df Flashcards
what is an exothermic reaction?
§energy is lost to surroundings (delta H reduces- negative value)
what is an endothermic reaction?
energy is taken from the surroundings to cool the reaction (delta H increases- positive)
what are standard conditions?
298k (25c) and 101kpa (1atm) 1moldm-3
equation for measuring enthalpy changes:
q=mcAT
kj=jg-1k-1 x g x k
what is the value for specific heat capacity?
4.18jg-1k-1
what is definition for standard enthalpy chance of COMBUSTION?
enthalpy change that occurs when one mole of substance is burnt completely in oxygen (done under standard conditions)
what is the definition for the standard enthalpy change of FORMATION?
enthalpy change when one mole of a compound is formed from its elements (down under standard conditions)
what is the standard enthalpy change of NEUTRALISATION?
measured using the amount of energy given out when acid reacts with alkali in aqueous solution
what is specific heat capacity?
the specific heat capacity of a substance is the amount of energy needed to raise the temperature of 1kg of substance by 1k
enthalpy change of combustion
reactants to products
co2 + h2o
enthalpy change of formation
reactants to products
each element involved
what is an aromatic compound?
compound that contain one or more benzene ring
what is an aliphatic compound?
compounds that do not contain benzene rings
what is the purpose of the functional group?
modifier thats responsible for there characteristics of the chemical
define activation enthalpy
minimum amount of energy required for a reaction to take place
practical: What are the causes for less energy being transferred than expected when working out ΔH(c)
heat lost to surroundings
incomplete combustion of ‘alcohol’
evaporation of ‘alcohol’ from wick
not unde standard conditions
define average bond enthalpy
energy required to break one mole of a specific type of bond in a gaseous molecule
what are properties of bond enthalpies?
energy is always required to break bonds
they’re always endo reactions
always have positive enthalpy value
what kind of energy and enthalpy changes are bond formations?
exothermic
releases energy
what kind of energy and enthalpy changes are bond breaking?
endothermic
absorbs energy
enthalpy change of combustion:
describe an experiment
1- measure 150cm3 of water using measuring cylinder and pour it into a beaker recording its initial temperature (nearest o.5c)
2- weigh the spirit burner containing ‘alcohol’
3-place spirit burner under the beaker and then light it - stir the water whilst whilst burning with the thermometer
4- after 3 mins extinguish the flame (placing cap over) then immediately record the maximum temp reached by water
5- re-weigh the spirit burner containing ‘alcohol’ (assume wick has not been burnt- decreases weight)
suggest how to improve the experiment for determining enthalpy change of a reaction
add lid
add an insulating layer around beaker
use draft shield around apparatus
calculating the enthalpy changes from average bond enthalpies
delta h = sum of bond enthalpies in reactants - sum of bond enthalpies in products
define the standard enthalpy change of reaction
enthalpy change that accompanies a reaction in their molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.
what makes an effective collision?
particles must have sufficient energy
must collide in correct orientation
what factors can affect the rate of reaction?
concentration
use of catalyst
temperature
pH
what effect does increasing temp have on a reaction?
increase rate as it increases the kinetic energy of the particles so they will have more frequent successful collisions
what effect does increasing pressure (g) / concentration have on rate of reaction?
it will increase rate
more particles per unit of volume so more frequent successful collisions
what do catalysts do?
lowers the activation enthalpy of a reaction without undergoing any permanent changes itself (can be used multiple times)
properties of catalysts?
not used up in reaction
provide a surface for a reaction to take place on
they are regenerated at the end of reaction
what are the 2 types of catalysts?
homogeneous = same physical state as reactants
heterogeneous = different physical state to reactants
how do homogenous catalysts interact with reactants?
they form an intermediate with reactants which then breaks down to give the product and regenerated catalyst
examples of homogeneous catalysts are:
sulphuric acid in the production of ester
chlorine radicals in ozone depletion
how do heterogeneous catalysts interact with reactants?
they’re usually solids in contact with gaseous reactants or reactants in solution
act as surface for reaction to take place on
steps to reaction catalysed by a heterogenous catalyst?
1- reactants form bonds with atoms on the surface of the catalyst = ADSORBED
2- bonds in the reactants are weakened and break
3- new bonds form between the reactants to form the products
4- the bonds to the catalyst surface weaken and are broken releasing the products = DESORPTION
how are alkenes distinguished from other hydrocarbons?
by the presence of a carbon-carbon double bond
implies its an unsaturated hydrocarbon
what happens when length if alkene change increases?
boiling point increases
what is the general formula of an alkene?
CnH2n
what prevents rotation in alkenes?
the pi bond locks the 2 carbon atoms into position
what are key features of C=C?
one sigma bond and one pi bond
they are exposed and have high density
vulnerable to attack by electrophiles (electron seeking species)
how are C=C formed in alkenes?
one sp2 orbital from each carbon overlap to form a single c-c (sigma = rotation can occur around this bond)
the pi and is then formed by the two p orbitals which overlap each other sideways above and below the plane of the molecule
what is the arrangement of bonds around the C=C bond?
planar
120^
what is stereoisomerism?
have same structural formula but different spacial arrangement of atoms
2 types of stereoisomerism:
E-stereoisomers and Z-stereoisomers
how do stereoisomers occur?
there is restricted rotation around the C=C double bond
there are two different atoms attached to both ends of the double bond
what are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?
higher atomic number is higher priority. Groups of higher priority on the same side of the double bond, then Z isomer.
effects of EZ stereoisomerism on physical properties:
Z =
higher boiling point
molecule will be polar due to the atoms being on the same side (one side will be slightly more negative and vice versa)
intermolecular forces are both id-id (weakest IMF) and pd-pd
E =
lower boiling point
molecule will be non-polar as bonds of different charges are on opposite sides
dipoles cancel out
intermolecular forces are only id-id (low bp)
what type of addition do alkenes undergo?
electrophilic addition and addition polymerisation (draw mechanism out on whiteboard)
what is the industrial importance of alkenes?
formation of polymers from ethene based monomers
alkanes: fractional distillation
oil is preheated then passed into he column
the fractions condense at different temperatures
temp of column decreases upwards
separation depends on boiling points and size of molecules
its a physical process involving the splitting of weak Van Der Waals forces
ho do you conduct fractional distillation in lab conditions?
1- heat flask wit bunsen burner or electric mantle
2- causes vapours of all components to be mixed
3- vapours pass up fractionating column
4- vapours of the substance with the lowest point reaches top of the fractionating column first
5- vapours of highest boiling points condense back into the flask
6- only the most volatile vapour passes into the condenser
7- condenser cools the vapours and condenses to a liquid an is collected
what is cracking?
conversion of large hydrocarbon chains into smaller hydrocarbons by breaking C-C bonds
high Mr alkane => smaller Mr alkane + alkene + hydrogen
economic reasons for cracking:
petroleum fractions with shorter C chains are in high demand
high demand for shorter hydrocarbons and so larger chains must be cracked
products of cracking are more valuable then stating materials
what are the main types of cracking?
thermal and catalytic
conditions required for thermal cracking:
high pressure (7000kPa)
high temperature (400-900)
conditions required for catalytic cracking:
slight or moderate pressure
high temperature (450)
zeolite catalyst
2 types of combustion:
complete = in excess oxygen alkanes burn
incomplete = limited amount of oxygen (producing CO - TOXIC and/or C - sooty flame)
example of a complete combustion reaction?
C8H18 + 12.5O2 => 8CO2 + 9H2O
example of incomplete combustion reaction?
CH4 + 3/2O2 => CO + 2H2O
