5.2 Energy Flashcards
Define lattice enthalpy
The enthalpy change that accompanies the formation of one mole of an ionic lattice from its gaseous ions under standard conditions
Define standard enthalpy change of formation
The enthalpy change the accompanies the formation of 1 mole of a compound from its constituent elements in their standard states under standard conditions
Define first ionisation energy
The energy change that accompanies the removal of 1 mole of electrons from one mole of gaseous atoms
Define the standard enthalpy change of solution
the enthalpy change that takes place when one mole of a solute is completely dissolved in water under standard conditions
Define the standard enthalpy of hydration
the enthalpy change that takes place when dissolving one mole of gaseous ions in water
What is the standard electrode potential of a half cell?
the e.m.f. of a half cell compared with a standard hydrogen half cell, measured at 298 K with solution concentrations of 1 moldm-3 and gas pressure of 100kPa
What does a half cell consist of?
An element in two oxidation states
What is the simplest half cell?
metal placed in an aqueous solution of its ions
What is the simplest half cell containing non-metal and non-metal ions?
hydrogen half cell, comprised of H2 and H+ ions with a coated platinum electrode
What is the purpose of platinum in a hydrogen half cell?
to be in contact with both the H2 and the H+ ions a
to allow the transfer of electrons into and out of the half cell via a connecting wire
What is the platinum electrode in a hydrogen half cell coated in?
platinum black, a spongy coat which can transfer electrons
What is a standard hydrogen half cell composed of?
- HCl(aq) of conc. 1 mol dm-3, as the source of H+
- hydrogen gas, at 100kPa pressure
- inert platinum electrode
What acts as an electrode in metal half cells and why?
platinum as there is no other solid piece of metal that could act as an electrode
What is the e.m.f of a hydrogen half cell?
0V
What are salt bridges usually made from?
a piece of filter paper soaked in an aqueous solution of ionic substance, usually KNO3 or NH4NO3
What does a more negative Estandard value mean?
greater tendency for the half cell to undergo oxidation
What does a more positive Estandard value mean?
greater tendency for the half cell to undergo reduction
What does a positive Estandard mean?
half cell accept electrons from the hydrogen half cell
What does the voltmeter that joins the half cells measure?
potential difference of the cell - the difference between the electrode potentials of the cells
Equation for standard electrode potential
Estandardcell = Estandard (positive terminal)- E standard (negative terminal)
Limitations of standard conditions of electrode cells
- predictions can be made about the equilibrium position but not the reaction rate, which may be extremely slow due to a high activation energy
- may not be standard conditions
- standard electrode potentials apply to aqueous equilibria - may not be aqueous
What does a large difference between Estandard values indicate?
reaction is more likely to take place
What happens when the difference in Estandard values is less than 0.4?
the reaction is unlikely to take place
What three types can cells be divided into?
- non-rechargeable cells
- rechargeable cells
- fuel cells
Describe non-rechargeable cells
provide electricity until chemicals have reacted to such an extent that the voltage falls
Describe rechargeable cells?
the cell reaction can be reversed during charging
Common examples of rechargeable cells
- nickel and cadmium, used in rechargeable batteries
- lithium ion and lithium polymer batteries, used in laptops
Describe fuel cells
cell reaction uses external supplies of a fuel and an oxidant
-consumed and need to be continuously supplied
What does a fuel cell use?
energy of the reaction of a fuel with oxygen to create a voltage
What happens in an alkaline hydrogen fuel cell?
- H2 and O2 flow in and react water flows out
- OH- ions flow from O2 end of fuel cell towards H2 end
What do more exothermic lattice enthalpy values indicate?
- stronger ionic bonds
- higher melting and boiling points
What is the standard enthalpy change of atomisation?
when one mole of gaseous atoms is formed from its elements in its standard state
Is enthalpy change of atomisation endo or exothermic?
ALWAYS endothermic
-bonds have to be broken
Define second ionisation energy
one mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions
Is first electron affinity always endothermic or exothermic?
exothermic
-electron is attracted to the outer cell of an atom by its nucleus
Define atomisation
The enthalpy change when one mole of gaseous atoms is formed from one element in its standard state
Define first electron affinity
The enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
Define second electron affinity
The enthalpy change accompanying the additions of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
Do we expect lattice enthalpy to be exothermic or endothermic?
Exothermic as bonds are being formed
What can lattice enthalpy tell us about?
Strength of ionic bonds
-more energy released = harder to break up lattice
Is the first ionisation energy always exothermic or endothermic and why?
- endothermic
- we must overcome the attraction between the nucleus and the electron
Is second electron affinity always exothermic or endothermic?
- endothermic
- combining electron with a 1- ion and must put in energy to overcome repulsion
Why can we not measure lattice enthalpy directly?
It is not possible to form one mole of ionic solid from gaseous ions in practice
Two stages of a solid dissolving
- ionic lattice breaks down
- free ions become part of solution (hydration)
What is the enthalpy of breaking down an ionic lattice equal to?
