Chemistry Unit 4 Definitions & Common Questions

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Define 'order of reaction'
The power of the concentration term (in the rate equation)
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Define a Bronsted-Lowry acid
Proton donor
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Define a Bronsted-Lowry base
A proton acceptor
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Define pH
-log(H+)
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Define the ionic product of water
Kw= (H+)(OH-)
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Define a weak acid
An acid that partially dissociates
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Define a weak base
A base that partially dissociates
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Define a buffer
A solution that maintains a constant pH, despite small additions of acid, base or water
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Define a chiral carbon
A carbon atom with four different groups attached
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Define an enantiomer
Molecules with a chiral carbon that exist as two non-superimposable mirror images
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Define a racemic mixture
Equal mixture of enantiomers
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Define a nucleophile
Electron pair donor
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Define an electrophile
Electron pair acceptor
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Define a stereoisomer
Molecules with the same structural formulae but with the atoms arranged differently in space
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Define 'retention time'
The time taken for a particular substance to pass through a chromatography column
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Explain why this is the rate determining step
By this step all the species in the rate equation have appeared and in the correct ratio
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What change in reaction conditions would cause the value of the rate constant (k) to change?
Temperature
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Using the equations, how can you tell that substance X acts as a catalyst in this reaction?
It appears in the rate equation but does not appear in the overall equation
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Explain why this Kc value has no units
Equal number of moles on each side of the equation so the units cancel
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State why the volume V need not be known in calculating the value of Kc for the reaction
Equal number of moles on each side of the equation so the volumes cancel
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Suggest why this pure water is not acidic
(H+) = (OH-)
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Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of acid
HA ⇌ H+ + A-, added H+ reacts with A-, acid eqm shifts to left to decrease H+, ratio of (HA)/(A-) remains constant
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Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of sodium hydroxide
HA ⇌ H+ + A-, added OH- reacts with H+, acid eqm shifts to right to replace H+, ratio of (HA)/(A-) remains constant
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Explain why a basic buffer (CH3CH2NH2/CH3CH2NH3+) maintains an almost constant pH when a small amount of HCl is added to it
CH3CH2NH2 + H2O ⇌ CH3CH2NH3+ + OH-, added H+ reacts with OH-. equilibrium shifts to RHS to replace OH-
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Explain why a basic buffer (CH3CH2NH2/CH3CH2NH3+) maintains an almost constant pH when a small amount of NaOH is added to it
CH3CH2NH2 + H2O ⇌ CH3CH2NH3+ + OH-, added OH- reacts withCH3CH2NH3+, equilibrium will shift to the LHS to decrease OH-
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Suggest a substance that could be added to aqueous methylamine to produce a basic buffer
CH3NH3Cl
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Identify a solvent in which a sample can be dissolved before obtaining its H n.m.r. spectrum
CDCl3
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Give the structural formula of TMS
Si(CH3)4
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Why is TMS used as a standard in recording n.m.r. spectra?
It gives one peak, low boiling point so can easily be removed, inert, non-toxic, all hydrogens equivalent, 1 clear peak at d=0 (upfield of most other peaks)
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What can be deduced about the 2 triplet peaks, both with an integration value of 2?
-CH2-CH2-
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State why the mass spectrum of acyl chlorides contain two molecular ion peaks
Cl has 2 isotopes (Cl35 and Cl37, 3:1 ratio)
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Name a suitable method by which the mixture of amino acids formed by hydrolysis of the protein can be separated
Chromatography
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How does chromatography separate mixtures?
There is a mobile phase, there is a stationary phase, separation depends on the attraction to each phase and each component will have a different retention time
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Explain how this racemic mixture is formed
The C=O bond is planar, there is equal chance of attack by the nucleophile from above or below the bond
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Suggest one advantage and one disadvantage of using a racemate rather than a single enantiomer in medicines
