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chemistry ocr b > df > Flashcards

df Flashcards

1
Q

what is an exothermic reaction?

A

§energy is lost to surroundings (delta H reduces- negative value)

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2
Q

what is an endothermic reaction?

A

energy is taken from the surroundings to cool the reaction (delta H increases- positive)

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3
Q

what are standard conditions?

A

298k (25c) and 101kpa (1atm) 1moldm-3

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4
Q

equation for measuring enthalpy changes:

A

q=mcAT
kj=jg-1k-1 x g x k

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5
Q

what is the value for specific heat capacity?

A

4.18jg-1k-1

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6
Q

what is definition for standard enthalpy chance of COMBUSTION?

A

enthalpy change that occurs when one mole of substance is burnt completely in oxygen (done under standard conditions)

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7
Q

what is the definition for the standard enthalpy change of FORMATION?

A

enthalpy change when one mole of a compound is formed from its elements (down under standard conditions)

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8
Q

what is the standard enthalpy change of NEUTRALISATION?

A

measured using the amount of energy given out when acid reacts with alkali in aqueous solution

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9
Q

what is specific heat capacity?

A

the specific heat capacity of a substance is the amount of energy needed to raise the temperature of 1kg of substance by 1k

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10
Q

enthalpy change of combustion

A

reactants to products
co2 + h2o

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11
Q

enthalpy change of formation

A

reactants to products
each element involved

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12
Q

what is an aromatic compound?

A

compound that contain one or more benzene ring

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13
Q

what is an aliphatic compound?

A

compounds that do not contain benzene rings

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14
Q

what is the purpose of the functional group?

A

modifier thats responsible for there characteristics of the chemical

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15
Q

define activation enthalpy

A

minimum amount of energy required for a reaction to take place

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16
Q

practical: What are the causes for less energy being transferred than expected when working out ΔH(c)

A

heat lost to surroundings
incomplete combustion of ‘alcohol’
evaporation of ‘alcohol’ from wick
not unde standard conditions

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17
Q

define average bond enthalpy

A

energy required to break one mole of a specific type of bond in a gaseous molecule

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18
Q

what are properties of bond enthalpies?

A

energy is always required to break bonds
they’re always endo reactions
always have positive enthalpy value

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19
Q

what kind of energy and enthalpy changes are bond formations?

A

exothermic
releases energy

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20
Q

what kind of energy and enthalpy changes are bond breaking?

A

endothermic
absorbs energy

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21
Q

enthalpy change of combustion:
describe an experiment

A

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)

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22
Q

suggest how to improve the experiment for determining enthalpy change of a reaction

A

add lid
add an insulating layer around beaker
use draft shield around apparatus

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23
Q

calculating the enthalpy changes from average bond enthalpies

A

delta h = sum of bond enthalpies in reactants - sum of bond enthalpies in products

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24
Q

define the standard enthalpy change of reaction

A

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.

25
Q

what makes an effective collision?

A

particles must have sufficient energy
must collide in correct orientation

26
Q

what factors can affect the rate of reaction?

A

concentration
use of catalyst
temperature
pH

27
Q

what effect does increasing temp have on a reaction?

A

increase rate as it increases the kinetic energy of the particles so they will have more frequent successful collisions

28
Q

what effect does increasing pressure (g) / concentration have on rate of reaction?

A

it will increase rate
more particles per unit of volume so more frequent successful collisions

29
Q

what do catalysts do?

A

lowers the activation enthalpy of a reaction without undergoing any permanent changes itself (can be used multiple times)

30
Q

properties of catalysts?

A

not used up in reaction
provide a surface for a reaction to take place on
they are regenerated at the end of reaction

31
Q

what are the 2 types of catalysts?

A

homogeneous = same physical state as reactants
heterogeneous = different physical state to reactants

32
Q

how do homogenous catalysts interact with reactants?

A

they form an intermediate with reactants which then breaks down to give the product and regenerated catalyst

33
Q

examples of homogeneous catalysts are:

A

sulphuric acid in the production of ester
chlorine radicals in ozone depletion

34
Q

how do heterogeneous catalysts interact with reactants?

A

they’re usually solids in contact with gaseous reactants or reactants in solution
act as surface for reaction to take place on

35
Q

steps to reaction catalysed by a heterogenous catalyst?

A

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

36
Q

how are alkenes distinguished from other hydrocarbons?

A

by the presence of a carbon-carbon double bond
implies its an unsaturated hydrocarbon

37
Q

what happens when length if alkene change increases?

A

boiling point increases

38
Q

what is the general formula of an alkene?

A

CnH2n

39
Q

what prevents rotation in alkenes?

A

the pi bond locks the 2 carbon atoms into position

40
Q

what are key features of C=C?

A

one sigma bond and one pi bond
they are exposed and have high density
vulnerable to attack by electrophiles (electron seeking species)

41
Q

how are C=C formed in alkenes?

A

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

42
Q

what is the arrangement of bonds around the C=C bond?

A

planar
120^

43
Q

what is stereoisomerism?

A

have same structural formula but different spacial arrangement of atoms

44
Q

2 types of stereoisomerism:

A

E-stereoisomers and Z-stereoisomers

45
Q

how do stereoisomers occur?

A

there is restricted rotation around the C=C double bond
there are two different atoms attached to both ends of the double bond

46
Q

what are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?

A

higher atomic number is higher priority. Groups of higher priority on the same side of the double bond, then Z isomer.

47
Q

effects of EZ stereoisomerism on physical properties:

A

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)

48
Q

what type of addition do alkenes undergo?

A

electrophilic addition and addition polymerisation (draw mechanism out on whiteboard)

49
Q

what is the industrial importance of alkenes?

A

formation of polymers from ethene based monomers

50
Q

alkanes: fractional distillation

A

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

51
Q

ho do you conduct fractional distillation in lab conditions?

A

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

52
Q

what is cracking?

A

conversion of large hydrocarbon chains into smaller hydrocarbons by breaking C-C bonds
high Mr alkane => smaller Mr alkane + alkene + hydrogen

53
Q

economic reasons for cracking:

A

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

54
Q

what are the main types of cracking?

A

thermal and catalytic

55
Q

conditions required for thermal cracking:

A

high pressure (7000kPa)
high temperature (400-900)

56
Q

conditions required for catalytic cracking:

A

slight or moderate pressure
high temperature (450)
zeolite catalyst

57
Q

2 types of combustion:

A

complete = in excess oxygen alkanes burn
incomplete = limited amount of oxygen (producing CO - TOXIC and/or C - sooty flame)

58
Q

example of a complete combustion reaction?

A

C8H18 + 12.5O2 => 8CO2 + 9H2O

59
Q

example of incomplete combustion reaction?

A

CH4 + 3/2O2 => CO + 2H2O

chemistry ocr b (32 decks)

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