F325 - Energy Flashcards
What does Hess’s law state?
That if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
What is lattice enthalpy?
The enthalpy change that accompanies the formation of one mole of an ionic compound from it’s gaseous ions under standard conditions.
What is the standard enthalpy change of formation?
The enthalpy change that takes place when one mole of a compound is formed from to constituent elements in their standard states under standard conditions.
What is the enthalpy change of atomisation?
The enthalpy change that takes place when one mole of gaseous atoms forms from the elements in it’s standard state.
What is the first ionisation energy?
The enthalpy change accompanying the removal of one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
What is the second ionisation energy?
The enthalpy change accompanying be removal of one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.
What is the 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.
What is the second electron affinity?
The enthalpy change accompanying the addition of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions.
Why is the second electron affinity endothermic?
Have to overcome the repulsion between the negative ion and the electron, more energy is needed.
Born Haber stuff
Page 168-172
What is the standard enthalpy change of solution?
The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions.
What two stages are involved when a solid dissolves?
Breakdown of the ionic lattice -
Exact opposite of producing the lattice enthalpy.
-Delta H LE
Hydration -
Delta H hyd
Standard enthalpy change if hydration.
Positively charged ions attracted to negative oxygen atoms in water, and vice versa.
What is the standard enthalpy change of hydration?
The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions.
What governs the size of lattice enthalpy and the size of hydration enthalpy?
Ionic size
Ionic charge
How does size affect lattice enthalpy?
As ionic radius increases:
The attraction between the ions decrease.
The lattice enthalpy becomes less negative/less exothermic.
How does ionic charge affect lattice enthalpy and hydration enthalpy?
Lattice enthalpy becomes more exothermic/more negative as the ionic charge increases.
Higher charged ions attract more strongly.
More energy to overcome.
What is the result of a highly negative lattice enthalpy and hydration enthalpy?
Strong attraction between it’s ions, it will have a high melting point.
What is entropy?
S
The Quantative measure of the degree of disorder in a system.
What values can entropy be?
Always a positive number because all systems possess some degree of disorder because particles are always in constant motion.
How does entropy increase?
With increasing temperature.
Molecules move faster.
Solid —> liquid —> gas
If a reaction produces more gaseous molecules entropy increases.
What is the standard entropy change of reaction?
The 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 free energy change?
Delta G
The balance between enthalpy, entropy and temperature for a process.
Delta G = delta H - T delta S.
A process can take place simultaneously when delta G < 0
What makes a process spontaneous?
If a chemical system becomes more stable and it’s overall energy decreases.
The overall energy decrease is due to both entropy and enthalpy.
What is T delta S?
Energy derived from entropy.
T is measured in kelvin.
As temperature increases, the energy derived from entropy becomes more significant.
What factors affect whether or not a process is spontaneous?
Temperature, T, in kelvin.
Entropy change, Delta S.
Enthalpy change. Delta H, with the surroundings.
Free energy change
G = H - T/\S
When is a reaction spontaneous?
When /\G is negative, as there must be a decrease in the overall energy served from both the enthalpy change and the entropy of a system.
How do you calculate /\S?
Sum of S (products) - sum of S (reactants)
What makes an endothermic reaction spontaneous?
/\S must be positive.
The temperature must be high enough so that T/\S > /\H
How do you convert *C into K?
Add 273
What is electrode potential?
Tendency to lose it gain electrons in the equilibrium.
What is the job of the wire connection and salt bridge in a half cell?
The wire connects the two metals, allowing electrons to be transferred between the two half cells.
Usually also has a voltmeter with a high resistance to minimise the current that flows.
The salt bridge connects the two solutions, allowing ions to be transferred between the Half cells. Usually KNO3 (aq) or NH4NO3 (aq) is used
How do you make a salt bridge?
Using a strip of filter soaked in an aqueous solution of an ionic compound that does not react wig either of the Half cell solutions.
Describe a metal/metal ion half cell.
Eg Zn and Cu.
Zn releases electrons more readily. This is the negative electrode.
It releases electrons into the wire, which flow to the Cu electrode.
Zn loses electrons and it’s equation moves to the left.
What does the reading on a voltmeter show?
The potential difference of the cell, which is the different between the electrode potentials of the half cells.
Describe a non metal/non metal ion half cell.
Eg hydrogen:
2H+ (aq) + 2e- —> H2 (g)
Used as a reference standard for the measurement of cell electromotive force.
Non metal and it’s aqueous ions.
There is no electrode to connect the wire to, so a platinum electrode is placed in solution in contact with both H2 (g) and H+ (aq).
The platinum is inert so does not react. It only transfers electrons in and out of the half cell via a connecting wire.
Coated in platinum black.
1 mol/dm3 HCl for H+ ions.
H2 (g) at 100kPa/1 atmosphere.
Describe a metal ion/metal ion half cell.
Ions of the same element in different oxidation states.
Eg Fe2+ + e- —> Fe3+
A solution contains both Fe2+ and Fe3+ of equal concentrations - equimolar.
An inert platinum electrode allows electrons to pass into or out if the half cell via a connecting wire.
What is the use of a platinum electrode?
To allow electrons to pass into or out of the Half cell via a connecting wire.
Inert so does not react.
What is the standard electrode potential?
The electromotive force of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1 mol/dm3 and a gas pressure of 100kPa/1 atmosphere.
What Is e.m.f?
Electromotive force.
The voltage produced by a cell when no current flows.
What is the standard electrode potential of a hydrogen half cell?
0V
What I the cell reaction?
The sum of the reduction and oxidation half reactions taking place in each half cell.
How do you calculate the standard cell potential?
Te difference between the standard electrode potentials of the half cells.
Positive - negative.
Why may a feasible reaction not take place?
The reaction rate is very slow due to a high activation energy.
The conditions are not the standard conditions.
Conditions may vary, pH temp etc.
Standard electrode potentials apply to aqueous equilibria only.
What are the three types of cells/batteries?
Non rechargeable cells.
Rechargeable cells.
Fuel cells
What are the advantages of fuel cell vehicles?
Less pollution & CO2:
Combustion of hydrocarbon fuels creates CO2, incomplete combustion creates CO.
Hydrogen rich fuels only produce a small amount Of CO2.
Greater efficiency:
In petrol energy 80% of the chemical energy is wasted as heat.
Hydrogen fuel cell vehicles are 40-60% efficient in converting the fuels energy.
This drops fuel consumption.
What methods of fuel cell vehicles have been developed?
Hydrogen rich fuel cells:
CH3OH + H2O —> 3H2 + CO2
The hydrogen is then fed into the fuel cells.
Fuel cells using methanol have also been developed.
Liquid is easier to store than hydrogen gas.
Methanol can be generated from biomass.
