oceans ocr

List 5 sources of error/improvements for calorimetry experiments

1. heat loss to the surroundings (use a draught shield)
2. non standard conditions
3. incomplete combustion
4. heat loss by the metal calorimeter (use a lid)
5. 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

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)

How do acidic buffers (CH3COOH

- 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+]

How do acidic buffers (CH3COOH

- 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+]

How do basic buffers (NH3 + H20

- 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-]

How do basic buffers (NH3 + H20

- 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

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?

1) the sun emits electromagnetic radiation mainly as visible light, UV radiation and infrared radiation
2) most the UV and IR are absorbed by the earth's atmospheric gases and some radiation is reflected back into space from clouds
3) 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
4) the earth then radiates infrared radiation
5) some of the IR escapes through the IR window
6) 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