5.2 Energy
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Define lattice enthalpy
formation of 1 mol of ionic lattice from gaseous ions, ∆LEH, under standard conditions
What is the use of lattice enthalpy?
Measure of the strength of ionic bonding in a giant ionic lattice
Define enthalpy change of solution
enthalpy change of solution is dissolving of 1 mol of solute in water, ∆solH
Define enthalpy change of hydration
dissolving of 1 mol of gaseous ions in water, ∆hydH
Give the equation for lattice enthalpy change of NaCl
Na+(g) + Cl-(g)—> NaCl(s)
Give the equation for the enthalpy change of hydration of Na+(g) + Cl-(g)
Na+(g) + Cl-(g) —> Na+(aq)+ Cl-(an)
Give the equation for the enthalpy change of solution of NaCl(s)
NaCl(s) —> Na+(aq) + Cl-(aq)
What happens in terms of bonding when an ionic lattice dissolves in water?
Bond between ions break to give gaseous ions (endothermic) Bonds between gaseous ions and water are made (exothermic)
Describe two things that affect lattice enthalpy
Ionic Charge-higher charge, the more energy released when an ionic lattice forms. This is due to stronger electrostatic attraction Size-smaller the ionic radii of the ions involved, the more exothermic the lattice enthalpy. Smaller ions have higher charge density and their smaller ionic radii mean that the ions can sit closer together in the lattice. Meaning attraction between ions are greater.
Give the first electron affinity of chlorine in Na+(g) + Cl(g) + e-
Na+(g) + Cl(g) + e- —>Na+ + Cl-
Define first electron affinity
energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions.
Give the equation for the atomisation of chlorine in Na(s) + 1/2Cl2(g)
Na(s) + 1/2Cl2(g) —> Na(s) + Cl(g)
What is entropy?
Measure of the dispersal of energy in a system which is greater, the more disordered the system
Explain the difference in entropy of a system of solids liquids and gas
Solids: lowest entropy Gas: have the most random arrangements so highest entropy
What else other then state results in more entropy?
More particles, more ways they and their energy can be arranged So entropy increases as number of moles increase
How do you calculate the entropy change of a system?
Entropy of products - reactants
What is the feasibility of a process dependent on?
The feasibility of a process depends upon the entropy change and temperature in the system, T∆S, and the enthalpy change of the system, ∆H
How do you calculate the free energy of a system?
∆G = ∆H – T∆S (the Gibbs’ equation) A process is feasible when ∆G has a negative value
What are the limitations of predictions made by ∆G about feasibility, in terms of kinetics.
Even if ∆G shows a reaction is theoretically feasible, the activation energy might be really high, or the reaction happens so slowly you don’t notice it happening at all.
How do you calculate when the reaction is just feasible?
When ∆G is zero
Do substances like disorder?
Yes, they’re more energetically stable, so they try to increase entropy