Studied by 1 personList 5 sources of error/improvements for calorimetry experiments
heat loss to the surroundings (use a draught shield)
non standard conditions
incomplete combustion
heat loss by the metal calorimeter (use a lid)
stirring of water (even distribution of heat)
What are standard conditions?
25°c, 298k 101Kpa, 1atm 1 moldm³ for solutions
Hess's law given ∆fH?
∆H= P-R
Hess's law given ∆cH?
∆H= R-P
When finding ∆fH using combustion data what do you put in the box?
the products of combustion of your reactants
When finding ∆cH using formation data what do you put in the box?
the individual elements the reactants are made of
What are ion-dipole interactions?
an attractive force between an ion and a polar molecule such as salt and water
What is a solvent ?
something that hydrates or surrounds the compound or solute you are trying to dissolve
What is a solute?
the substance or compound you want to dissolve
Describe the dissolving process
When something dissolves, bonds within the solvent and solute break and new bonds between the solvent and solute are made
What determines whether a solute dissolves or not?
generally speaking, substances won't dissolve if the bonds to be broken are stronger than those that will be formed
if less energy is released in making the solute-solvent intermolecular forces than required to break the intermolecular forces within the solute or solvent then it does not dissolve
What are the two types of solvent?
polar (e.g. water)
non polar (e.g. hexane)
What do most ionic substances only dissolve in and why?
most ionic substances only dissolve in polar solvents
the ion-dipole bonds formed between ionic substances and polar solvent molecules are strong enough to dissolve the solid
What solvent do ionic substances NOT dissolve in and why?
non-polar solvents
non-polar solvents don't interact strongly enough with ions to pull them away from an ionic lattice
electrostatic forces between the ions are way stronger than any bonds that could form between the ions and non-polar solvent molecules
What is an ion-dipole interaction
an ion in solution and the dipole of a polar solvent interact
the ions becomes surrounded in the polar solvent via the attractive force between the ion and the molecule with a permanent dipole
e.g. NaCl and Water
Hδ+ attracted to Cl-
Oδ- attracted to Na+
Explain how ionic solids dissolve in water
water is polar
ion dipole bonds form between the water molecules and the dissolved ions
the ions separate from the ionic lattice and become surrounded by water molecules (hydration)
unless the ionic bonds in the lattice are really strong, the ion dipoles will be strong enough to pull the lattice apart which is why most ionic substances are soluble in water
What is hydration?
the process in which an ion is surrounded by water molecules arranged in a specific manner
What is solvation?
The process of surrounding solute particles with solvent particles to form a solution
What is a hydrated ion?
An ion surrounded by water molecules
What do most covalent substances dissolve in and why?
most only dissolve in non-polar solvents
intermolecular bonds between covalent molecules tend to be pretty weak and can be broken by non-polar solvent molecules
What do most covalent substances NOT dissolve in and why?
don't dissolve in polar solvents
e.g. iodine doesn't dissolve in water much
the hydrogen bonds between water molecules are stronger than the bonds that would form between water molecules and iodine molecules
What is ∆LEH?
lattice enthalpy of formation
always negative because energy is released
the more negative the lattice enthalpy, the stronger the bonding/ionic bond strength
Na+ (g) + Cl-(g) --> NaCl(s) -∆H1
Define lattice enthalpy of formation
the enthalpy change when one mole of a solid is formed from it's gaseous ions under standard conditions
What is ∆LEH dependent on?
the strength of ionic bonding which is dependent on smaller ions and highly charged ions (charge density)
What is ∆HydH?
enthalpy of hydration
going from gaseous ions to aqueous ions
releases energy making this process exothermic
bonds between ions and water are made (ion-dipole?)
∆H2
Define enthalpy of hydration
the enthalpy change for the formation of solution of ions from one mole of gaseous ions
Is the process of ∆HydH exothermic or endothermic and why?
Exothermic because it goes from gaseous (lots of energy) to aqueous which releases energy
What is ∆SolH
enthalpy of solution
the overall effect on the enthalpy when something dissolves
the net effect of ∆LeH and ∆HydH
NaCl(s) --> NaCl (aq)
∆H3
Define enthalpy of solution
Enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution. Goes from solid to aqueous
∆solH (∆H3)=
[∆HydH(+ ion) + ∆HydH(- ion)] - ∆LEH (= ∆H1 + ∆H2)
What happens if the ∆SolH is negative (exothermic)?
the solute or compound dissolves
What happens if the ∆SolH is positive (endothermic)?
the solute or compound doesn't dissolve
What happens if the ∆SolH is slightly positive?