Advantage: Cheaper medicine due to cost/difficulty of separation Disadvantage: May be side effects from one enantiomer
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Give one advantage of the use of propanoyl chloride instead of propanoic acid in the preparation of methyl propanoate from methanol
Faster, not reversible, bigger yield, no catalyst required, purer product
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Give one advantage of the use of propanoic anhydride instead of propanoyl chloride in the manufacture of methyl propanoate from methanol
Reaction less violent/exothermic, no corrosive HCl fumes formed, safer, less toxic/dangerous, anhydride cheaper, anhydride less vulnerable to hydrolysis
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State how separate samples of the stereoisomers could be distinguished
A plane of polarised light, rotated in opposite directions
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Give the IUPAC name for glycerol
propane-1,2,3-triol
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Give a use for the mixture of sodium salts
Soaps
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Give a use for the mixture of methyl esters
Biodiesel
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Use the following data to state and explain the stability of benzene compared with the hypothetical cyclohexatriene
Benzene is more stable than cyclohexatriene, due to benzene's delocalised ring of electrons, with cylohexatriene the expected ΔH is -360kJmol-1, the actual ΔH is -208kJmol-1 which is 152kJmol-1 less exothermic than expected
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CH3COCl reacts with benzene. Write an equation for this reaction. Write an equation to show how a catalyst reacts with CH3COCl to produce a reactive intermediate and how the catalyst is reformed
CH3COCl + C6H6 --> C6H5COCH3 + HCl, CH3COCl + AlCl3 --> CH3CO+ + AlCl4-, AlCl4- + H+ --> AlCl3 + HCl
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The mechanism for the nitration of benzene is electrophilic substitution. Give the reagents used to produce the electrophile for this reaction. Write an equation/equations to show the formation of the electrophile and how the catalyst is reformed.
Conc HNO3, Conc H2SO4, H2SO4 + HNO3 --> HSO4- + NO2+ + H2O, HSO4- + H+ --> H2SO4
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Explain why butylamine is a stronger base than ammonia
The alkyl group has a positive inductive effect and releases electrons towards the N, making the lone pair more available and more likely to attract a proton
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Explain why phenylamine is a weaker base than ammonia
The lone pair on the nitrogen overlaps with the delocalised ring (negative inductive effect) which makes the lone pair on N less available and less likely to attract a proton
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Name the type of compound produced when a large excess of CH3Br reacts with CH3NH2. Give a use for this type of compound.
Quaternary ammonium salt, used in fabric/hair conditioner
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Explain why the melting point of the amino acid is higher than the melting point of CH3CH2CH(OH)COOH
Ionic bonding in the amino acid is stronger than the hydrogen bonding between CH3CH2CH(OH)COOH
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Explain why polyamides can be made into fibres suitable for use in sewing and weaving, whereas polyalkenes usually produce fibres that are too weak for this purpose
Hydrogen bonding between molecules in polyamides is stronger than the Van der Waals forces between molecules in polyalkenes
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A student accidentally splashed aqueous sodium hydroxide on a shirt made from a polyester. Holes later appeared. Name the type of reaction.
Hydrolysis
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[H+] = ?
[H+] = 10 ^ -pH
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pKa = ?
pKa = -log(Ka)
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At half equivalence, pH= ?
pH= pKa
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For a neutral solution/pure water, Kw=
Kw= [H+]^2
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At 298K/25 degrees Celsius, Kw=
Kw= 1 x 10^-14
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To work out the pH of strong acid-strong base mixture?
1) Calculate moles of H+ 2) Calculate moles of OH- 2) Work out excess moles and convert to concentration (divide by new volume) 4) If excess H+ pH= -log[H+] If excess OH- [H+]= Kw/[OH-] then pH=-log[H+]
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For a weak acid Ka=
Ka= [H+]^2/[HA]
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For a buffer Ka=
Ka= [H+][A-]/[HA] (When calculating the pH of a buffer use moles, for a weak acid/strong base buffer [OH-]=[A-])
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For which type of buffer do you need to work out the excess moles of weak acid?
Excess weak acid with strong base buffers (as a reaction occurs to produce A-, with a weak acid/salt buffer no reaction occurs)
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Adding OH- does what to a buffer?
Decreases the moles of HA and increases the moles of A- (Remember: HA + OH- ⇌ A- + H2O, acid + base ⇌ salt + water)
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Define a Bronsted-Lowry acid

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Card 4

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Card 5

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Define the ionic product of water

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