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32 Cards in this Set
- Front
- Back
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Standard conditions |
Temp at 25 degree or 298k Pressure at 1 bar or 10,000 pa Solution conc is 1mol/dm3 |
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Standard enthalpy change of reaction |
The heat energy absorbed or evolved when molar quantities of reactants stated in the thermochemical equation react together under standard conditions Kj per mile |
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Standard enthalpy change of combustion |
- The energy change when one mole of substance is completely burnt in excess oxygen under standard conditions
-The more heat evolved from the combustion , the better the fuel is |
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Standard enthalpy change of neutralisation |
Energy change one mole of water is formed during the neutralisation of an acid and an alkali under standard conditions |
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Enthalpy change of combustion |
- heat evolved / amount of fuel used - assumption that heat evolved by fuel = heat absorbed by the water ( 100% efficient ) |
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Why is experimental value for enthalpy change of combustion less than theoretical value |
- incomplete combustion due to insufficient supply of oxygen leading to the formation of products thus causing less heat to be evolved
- not all the heat evolved is transferred to water as some heat is absorbed by the calorimeter and some heat is lost to the surrounding —> heat transferred to the water and temp registered by the thermometer would be lower than expected
- heat loss to the surrounding without the use of lid |
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Formula of enthalpy change of neutralisation |
Heat evolved = heat absorbed by the solution - heat evolved / amount of water formed
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Hess law of constant heat of summation |
Enthalpy change for a chemical reaction is the same regardless of the route taken , provided the initial states of the reactants and the final states of the products are the same |
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Enthalpy change of formation |
-The energy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions - hr = sum n ( hf products ) - sum m ( hf reactants ) - hr= sum a ( hcombustion reactants) - sum b ( hcombustion products ) - hf of an element is alw 0 - compound is energetically more stable than its constituent elements if hf < 0b |
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Bond dissociation energy |
The energy required to break one mole of covalent bond between two atoms in the gaseous state - positive value bcuz bond breaking requires energy - indicates strength of covalent bond —> greater the BE , stronger the bond —> more in reactive eg. N2 - diatomic molecules only have one value for BE bcuz there’s only 2 atoms in the molecule |
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Why are BE calculated different ? |
- Bond energies in data booklet are average values from many different molecules , hence it can only provide an estimate value - __ is liquid at standard conditions and hvap is not accounted for |
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Standard enthalpy change of atomisation |
Energy change when one mole of gaseous atoms is formed from its elements under standard conditions
- hat of a compound is the energy change when one mole of compound is broken to form gaseous atoms under standard conditions ( don’t need one mole) - hat is alw endo bcuz energy must be absorbed to pull the atoms far apart and break the bonds btwn them - hat of noble gases are zero bcuz the elements exists as gaseous separate atoms under standard conditions |
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First ie |
The energy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous single charged cations |
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First electron affinity ( ea) |
Energy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous anions |
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Standard lattice energy |
The energy change when one mole of an ionic compound is formed from its constituent gaseous ions at standard conditions
- alw exo as heat energy is evolved when the ions come tgt to form ionic bonds - energy associated with the reverse process is known as the reverse lattice energy or lattice dissociation energy which is endothermic |
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Factors affecting LE |
- LE prop |z+z|/ r+ + r- - greater the charge , the greater the MAGNITUDE of LE - smaller the ions , greater the MAGNITUDE of LE |
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Born haber cycle |
- only for ionic compounds 1) atomisation 2) formation of gaseous ions from atoms 3) formation of ionic lattice from gaseous ions 4) use Hess law to solve |
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Comparison btwn theoretical and experimental values of LE |
Eg. AgI—> e cloud of iodine is large and Polaris able by Ag+ readily , hence AgI shows covalent character —> not pure ionic cmpd —> LE differs from that predicted for a purely ionic cmpd |
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Enthalpy change of hydration |
The energy released when one mole of gaseous ions is surrounded by water molecules , forming a solution at infinite dilution under standard conditions
- alw exothermic bcuz energy is released when bonds are formed btwn the gaseous ions and the dipoles on the water molecules - Hhyd is prop to |z| / r - highly charged ions undergo hydration most easily , evolving a lot of heat |
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Standard enthalpy change of solution |
Energy change when one mole of compound is dissolved by solvent such that further dilution produces no more heat change under standard conditions - Hsol = Hhyd - Hlatt - can be exo or endo - Hsol more exo (Hhyd > Hlatt) —> more soluble - Hsol more endo ( Hlatt > Hhyd) —> more insoluble |
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Entropy |
A measure of the disorder of a system - J/k/mol |
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Factors affecting entropy |
1) change in temp - entropy increases when temp increases - an increase in temp increases the energy the system possesses - there are more ways to distribute the energy packets now —> implies an increase in disorder of the system |
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Factors affecting entropy ( change in phase ) |
2) change in phase - change in solid to liquid to gas involves molecules having more disordered or random arrangement within the system thus entropy increases - change from solid to liquid —> molecules are broken free from the orderly arrangement in solid lattice to become free from moving liquid molecules - change from solid to aq or dissolving a solid —> ions are broken free from the orderly arrangement in solid lattice to become free moving aqueous ions - increase in entropy from liquid to gas is greater than the increase in entropy from solid to liquid because increase in volume from liquid to gas is greater than increase in vol from solid to liquid |
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Factors affecting entropy ( change in number of particles ) |
Entropy increases in the reaction as one mole of Cl-Cl bond is broken to form 2 moles of Col gaseous radicals where there is an increase in disorder of the system due to the increase in the number of gaseous particles —> more ways of arranging the particles |
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Factors affecting entropy ( mixing ) |
Entropy increases bcuz there is mixing of gaseous particles , where there is an increase in disorder of the system as there are more ways of arranging the particles |
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Gibbs free energy change |
G = H - T S G , H —> Kj /mol T —> Kevin S —> J / K / mol G < 0 —> reaction takes place spontaneously in the forward reaction G > 0 —> reaction cannot take place simultaneously G = 0 —> reaction reaches equilibrium |
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What is the assumption made when calculating spontaneous |
H and S do not vary with temp |
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When is reactions feasible at all temp |
H is positive and s is positive |
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When is reaction not feasible at all temp |
H is positive and s is negative |
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How to make endothermic reaction feasible ? |
- H is positive bcuz … - S is positive because … - some endothermic rxt will not be feasible at rtp but will be when temp is high (eg. Melting boiling , decomp ) - T increases such that “ -T A “ value is large enough to make G < 0 ( | -TS | > | H | ) |
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How to make exo rxt feasible |
- H is negative bcuz … - S is negative bcuz .. - some exo rxt is feasible at low temp ( eg . Condensation , freezing ) —> may be slow thus catalyst must be used - temp must be low enough such that the “ -T S “ term becomes smaller than the H term which is -be to make G -ve |
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Assumptions of G |
Using G to predict spontaneity is valid only under standard conditions |
