until all the reducing agent has reacted at which point the colour will turn pink (colour of oxidisng agent)

Question 6

why are electrochemical cells a redox process

Answer 6

there are always 2 reactions (one oxidation and one reduction)

Question 7

what is an electrochemical cell

Answer 7

2 different metals dipped in salt solutions of their own ions and connected by a wire. oxidation happens at anode and reduction at cathode

Question 8

describe the flow of electrons in an electrochemical cell

Answer 8

electrons flow through the wire from the most reactive metal to the least

Question 9

what is the cell potential or Ecell

Answer 9

voltage between the 2 half cells. measured by a voltmeter in the external circuit and also measures the direction of flow of electrons

Question 10

method of setting up an electrochemical cell involving 2 metals

Answer 10

1. clean surface of each strip of metal with emery paper
2. clean electrodes using propanone
3. place each electrode in a beaker filled with a solution containing ions of that metal (sometimes you have to add acid to a half cell too)
4. create a salt bridge by soaking piece of filter paper in salt solution and drape between 2 beakers so it’s immersed in both
5. connect electrodes to voltmeter using wires and crocodile clips (if set up correctly, you’ll get a reading on voltmeter)

Question 11

what is the rule for writing half equations

Answer 11

the reduction reaction is always written in the forward direction with the electrons being added on the left hand side (even though the reaction is reversible)

Question 12

how would you set up a half cell between 2 ions of the same element in different oxidation states

Answer 12

conversion happens on the surface of the electrodes and as neither reactants or products are solid something that conducts electricity but is inert needs to be used for the electrodes eg platinum (expensive) or graphite

Question 13

what solutions can be used for a salt bridge

Answer 13

KCl or KNO3

Question 14

what is the role of the salt bridge

Answer 14

balances the charges between the 2 half cells and allows ions to flow through the cell completing the circuit

Question 15

what is the standard electrode potential

Answer 15

the voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode

Question 16

what are standard conditions for a standard electrode potential

Answer 16

1. concentration of 1 mol dm -3
2. 298K (25c)
3. 100 kPa

Question 17

what is the standard hydrogen electrode

Answer 17

half cell with a value of 0V

always shown on the left (doesn’t matter whether or not the other half cell is where oxidation happens)

Question 18

how do you work out Ecell from standard electrode potentials

Answer 18

Ecell = (Epositive - Enegative)

Question 19

what do mor reactive metals have

Answer 19

large negative electrode potentials as the more reactive the metal, the more it wants to lose electrons and form a + ion

Question 20

what do more reactive non metals have

Answer 20

more positive standard electrode potentials as it wants to gain electrons to form a negative ion

Question 21

which direction do different half equations go

Answer 21

the half equation with the more negative electrode potential moves to the left and the half equation with the more positive electrode potential moves to the right

Question 22

how can you tell if a reaction is feasible from the half equations

Answer 22

when the redox created from the half equations matches the reaction described in the question and the feasible directionn

Question 23

when can feasibility predictions be wrong

Answer 23

1. if the rate of the reaction is so slow that the reaction appears not to happen
2. if the reaction has a high activation enthalpy, this could stop it happening

Question 24

what are the 2 half equations involved in rust formation

Answer 24

Fe2+ + 2e- = Fe and

2H2O + O2 + 4e- = 4OH-

Question 25

why is rust less likely to form in alkaline conditions

Answer 25

adding OH- ions to the reaction causes the equilibrium to shift in the iron half equation to the right so theres less Fe2+ and less rust

Question 26

what are the 2 main ways to prevent rust

Answer 26

1. painting / coating with a polymer

2. oiling / greasing- used when moving parts are involved eg a bike chain

Question 27

what is the sacrificial method of preventing rust

Answer 27

placing a more reactive method with the iron so the water and oxygen react with this instead eg zinc is often used

Question 28

define what a transition metal is

Answer 28

a d block element that can form at least one stable ion with an incomplete d sub shell

Question 29

why are scandium and zinc not transition metals

Answer 29

1) scandium only forms one ion (Sc3+) which has an empty d sub shell
2) zinc only forms one ion (Zn2+) which has a full d sub shell

Question 30

what are the chemical properties of transition metals

Answer 30

1) can exist in variable oxidation states eg iron can exist as 2+ and 3+
2) they can form coloured ions in solution eg Cu2+ is blue, Fe2+ is pale green and Fe3+ is yellow

Question 31

why can transition metals have variable oxidation states

Answer 31

because the energy levels of the 4s and 3d sub shells are very close to one another so different numbers of electrons can be lost / gained using similar amounts of energy

Question 32

what is a complex ion

Answer 32

a central metal atom or ion surrounded by coordinately (datively) bonded ligands

Question 33

what is a coordinate bond

Answer 33

a dative bond- covalent bond where both electrons in the shared pair come from the same atom. in a complex they both come from the ligands

Question 34

what is a ligand

Answer 34

an atom, ion or molecule that donates a pair of electrons to a central transition metal ion to from a coordinate (dative covalent) bond (must have a lone pair of electrons)

Question 35

define monodentate ligand

Answer 35

ligands with one lone pair

Question 36

define bidentate ligand

Answer 36

ligands with 2 lone pairs so 2 coordinate bonds can be formed with a metal ion

Question 37

define polydentate ligand

Answer 37

ligands with more than 2 lone pairs eg EDTA 4- has 6 lone pairs so its hexadentate and can form 6 coordinate bonds with a metal ion

