Module 5.2 Flashcards
(29 cards)
Define lattice enthalpy
Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
Explain why you have to use a Born-Haber cycle to determine the lattice enthalpy of an ionic solid.
Lattice enthalpies cannot be measured directly because it is impossible to form one mole of an ionic lattice from gaseous ions experimentally.
Describe and explain the effect of ionic charge and ionic radius on the magnitude of lattice enthalpy.
As the ionic radius increases, the attraction between the ions decreases. This leads to the lattice enthalpy becoming less negative and the ionic bonds becoming weaker.
As ionic charge increases, attraction between ions increases. Therefore the lattice enthalpy becomes more negative and the ionic bonds become stronger.
Define enthalpy change of solution
The standard enthalpy change of solution is the enthalpy change that takes place when one mole of a solute is completely dissolved in a solvent/water under standard conditions.
Define enthalpy change of hydration
The standard enthalpy change of hydration is the enthalpy change that takes pace when dissolving one mole of gaseous ions in water to form 1 mole of aqueous ions.
Define entropy
the quantitative measure of the degree of disorder in a system
Define standard entropy
The standard entropy of a substance is the entropy content of one mole of the substance under standard conditions.
Define standard entropy change of reaction
Standard entropy change of reaction is 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 the free energy/Gibbs’ equation?
△G = △H - T△S
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.
State the limitations of making predictions about feasibility based on △G
It takes no account of the kinetics or rate of reaction.
State and explain the factors that affect the values of hydration enthalpies
The smaller the ionic size, the greater the charge density and attraction for water molecules, and the more exothermic the hydration enthalpy.
The larger the ionic charge, the greater the charge density and attraction for water molecules, and more exothermic the hydration enthalpy.
Define oxidising agent and reducing agent
An oxidising agent is the species that is reduced in a reaction and causes another species to be oxidised.
A reducing agent is the species that is oxidised in a reaction and causes another species to be reduced.
What is the standard electrode potential of a half cell?
The standard electrode potential of a half cell is the e.m.f. of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1moldm^-3 and a gas pressure of 100kPa.
State what a standard hydrogen half cell consists of
Hydrochloric acid, HCl, of concentration 1M, as the source of
H+ (aq);
Hydrogen gas, H2 (g), at 100kPa pressure;
An inert platinum electrode to allow electrons to pass into or out of the half cell via a connecting wire.
Suggest two reasons why a reaction may not take place even though they are feasible.
Conditions not standard/concentrations not 1M
High activation energy/slow rate
State two advantages of vehicles using fuel cells compared with the combustion of conventional fossil fuels.
Fuel cells produce less greenhouse gasses
Greater efficiency
Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for vehicles.
Methanol is a liquid and methanol is easier to store and transport.
Define ΔG
ΔG is the Gibb’s free energy change (measured in kJmol^-1) and predicts the feasibility of the reaction.
Explain how some endothermic reactions can become energetically feasible under certain conditions.
Endothermic reactions have a +∆H value.
For an endothermic reaction to be feasible ∆G must be negative. This means that if ∆S is positive and T is sufficiently large then T∆S will have a larger magnitude than ∆H and thus at high T an endothermic reaction will become feasible.
Explain why some reactions are never energetically feasible.
If ∆H is positive and ∆S is negative then no value of T will ever allow ∆G to become negative.
Chloride ions, Cl⁻, are oxidised to chlorine by acidified MnO₄⁻ ions. Suggest why sulfuric acid is used rather than hydrochloric acid in a manganate(VII) titration.
HCl contains chloride ions that would be oxidised instead.
Why is an excess of KI added when analysing oxidising agents in iodine-thiosulfate titrations?
All of the oxidising agent reacts with the excess KI producing iodine for the titration.
State the limitations of predictions for feasibility based on E⦵
values
The reaction may have a slow rate with high activation energy.
The concentration may not be 1M, changing the E⦵ value.
