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A2 chemistry 4.4

Edexcel chemistry - entropy

QuestionAnswer
What is the first law of thermodynamics? Energy can't be made or destroyed
What is entropy? What factors increase it? Entropy is a measure of the disorder of a system and of the number of ways of distributing the energy of a substance in the available energy levels, J/K/mol. Factors increasing: gas<liquid<solid, high temperature, more gas moles, more random arranegemnt
What does the standard entropy of a substance depend on? Physical state and complexity (number of atoms)
What are the 3 equations for entropy + precuations? ΔStotal = ΔSsystem + ΔSsurroundings. ΔSsystem = ΣSproducts - ΣSreactants (including stoichiometric numbers). ΔSsurroundings = -ΔHsystem/T. T must be in K - celcius + 273. ΔH kJ/mol -> J/mol
What is the 2nd law of thermodynamics? Spontaneous change results in an increase in disorder/entropy. Natural direction of change is of increasing total entropy. 1 ordered arrangement (crystalline substance) an be rearranged into many disordered arrangements, so P(disorder) is greater
What is a spontaneous reaction? Occurs without continuous intervention, although can be briefly heated/ catalysed
State ΔH, ΔSsystem, ΔSsurroundings, effect of increasing/ decreasing T for exothermic reaction -ΔH, - ΔSsystem, + ΔSsurroundings, increasing T makes ΔH/T less positive so ΔStotal is less positive
State ΔH, ΔSsystem, ΔSsurroundings, effect of increasing/ decreasing T for endothermic +ΔH, +ΔSsystem, -ΔSsurroundings, increasing T makes ΔH/T less negative so ΔStotal is more positive
What is the 3rd law of thermodynamics? Entropy of a perfect crystalline substance at 0K is 0, but entropy is gained as soon as heat/ defect is introduced into system
Explain temperature/ entropy graph 0 entropy @ 0K. At melting+boiling points, large increase in S as change in state allows particles to move more randomly +freely. No T change -energy is used to break bonds. Supplying heat to hot object smaller increase in S than same to cold object.
Explain how kinetics and thermodynamics affect reaction feasibility Kinetics - determines rate, large Ea means slow reaction because reactants are more kinetically stable than products, thermodynamically feasible reaction may not take place at RT due to high Ea. THermodyanmics- enthalpy = exothermic is favourable...
Cont. Entropy: ΔStot>200 reaction goes to completion, 0<ΔStot<200 equilibrium favours RHS, -200<ΔStot<0 equilibrium favours LHS, ΔStot<-200 reaction doesn’t go. ΔStot>0 reaction is thermodynamically feasible + spontaneous, products more stable than reactants
When can endothermic reactions occur at RT? Give some examples + equations If the magnitude of ΔSsys > ΔH/T. NH4NO3 (s) -> NH4+ (aq) + NO3- (aq). 2CH3COOH + (NH4)2CO3 -> CH3COONH4 + CO2 + H2O. Ba(OH)2.8H2O (s) + NH4Cl (s) -> 10H2O + BaCl2 + 2NH3
Why is the combustion of magnesium ribbon in air considered spontaneous? Exothermic reaction, the increase in entropy of the surroundings compensates for the decrease in ΔSsys (gas -> solid). Since ΔS total > 0, thermodynamically feasible and spontaneous (2nd law thermodynamics). Continues to burn when heat is removed
Define ΔH soln + implications for ionic solids The enthalpy change when 1 mol of solid dissolves in sufficient solvent to give an infinitely dilute solution, ions become hydrated when ionic solids dissolve in water.
Define ΔH hyd The enthalpy change when 1 mol gaseous ions dissolves in sufficient solvent to give an infinitely dilute solution, usually exothermic
What factors make ΔH hyd more exothermic? High charge density on ions (higher charge, smaller ionic radius) means stronger attraction between ions and solvent molecules, more bonds form releasing energy
What is ΔH LE? Enthalpy change when 1 mol of ionic solid is formed from gaseous constituent ions that start infinitely far apart, exothermic
What factors make ΔH LE more exothermic? Higher charge density (higher charge on ions, smaller ionic radii) = stronger electrostatic attraction between ions. More polarisation of anion - polarising cation (high charge density) + polarisable anion (high charge, large radius) = more covalency
What equation allows you to calculate theoretical values of ΔH soln? Give an example for CaCl2 ΔH soln = Δ H hyd - ΔH LE. e.g: ΔHsoln (CaCl2) = ΔHhyd (Ca2+) + 2ΔHhyd (Cl-) - ΔHLE (CaCl2)
When is ΔH soln exothermic? ΔHhyd is exothermic enough to compensate for large positive (-ΔHLE) i.e. energy released from making bonds between solvent+solute compensates for energy to break lattice. Ions have a high charge density: form strong attractions with solvent, lots of energ
When is ΔH soln endothermic? If magnitude of ΔHLE> ΔHhyd, so the energy released from forming bonds between solvent/solute can't compensate for energy required to break lattice. Tend to be ions with LOW CHARGE DENSITY- form weaker attractions, less energy released by hydration
Give an example of an ionic solid that dissolves endothermically and explain why it occurs at RT NaCl: low charge density, weak attractions between ions and solvent, less energy released by hydration so endothermic, but increase in Ssys compensates for this: ionic solid -> ions + solvent doesn't become much more ordered
What determines the extent of solubility of a solid? ΔS total for 1 mol of solid
Does solubility increase or decrease down a group? ΔH hydration and ΔH LE become less exothermic down a group- if ΔHhyd becomes less exothermic more quickly, solubility decreases down group. If ΔH LE becomes less exothermic more quickly, solubility increases down group
Created by: 11043