F325 EEE- Enthalpy and Entropy

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  • Created on: 18-11-15 21:39
Define Enthalpy
The heat content that is stored in a chemical system.
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Define Hess' Law
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.
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What is meant by standard conditions?
100 kPA, 298K, conc= 1 mol dm-3
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Define standard enthalpy change of formation
The enthalpy change that takes place when 1 mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions.
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Define standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation, under standard conditions, all reactants and products being in their standard states.
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Define standard enthalpy change of combustion
The enthalpy change that takes place when I mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
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Equation for the formation of NaCl
Na (s) + 1/2Cl (g) --> NaCl (s)
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Equation for the thermal decomposition of CaCO3
CaCO3 (s) --> CaO (s) + CO2 (g)
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Define first ionisation energy
The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of 1+ ions.
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Define lattice enthalpy
The enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions.
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Equation for lattice enthalpy of NaCl
Na+ (g) + Cl (g) - --> NaCl (s)
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Positive enthalpy values
Endothermic reaction
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Negative enthalpy values
Exothermic reaction
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Making bonds
Exothermic
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Breaking bonds
Endothermic
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What does lattice enthalpy show?
The strength of an ionic lattice, and is a measure of ionic bond strength.
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Define standard enthalpy change of atomisation
The enthalpy change when one mole of gaseous atoms is made from the element under standard conditions. e.g Li (s) --> Li (g)
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Define standard enthalpy change of first ionisation energy
The energy change when an electron is removed from every atom in one mole of gaseous atoms.
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Why is enthalpy change of first ionisation energy positive?
Energy is needed to overcome the nuclear attraction on the electrons from the nuclei.
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Why is enthalpy change of second ionisation energy even more positive?
Even more energy is required to remove a negative electron from a positive ion, as opposite forces attract.
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Define standard enthalpy change of first electron affinity
The energy change when one mole of gaseous atoms gains one mole of electrons under standard conditions.
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Why are lattice enthalpies not found directly?
It is impossible to form one mole of an ionic lattice from gaseous atoms.
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How do you calculate lattice enthalpy from a Born-Haber cycle?
Clockwise=Anticlockwise OR follow the arrow direction round, starting from the non-pointing end and change sign if the arrow is in the opposite direction.
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What factors affect lattice enthalpy?
Ionic radius (smaller IR= more exo. LE as nuclear attraction is greater) and ionic charge (greater IC= more exo. LE as nuclear attraction is greater).
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Define enthalpy change of solution
The enthalpy change when one mole of a compound is dissolved in water under standard conditions. Can be exo/endothermic.
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What 2 processes make up enthalpy of solution?
Ionic compounds breaking down to gaseous ions (opposite of LE) and standard enthalpy of hydration.
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Define standard enthalpy of hydration
The enthalpy change when one mole of aqueous ions is formed from their gaseous ions under standard conditions.
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Define entropy
The quantitative measure of the degree of disorder in a system, given the symbol S. (All systems contain some entropy because particles are always moving and entropy is always a positive value).
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When does entropy increase?
Melting solids, boiling liquids, dissolving ionic compounds in water, increasing temperature, increase the number of moles of gas
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Define standard entropy of a substance
The entropy content of one mole of the substance under standard conditions , given the unit J K-1 mol-1
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Define standard entropy change of a reaction
The entropy change which accompanies a reaction of molar quantities under standard conditions, with all reactants and conditions being in their standard states.
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Calculating ^S
^S= S(products)-S(reactants)
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System becomes more ordered
^S is negative
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System becomes more disordered
^S is positive
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Define spontaneous process
A spontaneous process is one that takes place on its own and leads to lower energy and increased stability. The overall energy decrease results from contributions from both enthalpy and entropy.
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Exothermic and spontaneous?
Exothermic are commonly spontaneous- ^H decreases, increasing stability
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What factors determine whether a reaction is spontaneous or not?
Temperature (in K), Entropy change in system, Enthalpy change with the surroundings
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Eqaution for spontaneity
^G=^H-T^S Feasible reactions occur when ^G (Gibbs free energy) is a negative value. Min. temp is when ^G=0
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-ive ^H, +ive ^S=
always -ive ^G, reaction always feasible
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+ive ^H, -ive ^S=
always +ive ^G, reaction never feasible
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-ive ^H, -ive ^S=
-ive ^G at low temps, reaction feasible at low temps
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+ive ^H, +ive ^S=
-ive ^G at high temps, reaction feasible at high temps
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Calculating ^G
Calculate ^H, then ^S then ^G ENSURE UNITS ARE CONVERTED FROM J TO kJ!
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Card 2

Front

Define Hess' Law

Back

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.

Card 3

Front

What is meant by standard conditions?

Back

Preview of the front of card 3

Card 4

Front

Define standard enthalpy change of formation

Back

Preview of the front of card 4

Card 5

Front

Define standard enthalpy change of reaction

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