5.2 Energy (completed) Flashcards
5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials
Define lattice enthalpy
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
What kind of energy change will lattice enthalpy always be?
Exothermic
Enthalpy change is negative
What is the Born-Haber cycle?
A way of indirectly calculating lattice enthalpy from other known energy changes.
What is the formation of gaseous atoms and what energy change is it?
Changing elements in their standard states into gaseous atoms.
Endothermic as it involves bond breaking.
npomkcd
Exothermic.
Define standard enthalpy change of formation
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of atomisation
The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
Define first ionisation energy
The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Why is ionisation energy always endothermic?
Energy is required to overcome the attraction between a negative electron and a positive nucleus.
Define first electron affinity
The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
Why is first electron affinity always exothermic?
Electron being added is attracted in towards the nucleus.
Why are successive electron affinities endothermic?
The negative ion repels the additional electron, so energy is required to force the electron onto it.
Define standard enthalpy change of solution
The enthalpy change that takes place when one mole of a solute dissolves in a solvent.
What happens for a solid ionic compound to dissolve in water?
Ionic lattice breaks up
Water molecules are attracted to and surround the ions.
Define enthalpy change of hydration
The enthalpy chnage that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
Describe the effect of ionic size on lattice enthalpy and melting point
Ionic radius increases
Attraction between ions decreases
Lattice enthalpy becomes smaller (less negative)
Melting point decreases
Describe the effect of ionic charge on lattice enthalpy and melting point
Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes greater (more negative)
Melting point increases
Describe the effect of ionic size on hydration enthalpy
Ionic radius increases
Charge density decreases
Attraction between ions and water molecules decreases
Hydration enthalpy becomes smaller (less negative)
Describe the effect of ionic charge on hydration enthalpy
Ionic charge increases
Attraction between ions and water molecules increases
Hydration enthalpy becomes greater (more negative)
Why is hydration enthalpy an inaccurate method of predicting solubility?
Many compounds with endothermic enthalpy changes of solution are soluble
Define entropy
A measure of the dispersal of energy and order within in a system
What are the units of entropy?
J/K/mol
How does entropy relate to physical states?
In order of size of entropy value:
Gases > liquids > solids
Define standard entropy
The entropy of one mole of a substance, under standard conditions (always positive)
How is an entropy change calculated for a reaction?
Products - Reactants
What is feasibility?
Whether a reaction is able to happen and is energetically feasible (or spontaneous)
What is the equation for free energy change?
ΔG = ΔH -TΔS
What is the requirement for the free energy change such that a reaction is feasible?
ΔG < 0
A decrease in free energy
Even though a reaction may have a negative value for ΔG, why might the reaction not take place at room temperature?
High activation energy resulting in a very slow rate
Why does one ion have a higher lattice enthalpy than another?
(3 Marks)
- Greater charge/charge density
- As it is a smaller ion
- Therefore a greater force of attraction and more energy required
Why does one ion have a higher enthalpy of hydration than another?
(3 Marks)
- Greater charge/charge density
- As it is a smaller ion
- Therefore a greater force of attraction to water
How do you work out the enthalpy of solution?
Born-Haber Cycle:
Solution = Enthalpy of Hydration(s) - Lattice Enthalpy
How do you write a redox equation in acidic conditions?
Write the overall unbalanced equation Write two half equations Balance the total electrons Combine Balance oxygen and hydrogen this with H+ and H2O
How do you write redox equations in alkaline conditions?
Balance and acidic conditions
Neutralise any H+ by adding OH- ions to both sides to produce water
Final balance check
What is the main difference between redox titrations and acid base titrations?
Redox titrations are self indicating and no indicator is needed
What occurs in MnO4-/Mn2+ titrations with Fe2+?
Aqueous manganate is dark purple but aqueous manganese is colourless manganate loses its colour as it enters the iron solution and the endpoint is when there is a permanent pink tinge as all the Fe 2+ has been used up and will not decolourise the manganate
What occurs in the I2/S2O3 2- titrations?
Iodide is a reducing agent and is colourless however aqueous iodine has a brown colour in atria solutions and deep blue black with a starch indicator
By titrating it with Fitbit it can be used to assess the concentration of iodine
What should you look for in titration equations?
Having to scale up for a different volume
What does a salt bridge allowed for?
Ions to be transferred between half cells
What are the conditions of a standard electrode?
298K
1atm
1.00 mol dm-3 ion solutions
How do you measure the standard electrode potential of a half cell?
You can’t
It is only possible to measure the difference of E between to have cells
All half cells are measured against the standard hydrogen electrode
SHE is given E of exactly 0
What does the standard hydrogen electrode look like?
Platinum electrode
H+ ion solution
Supply of hydrogen at 1 atm
Glass dome for hydrogen to bubble out
What does the E values that are most positive do the best?
Gain electrons
Reduce
Oxidising agents
What does the E values that are most negative do the best?
Lose electrons
Oxidise
Reducing agents
What must you remember when drawing a half celf?
Salt bridge
Platinum or other metal rod
Volt meter
Conditions
How do you calculate the E value of the overall cell?
E positive terminal - E negative terminal
How can use E values to decide whether something can reduce another?
Add the 2E values
If the reaction is positive it will be spontaneous
Remember to look to reverse E values if the equation is reversed
What is the limitations of using E values to predict if one substance can reduce another?
Predictions are only valid under standard conditions
High activation energy may mean reaction will take place very slowly
Standard electrode potential is only applied to aqueous equilibria
If the difference between E value is less than 0.4 V the reaction is unlikely to take place
What are the two types of commercial cells?
Rechargeable and non-rechargeable
What is a car battery consist of?
Several individual cells that are rechargeable
What are two advantages of fuel cells?
Carbon dioxide contribute to climate change and greenhouse affect
Fuel prices are rising and resources dwindling
What are two disadvantages of fuel cells?
Safety of storage of hydrogen
Limited life-cycle of a cell – economic considerations
What charge carriers are transferred through
a) the wire
b) the solution
a) electrons
b) ions
What is a fuel cell?
An electrochemical cell that converts energy from the reaction of a fuel with oxygen into a voltage?
What is the overall reaction for a hydrogen fuel cell?
2H2 + O2 → 2H2O