-(lattice enthalpy)
Is breaking down a lattice always endothermic or exothermic?
endothermic
What is the magnitude of lattice enthalpy dependent on?
- size of ions involved
- charge density
- ionic bond strength
Do large ions or small ions form stronger bonds with each other?
Smaller ions because they can get closer together
What does greater charge density mean?
- attractive force is stronger so bonds are stronger
- lattices of smaller, more highly charged ions will have stronger bonds and give out the most energy when they form
Equation for charge density
charge/size
What is the magnitude of the enthalpy of hydration affected by?
- size of ions
- charge density
What is entropy?
the quantitative measure of the degree of disorder in a system
What is the standard entropy?
the entropy content of one mole of the substance under standard conditions
What is the equation of entropy change?
Change in entropy of a system = (sum of standard entropy of products) - (sum of standard entropy of reactants)
Equation for total entropy change of universe?
(entropy change of system) + (entropy change of surroundings)
What is the standard entropy change of reaction?
entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states
What is the free energy change?
The balance between enthalpy, entropy and temperature for a process:
free energy change = change in enthalpy - temperature (in K) x change in entropy
What does G stand for?
Free energy
What does H stand for?
Enthalpy
What does S stand for?
Entropy
Why is free energy useful?
can tell us if a reaction is feasible
What values of delta G show us a reaction is feasible?
Negative ones
Equation for the entropy of the surroundings?
-change in enthalpy/ temperature
What things play a role in predicting weather or not a reaction will happen?
- thermodynamics (free energy)
- kinetics
What are examples of reaction kinetics that are used to predict feasibility?
- activation energy (may be too high for a reaction to happen)
- rate of reaction (may be very slow and not be seen to happen)
Oxidation number of hydrogen
+1
Oxidation number of oxygen
-2
What is an oxidising agent?
the species that is reduced in a reaction and causes another species to be oxidised
What is a reducing agent?
the species that is oxidised and causes another species to be reduced
In a redox titration, what value is known?
Either a reducing agent or the oxidising agent
What do we use as indicators in redox titrations?
- normal acid base indicator
- self indicate when changing oxidation states
Is MnO4- an oxidising agent or a reducing agent?
oxidising
What is the colour change when manganate (VII) is reduced and what is the change in oxidation states?
Deep purple at 7+ to colourless 2+
What is the equation for the reduction of manganate (VII) ions?
8H+ + MnO4- +5e- = Mn2+ + 4H2O
What do we use as a source of manganate (VII) ions?
Potassium permanganate
KMnO4
In an iron (II) and potassium permanganate titration, what ion is being oxidised?
Fe2+
Equation for the titration reaction between iron (II) and potassium permanganate
8H+ + MnO4- + 5Fe2+ = Mn2+ +4H2O + 5Fe3+
Where is the end point of a potassium permanganate and iron (II) titration?
-goes purple as MnO4- goes to Mn2+
What are the uses of a iron (II) and potassium permanganate titration?
- working out the percentage composition of iron in a sample
- assuming all of the iron has formed 2+ ions in solution
Aside from potassium permanganate, what else can be used to oxidise Fe2+ ions?
Potassium dichromate (VI) Cr2O72- ions
What is the overall equation for the titration between Fe2+ and chromium (VI)?
14H+ + Cr2O72- + 6Fe2+ = 2Cr3+ + 7H2O + 6Fe3+
How do you determine the end point of an Fe2+ and chromium (VI) titration?
- NO CLEAR COLOUR CHANGE
- indicator needed
What is an indicator used to determine the end point of De2+ and chromium (VI) titrations and what is its colour change?
diphenylamine sulfonate
gives a violet-blue end-point
How is the end point in an iodine titration determined and why?
- iodine has a blue-black colour in presence of starch
- iodide is colourless in starch
What titration uses the reduction of iodine?
iodine and sodium thiosulfate
What does sodium thiosulfate do to iodine?
reduces it
What is the titration equation for the reaction between iodine and sodium thiosulfate?
2S2O3- + I2 = S4O62- + 2I-
What is the formula for a thiosulfate ion?
S2O3-
What is the formula for a tetrathionate ion?
S4O62-
Uses of an iodine - sodium thiosulfate titration?
- work out the concentration of oxidising agent by letting it oxide excess I-
- use titration to find the amount of I2 present after oxidation
- work backwards to find the conc of the oxidising agent
First thing to do when working with unknown titrations?
find the full reaction equation by using two half equations of oxidation and reduction
Advantages of fuel cells over fossil fuels
- only H2O formed (non polluting)
- greater efficiency
Disadvantages of fuel cells over fossil fuels
- H2 difficult to store
- H2 difficult to initially manufacture
- limited life cycle of H2 absorber
What mass do you use when doing netball change of solution?
Mass of solution, not mass of water
How concordat does a titre have to be?
0.1cm3
