cap 4 - organic chemistry and reaction rates Flashcards
why is carbon able to form such a diverse range of compounds?
2, 4 electron configuration (4 valence) ⇒ ability to covalently form 4 more single bonds ⇒ variety of arrangements (single, double, triple) ⇒ forms many compounds
hydrocarbons
organic compounds, based on hydrogen and carbon atoms
aliphatic
carbon atoms arranged in open chains ⇒ can be saturated or unsaturated
aromatic
carbon atoms arranged in closed rings
functional groups
atom or group of atoms responsible for typical chemical reactions, determining patterns of reactivity
homologous series
group of molecules with same functional group, but different number of carbons in the main chain
alkanes
single bonds only
saturated compounds
C(n)H(2n+2)
alkenes
at least one double bond
unsaturated compound
C(n)H(2n)
benzenes
unsaturated hydrocarbon of a perfectly hexagonal, flat ring structure consisting of 3 double and single bonds (rapid equilibrium model), and a cloud of delocalised electrons above and below the ring; colourless liquid at room temp
formula for cycloalkanes
C(n)H(2n)
formula for cycloalkenes
C(n)H(2n-2)
saturated hydrocarbons
consisting of only single bonds (bonded to the maximum number of hydrogens)
unsaturated hydrocarbons
consisting of at least one double or triple bond, thus being more reactive (double bond broken to accommodate addition of more hydrogen atoms)
isomerism
same molecular formula but a different spatial arrangement (structural or geometrical)
structural isomerism
varying physical structure (can be chain, positional, functional group)
chain isomerism
consisting of branches and additional substituents
substituents
additional “branches” or “stems”, named as according to where they are and how many carbons they have
positional isomerism
location of double bonds
functional group isomerism
location of functional groups
geometrical isomerism (cis-trans)
structural rotations around C-C double bonds
sigma bonds
- single bonds, strongest out of all bonds
- rotational symmetry (NO geometrical isomerism)
pi bonds
- double or triple bonds
- weaker and more reactive
- rotation causes weaker bond to “snap” or break
- has different symmetry (geometric isomerism)
- much more electron deficient ⇒ much more susceptible to reactions
reactivity of hydrocarbons
unsaturated hydrocarbons are MORE reactive than saturated hydrocarbons;
for alkanes to react, highly stable sigma bonds must be broken (requiring large amounts of energy to break the stronger bonds) ⇒ large input energy ⇒ relatively higher activation energy ⇒ lower rate of reaction ⇒ lower reactivity
substitution reactions
type of chemical reaction where an atom or functional group of a molecule is replaced by another atom or functional group
substitution of alkanes (energy)
UV light supplies sufficient energy to meet the activation energy of the reaction => breaking bonds between halogens (diatomic molecules) => atoms in high energy state can react with alkane to substitute with hydrogen
substitution of benzenes (catalyst)
halogens involved in substitution aren’t electrophilic enough to break the aromatic nature of benzene, so a catalyst is used to attract electrons for substitution
e.g. nickel catalyst for hydrogenation
addition reaction
type of chemical reaction where atoms are added across the double bond of an alkene to form an alkane or a substituted alkane, breaking pi bond to form another sigma bond
why do addition reactions happen so quickly
sigma is stronger and more energetically favourable to form, being more stable, so weaker pi bond will more readily break to form a stabler sigma bond
test for unsaturation
add few drops of bromine water (orange) or iodine water (brown) into the tested liquid, and shake
two layers would form, since nonpolar hydrocarbons are insoluble in the polar solution (halogens bottom layer)
if unsaturated, would undergo addition reaction and decolorise
complete combustion reactions
hydrocarbon + excess oxygen → carbon dioxide + water + energy
incomplete combustion reactions
hydrocarbon + insufficient oxygen → carbon monoxide + water + energy
hydrocarbon + insufficient oxygen → carbon + water + energy
less exothermic than complete combustion, i.e. releasing lots of energy per unit; soot is air pollution, carbon monoxide is poisonous gas
pressure
force exerted per unit area by collisions between gas particles and interior surfaces of container (gases consist of particles in constant random motion), measured in pascals (Pa)
temperature
measure of average molecular kinetic energy
activation energy
minimum amount of energy required for particles to react
minimum amount of energy required to form the activation complex
(energy needed for the bonds of the reactants to break)
transition state (activation complex)
state corresponding to the highest potential energy (particular configuration) along the reaction coordinate
activation complex = temporary, highly unstable formation (bonds of reactants broken, bonds of products not yet formed)
collision theory
- reactants must collide
- with sufficient, activation energy
- at appropriate orientation
rate of reaction depends on
- frequency of collisions
- proportion of successful collisions
rate of reaction
NUMBER OF SUCCESSFUL COLLISIONS OVER A GIVEN TIME
change in concentration of reactants or products per unit time (speed of reaction)
measured using indicators of reactions (mass, colour, volume, pH, concentration) => volume of gas produced, loss of mass, change in transparency
slows over time (concentration of reactants decreases, reduced frequency of collisions)
key factors for rate of reaction
temperature
surface area
volume / pressure
presence of catalysts
concentration
concentration of dissolved reactants
higher concentration ⇒ more particles in the same amount of space ⇒ increased frequency of collisions, same proportion of collisions that are successful ⇒ more successful collisions (same chance but more collisions)
pressure of gaseous reactants (volume of container)
decreased volume, increased pressure ⇒ smaller space for gas particles, closer together ⇒ increased frequency of collisions, same proportion of collisions that are successful ⇒ more successful collisions (same chance but more collisions)
surface area of solid reactants
(all reactions with solids occur on the surface) split into several pieces ⇒ increased surface area ⇒ increased area for reactants particles to collide with ⇒ increased frequency of collisions, same proportion of collisions that are successful ⇒ more successful collisions (same chance but more collisions)
temperature
higher temperature ⇒ higher average kinetic energy of particles
increased average velocity of particles ⇒ increased frequency of collisions ⇒ more successful collisions (same chance but more collisions)
colliding with more energy (increased number of collisions having sufficient activation energy) ⇒ increased proportion of collisions that are successful
catalyst
a substance that speeds up a chemical reaction without being consumedz
presence of a catalyst
offers an alternate pathway for the reaction with a lower activation energy, without directly lowering the activation energy
greater proportion of collisions have sufficient energy to overcome activation energy and form transition complex ⇒ greater proportion of successful collisions
maxwell-boltzmann distribution curve
demonstrates the proportion of particles that are expected to successfully undergo a collision and thus a chemical reaction
sources of error for gas experiment (inverted cylinder)
(random) state of subdivision and surface area : volume ratio of marble chips
(random) parallax error in reading inverted measuring cylinder
(systematic) loss of gas produced; systematic delay of placing stopper
(systematic) loss of gas by dissolution in water
sources of error for “cross” concentration experiment
(random) parallax error, dependent on perception, to determine when cross disappears
(systematic) “degree” of swirling, where agitation increases frequency of collisions
(systematic) human reaction time
sources of error for the “cross” temperature experiment
(random) parallax error, dependent on perception, to determine when cross disappears
(systematic) “degree” of swirling, where agitation increases frequency of collisions
(systematic) calibration of the water bath, assuming exact temperature of thermometer
(systematic) temperature lost to surrounds
(systematic) human reaction time
