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  • Created by: Gemma
  • Created on: 13-04-14 18:37
Definition of Enthalpy
The heat content of a substance
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Definition of Enthalpy change of solution
The enthalpy change when 1 mole of a compound dissolves to form an infinitely dilute (have enough water to dissolve the ion) solution in water
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Definition of Enthalpy change of hydration
Enthalpy change for formation of an aqueous solution of ions from 1 mole of gaseous ions- when water molecules surround an ion
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Definition of Lattice enthalpy
enthalpy change when 1 mole of a solid is formed by the coming together of seperate gaseous ions
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How many H bonds can water molecules form compared to NH3 + HF
2X as many
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What is the boiling point of water like , compared to those of the hydrides in group 6?
much higher- doesn't follow the set pattern
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What is the definition of specific heat capacity
The energy needed to raise 1g of a substance by 1K
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What is the specific heat capacity of water like + why
very high compared to most substances, because the energy is being used to break the H bonds between molecules instead of being used to increase KE of molecules
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What is the enthalpy change of vapourisation
A measure of the energy we have to put into a liquid to overcome the intermolecular bonds + turn 1 mole of molecules from liquid to vapour
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What is the enthalpy change of vapourisation of water like + why
high due to the strong H bonds holding the water molecules together more strongly than other IM bonds + so more energy is needed to overcome them b4 boiling/vapourisation can occur
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What is significant about the melting and boiling point of water?
Both 0degrees , so can exist both as a liquid and solid
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What happens to the density of water on melting?
From the open tetrahedral structure of ice , the density increases on melting due to the water molecules falling into the open spaces
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What temp does density decrease in water?
4 degrees
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Formula for the enthalpy change of solution?
(formula)
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What is the value of LE?
negative- bond breaking
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What do you do to delta LE when you break the bonds in a lattice
place a negative sign in front of the LE
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What is sign of the enthalpy change of hydration
negative - bond breaking
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What is the enthalpy change of hydration and lattice enthalpy increased by?
Ionic charge & smaller ion
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How does the ionic charge affect the enthalpy change of hydration + lattice enthalpy
higher the charge the greater the attraction
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How does the size of the ion charge affect the enthalpy change of hydration + lattice enthalpy
the smaller the ion , the greater the attraction because they're closer together
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If Delta H solution is small + positive will it dissolve?
it may
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If Delta H solution is large + positive will it dissolve?
no
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If Delta H solution is exothermic (-) will it dissolve
yes
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Why doesn't CaCO3 dissolve even though its exothermic
because the solvent is more ordered
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Why does NaCl dissolve even though it is endothermic?
because the solute is more disordered- the disorder of the solute is outweighed by the decrease in entropy of the solvent
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What are the units for enthalpy?
kJ mol-1
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Why is the enthalpy change of solution positive
Because the hydration of ions provide less energy than that needed to break up the lattice
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What is entropy?
A measure of disorder
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What are the units of entropy?
JK-1mol-1
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What is the formula for entropy?
(formula)
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What does a positive total entropy change mean?
entropy has increased
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What is zero degrees in Kelvin?
273K
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What is the formula for entropy change of the system
(formula)
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What is the formula for entropy change of the surroundings
(formula)
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What is the characteristic of a strong acid?
Fully dissociates in water
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What is the characteristic of a strong base?
a base which is fully ionised in solution
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What is the equation for a strong acid?
HA ==> H+ + A-
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What is the characteristic of a weak acid?
(substances) acids which have a weak tendency to donate
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Which side does the equilibrium lie in a weak acid?
LHS
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What is the formula for the acidity constant Kc
(Formula)
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What is the pH a measure of?
The concentration of H+ ions in a solution
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What is pKA a measure of?
The strength of the substance - the extent to which an acid can donate H+
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Assumption of a weak acid
[H+] = [A-] - we can ignore the H+ from the water it is a low conc. The amount of HA at equilibrium is equal to the amount of HA put into the solution
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What is the formula of pH
(Formula)
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What is the formula of pKa
(formula)
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Assumption of strong acid
Amount of moles of H+ ions = the amount of HA put into the solution for a MONOPROTIC ACID
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What is the definition of a monoprotic acid
An acid that donates one proton/H atom per molecule to an aqueous solution
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What is the definition of diprotic acid
An acid than donates 2 H atoms per molecule to an aqueous solution (conc of H+ is 2X that of HA)
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What does a high pH indicate?
Alkaline solution (weaker the acid)
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What does a high Ka value indicate
the stronger the acid
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To get the H+ conc from pH
10^(-pH)
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What does a high pKa value indicate?
Alkaline solution (weaker the acid)
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For strong bases which equation do you use?
(formula)
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What is a buffer?
a substance which resists change in pH despite the addition of a small amount of acid/alkali- maintains constant pH
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What is a buffer made up of?
A weak acid + its salt // a weak base + its salt
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What are the assumptions of a buffer
All the A- ions come from the salt Almost all the HA molecules put into the solution remain unchanged (until H+/A-)
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How can you use the Ka value to coarse tune a buffer?
Can choose a particular weak acid determines which region on the pH range the buffer is in . Ka=[H+]
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How can you use the Ka value to fine tune?
Find the ratio of salt:acid - can change it from 3:1 ratio to 1:3 ratio = changes H+ conc by a factor of 9 . Changes pH by factor of 1. The ratio shouldnt be outside this range otherwise there will be insufficient HA/A- for buffer to be effective
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How can you reduce the atmospheric CO2?
use of alternate fuels, capture + storage of CO2 in deep sea trenches or injecting it into sea floor at a low depth so it turns to liquid , increased photosythesis (grow more trees + increase content of soil)
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Other cards in this set

Card 2

Front

Definition of Enthalpy change of solution

Back

The enthalpy change when 1 mole of a compound dissolves to form an infinitely dilute (have enough water to dissolve the ion) solution in water

Card 3

Front

Definition of Enthalpy change of hydration

Back

Preview of the front of card 3

Card 4

Front

Definition of Lattice enthalpy

Back

Preview of the front of card 4

Card 5

Front

How many H bonds can water molecules form compared to NH3 + HF

Back

Preview of the front of card 5
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