The solute could dissolve
Describe an enthalpy level diagram
gaseous ions at the top
solid ions in the middle
aqueous ions at the bottom
What is solubility?
Maximum amount of a solute that can be dissolved in a given amount of solvent at a given temperature
What is a saturated solution?
a solution in which no more solute can dissolve
How do do an enthalpy change of solution experiment
reaction solution in polystyrene cup with lid and thermometer
measure temperature change
calculate the number of moles of what's reacting
need mass of total solution
q=mcat ÷1000
N=mass/me ∆SolH= q/n kjmol-¹
What effects both LeH and HydH?
charge density
How does charge density effect LeH?
the ions with higher charge density are better at attracting each other in ionic lattices than those with lower charge densities
this means the ionic bonds are stronger, meaning more energy is released when the bonds are formed, giving them a more exothermic LeH
How does charge density effect HydH?
ions with a higher charge density are better at attracting water molecules than those with lower charge densities
this means the intermolecular bonds are stronger, meaning more energy is released when the bonds are made, giving them a more exothermic HydH
What is the Bronsted-Lowry acid definition?
acid is a proton donor
What is the Bronsted-Lowry base definition?
base is a proton acceptor
What is s Bronsted-Lowry
a reaction involving the transfer of a proton
What decimal place should you use for pH?
two decimal places
What are strong acids?
acids that completely dissociate into ions e.g. HCl --> H+ + Cl-
What are weak acids?
acids that partially dissociate into ions e.g. CH3COOH --> CH3COO- + H+
What is monoprotic acid?
an acid that releases one H+ per molecule e.g. HCl, HNO3, Ch3COOH
What does monoprotic mean?
an acid that releases one H+ per molecule
a base that can only react with/accept one proton per molecule
What is a diprotic acid?
an acid that releases two H+ per molecule e.g. H2SO4, H2C2O4
What is a triprotic acid?
an acid that releases three H+ per molecule e.g. H3PO4
What is the definition of pH?
pH= -log10[H+]
How do you work out [H+]
10^-pH
What is Kw?
ionic product of water
kw=[H+] [OH-]
kw=10^-14
How do you work out pOH?
pOH = -log[OH-]
How do you work out pH of (strong) bases?
pH = 14-pOH
What is the effect of temperature on the pH and neutrality of water?
H2O <--> H+ + OH- = endo
as temperature increases, the equilibrium moves right to oppose the increase in temp
[H+] and [OH-] inceeas
kw increases
pH increases
water is still neutral as [H+] = [OH-] and that is the definition of neutral
What is the definition of neutral?
[H+] = [OH-]
How do you calculate the pH of a diluted acid solution?
(conc X original vol)÷new total volume = X
pH= -log10(X)
What's important to look out for when doing pH calculations?
Whether it is a monoprotic, diprotic, triprotic etc. You may need to multiply or divide by two (or on occasion three) to work out the [H+] or [OH-]
How do strong bases ionise in water?
fully ionise in water e.g. NaIH +H2O --> Na+ + OH-
What is a conjugate base pair?
pair made of acid + base in an reaction
acid reactant pairs with base product
base reactant pairs with acid product
What is Ka?
acid dissociation constant
Ka= [H+] [A-] ÷ [HA]
What units do Ka have?
moldm-³
What does a large Ka value mean for strength of acid?
the bigger the value, the stronger the acid
What does the pKa value mean for strength of acid?
the smaller the pKa value the stronger the acid
How do you work out pKa?
pKa = -logKa
How do you work out Ka using pKa?
Ka= 10^-pKa
How do you work out the pH of mixtures of strong acids and bases?
check if monoprotic/diprotic etc
calculate moles H+
calculate moles OH-
which is in excess
work out excess
convert to concentration by dividing by total volume in dm³
calculate pH
if excess H+ calculate pH using -log[H+]
if excess OH- calculate pH using 14-(-log10 [OH-])
What can be said about a neutral solution and the conc of H+ and OH-?
[H+]=[OH-]
What makes a solution acidic?