Question 38

what is the shape of a complex ion with 6 coordinate bonds

Answer 38

octahedral shape with all bond angles 40degrees

Question 39

what is the shape of a complex ion with 4 coordinate bonds

Answer 39

tetrahedral shape with all bond angles 109.5 degrees but .. sometimes forms a square planar shape eg cisplatin where all bond angles are 90

Question 40

what happens when ligands bond to transition metal ions

Answer 40

normally all the 3d orbitals of transition metal ions have the same energy but when ligands come and bond to the ions some of the orbitals gain energy so the 3d orbitals then split into 2 different energy levels

Question 41

what things affect the size of the energy gap that electrons have to jump

Answer 41

1) the central metal ion
2) oxidation state
3) ligands
4) coordination number

Question 42

what causes transition metals to be coloured

Answer 42

some frequencies of visible light are absorbed when electrons jump to higher orbitals. the freq absorbed depends on the energy gap. the rest of the frequencies are transmitted or reflected and these combine to make the complement of the colour of the absorbed frequencies (the colour you see)

Question 43

when does a compound look white or colourless

Answer 43

when there are no 3d electrons or the 3d subshell is full, then no electrons will jump so no energy will be absorbed and compound appears white or colourless

Question 44

what is ligand substitution

Answer 44

when one ligand is swapped for another ligand- usually results in a colour change (if ligands are of a similar size then the coordination number and shape doesn’t change)

Question 45

why do transition metals make good catalysts

Answer 45

because they can change oxidation states by gaining or losing electrons within their d orbitals so they can transfer electrons to speed up reactions

Question 46

what are the risks of using transition metal catalysts

Answer 46

many of the metals and their compounds are toxic eg long term exposure to copper can damage liver and kidneys and exposure to manganese can cause psychiatric problems

Question 47

why do transition metals make good HETEROGENEOUS catalysts

Answer 47

because they can use their s and d orbitals for bonding to the reactant molecules (strongly enough so they are held to the surface long enough to react but not attract too strongly so products don’t desorb)

Question 48

why do transition metals make good HOMOGENOUS catalysts (peroxodisulfate + iodide example)

Answer 48

this reaction is really slow as both ions are negatively charged so they repel each other and it’s unlikely they’ll collide/ react. Adding Fe2+ causes them to be oxidised to Fe3+ by the S2O8 2- and the Fe3+ can then oxidise the I- to iodide

Question 49

how is the colour of a transition metal determined

Answer 49

when white light hits a transition metal, one freq of light is absorbed and the rest of the frequencies are transmitted (the transmitted frequencies correspond to the colour you see) THE COLOUR OF THE TRANSITION METAL SOLUTION IS THE COMPLEMENT OF THE COLOUR ABSORBED)

Question 50

how does a colorimeter work

Answer 50

1) filters a source of white light into monochromatic light (narrower freq range, one colour)
2) pick a filter that produces a colour thats absorbed by the transition metal you want to look at
3) set colorimeter to 0 by measuring absorbance of a blank sample
4) samples should be placed in a clean cuvette
5) light passes through filter and sample (where some of it will be absorbed) remaining light travels to a detector which compares absorbance of sample to the blank
6) high absorbance means a lot of light has been absorbed and sample is very concentrated

Question 51

how can visible absorption spectroscopy be used to find concentrations of ions

Answer 51

1) beam of monochromatic light is passed through a solution of the complex. detector measures intensity of light before and after its passed through the solution (calc absorbance from this)
2) different freq of light are passed through solution to produce visible absorption spectrum (peaks in graph tell you which colours the complex absorbs most strongly)
3) calibration curve can be used to convert absorbance data into concentration

Question 52

why does Co (cobalt) have 3 unpaired electrons

Answer 52

because the 4S subshell fills up before the 3D does so Co has 27 electrons and the electron configuration ends with 3d7 4s2. also the 3d orbitals are occupied singly at first and only double up when they have to

Question 53

what are the chromium and copper exceptions

Answer 53

chromium- 4s orbital does start to fill up before 3d is full but electron configuration is 3d5 4s1 because it is more stable to have one electron in each orbital of the 3d sub shell
copper- electron configuration of 3d10 4s1 (more stable to have a full 3d sub shell and just one in 4s) when it forms Cu2+ it becomes 3d9

Question 54

give the half equation that shows how hydroxide ions are formed in rusting

Answer 54

2H2O + O2 + 4E- = 4OH-

Question 55

what is the oxidation state of iron in red-brown rust

Answer 55

+3

Question 56

what do you get if you mix 2 complementary colours of light

Answer 56

white light

Question 57

why do transition metals form coloured complexes but other elements don’t

Answer 57

when ligands bond with transition metal ions there is repulsion between the electrons in the ligands and the electrons in the d orbitals. the d orbital electrons are split into 2 groups and the energy difference between these 2 groups is directly proportional to the freq of visible light that this complex can absorb and the colour we see is the complementary colour

Question 58

what does the size of the energy gap between the 2 groups of d block electrons depend on

Answer 58

1) the nature of the transition metal ion
2) its oxidation state (as oxidation state increases so does the amount of splitting of the d orbitals)
3) the nature of the ligands (ligands with strong electrical fields cause a large energy gap when the d orbitals split)
4) coordination of the ion (splitting is greater in octahedral than tetrahedral)

Question 59

why can a titration with manganate be done without indicator

Answer 59

because MnO4^2- is pink coloured so induces a colour change of colourless to pink solution to mark the end point