[H+] > [OH-]
What are buffer solutions?
solutions that resist changes in pH when small amounts of acid (H+) or alkali (OH-) are added
What are acidic buffer solutions?
pH higher less than 7
[HA] and [A-(salt)] are much higher than [H+]
What are basic buffer solutions?
pH more than 7
[Base] and [salt] much higher than [OH-]
What are the two routes that acidic buffers are formed?
route 1- mixture of weak acid and one of its salts e.g. ethanoic acid and sodium ethanoate route 2- mixture of an excess of weak acid and a strong base e.g. excess ethanoic acid and sodium hydroxide
What are the two routes that basic buffers are formed?
route 1- mixture of weak base and one of its salts e.g. ammonia and ammonium chloride route 2- mixture of an excess of weak base and a strong acid e.g. excess ammonia and hydrochloric acid
How do buffers maintain pH?
when small amounts of acid or alkali are added, the ratio remains roughly constant and so the pH hardly changes
if large amounts of acid or alkali are added, the ratio would change and so the pH would change significantly (- moves or release hydrogen ions)
adding H+ ions
CH3COO- reacts with the H+ ion to form CH3COOH
equilibirum shifts left
reduces [H+] to close to original value
[CH3COOH] increases slightly
[CH3COO-] decreases slightly
pH doesn't change much
[CH3COOH]/[CH3COO-] >> [H+]
CH3COOH reacts with OH- to form CH3COO-
equilibrium shifts right
[H+] increases to close to original value
[CH3COO-] increases slightly
[CH3COOH] decreases slightly
pH doesn't change much
[CH3COOH]/[CH3COO-] >> [H+]
OH- reacts with the H+ to form water
equilibrium shifts to the right to increase the [OH-] to close to its original value
pH doesn't change much
[NH3]/[NH4+] >> [OH-]
NH4+ reacts with OH- to form NH3 and water
equilibrium shifts to the right
extra OH- is consumed
[NH3] increases slightly
[NH4+] decreases slightly
pH doesn't change much
[NH3]/[NH4+] >> [OH-]
What are the industrial uses of buffers?
shampoos and soaps (most shampoos contain a pH 5.5 buffer as soap is very alkaline makes your hair dull)
biological washing powders (to keep the pH at the correct level for enzymes to work)
food products (such as sodium citrate)
fermentation
What are the natural buffers in our bodies?
blood (needs to be kept at 7.4. It is buffered by carbonic acid H2CO3 <--> H+ + HCO3¯ (kidneys) H2CO3 <--> H2O + CO2 (respiration)
cells need a constant pH to allow biochemical reactions to take place H2PO4¯ <--> H+ + HPO4²¯
What do you use to calculate the pH of acidic buffers?
rearrange Ka= [H+][A-]/[HA] to find [H+]
pH= - log[H+]
What are the units for entropy?
Jmol-1K-1
What is entropy?
measure of disorder
a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared between the particles
What is the order of entropy for solids, liquids and gases?
gas > liquid > solid
Which has higher entropy, mixtures or seperate?
mixtures
Which has higher entropy, large molecules or small molecules?
large molecules
Are substances energetically stable when there's high or low disorder?
Substances like disorder so are more energetically stable when there's more disorder
What can affect entropy?
physical state
amount of energy a substance has
amount of particles
How does physical state affect entropy?
solid particles wobble about a fixed point so have hardly any disorder and have the lowest entropy
gas particles have the most disordered arrangements of particles and so have the highest entropy
How does amount of energy a substance have affect entropy?
the more quanta (fixed package of energy) a substance has the more ways they can be arranged and the greater the entropy
How does amount of particles affect entropy?
the more particles, the more ways they and their energy can be arranged
e.g. N2O4 --> 2NO entropy increases because the number of moles increases
How do you calculate entropy change of system?
entropy of products - entropy of reactants
How do you calculate entropy change of surroundings?
-ΔH/T
ΔH in jmol-1 (convert from kjmol-1 by x1000)
T in kelvin (convert from C by + 273)
How do you work out total entropy?
ΔtotalS = ΔsysS + ΔsurrS
When is a chemical change feasible?
When ΔtotalS has a positive value
What is the greenhouse effect?
the sun emits electromagnetic radiation mainly as visible light, UV radiation and infrared radiation
most the UV and IR are absorbed by the earth's atmospheric gases and some radiation is reflected back into space from clouds
the energy that reaches the earth's surface is mainly visible light and UV. Some is reflected into space by light coloured,shiny surfaces like ice and snow. The rest is absorbed by the earth causing it to heat up
the earth then radiates infrared radiation
some of the IR escapes through the IR window
greenhouse gases in the troposphere absorb some of the remaining IR in the IR window and re-emit in all directions
What is the IR window?
the range of IR frequencies that are not absorbed by atmospheric gases
What are the main greenhouse gases?
carbon dioxide, water vapor, methane
Note 
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