DF 4, 5

Question 1

what is a bond enthalpy

Answer 1

the energy needed to break one mole of bond into separate atoms all in a gaseous state

Question 2

why are bond enthalpies positive values

Answer 2

because breaking a bond is an endothermic process that requires energy

Question 3

what does a bond enthalpy indicate

Answer 3

how strong bonds are

Question 4

how does a bond form between atoms

Answer 4

-atoms move together due to the attractive forces between the nuclei and the electrons
-there are repulsive forces between the two nuclei
-repulsion gets bigger, the closer they get together until they stop moving closer
-the ultimate distance between them is the equilibrium bond length

Question 5

what does a short bond length mean

Answer 5

• the stronger the attraction between atoms, the higher the bond enthalpy

Question 6

what is chemical data of bond enthalpies deduced from, and why

Answer 6

• they are average bond enthalpies because the exact value of a bond enthalpy depends on the compound its in

Question 6

what is chemical data of bond enthalpies deduced from, and why

Answer 6

• they are average bond enthalpies because the exact value of a bond enthalpy depends on the compound its in

Question 7

why is it hard to measure bond enthalpies

Answer 7

• because there is often more than one type of bond in a compound
• Its also very difficult to make measurements when everything is in a gaseous state
• so they are measured indirectly using enthalpy cycles

Question 8

define cracking

Answer 8

any reaction which a larger molecule is made into smaller molecules

Question 9

how is petrol formed

Answer 9

the cracking if heavy oils in the presence of a catalyst

Question 10

define catalyst

Answer 10

a substance that speeds up the rate of reaction that can be recovered without being chemically unchanged at the end

Question 11

what is homogeneous catalysis

Answer 11

when reactants and catalysts are inthe same physical state e.g. both are in aqueous solution

Question 12

what is heterogeneous catalysis and how does it work

Answer 12

• where the catalysts and the reactants are in physically different states
  1. the reactants form bonds with the atoms on the surface of the catalyst, they are adsorbed onto the surface
  2. as a result, the bonds in the reactant weaken and break
  3. new bonds form between the reactants, held together close on the surface, to form the products
  5. this weakens their bond to the catalyst and the products are released

Question 13

what is an important feature in heterogeneous catalysis

Answer 13

• that the catalyst has a large surface area for contact with reactants
• so solid catalysts are usually used in a finely divided or as a wire mesh
• sometimes supported on a porous material to increase the surface area and prevent it from crumbling
• transition metals are often used

Question 14

what is catalyst poisoning and how does it work in heterogeneous catalysis

Answer 14

• the poison molecules are adsorbed more strongly onto the catalyst surface than the reactant molecules, catalyst can’t bond with the reactant, reactant can’t access their active sites so it becomes inactive

Question 15

why is it not possible to replace expensive metals used in catalytic converters with cheap metals

Answer 15

• the cheaper metals are very vulnerable to catalyst poisoning by sulfur dioxide present in car exhausts
• once it becomes inactive, it cannot be regenerated, a new converter must be fitted which is very costly

Question 16

how is catalyst poisoning a problem in the Haber process

Answer 16

• nearly all the hydrogen in the Haber process is prepared by steam reforming of methane
  CH4 + H2O —-> (Ni) —-> CO + 3H2
• sometimes the feedstock contains sulfur compound which must be removed to prevent severe catalytic poisoning

Question 17

how is catalytic poisoning fixed in the cracking of long-chain hydrocarbons

Answer 17

• carbon is produced and the surface of the catalyst becomes covered in a layer of soot
• this blocks the adsorption of the reactant molecules and the activity of the catalyst is reduced
• hot air is blown through the powder and the oxygen in the air is converted to carbon dioxide and cleans the catalyst surface

Question 18

define electrophile

Answer 18

• a positive ion or molecule with a partial positive charge, that will be attracted to a negatively charged region and ACCEPT A LONE PAIR of electrons to form a covalent bond

Question 19

describe the bonding in alkenes

Answer 19

• consists of single bond and double bonds
• • they are both regions of high electron density
• single bond is called sigma bonds
• double bonds are pi bonds that consist of two regions of negative charge, above and below

Question 20

define addition reaction

Answer 20

a reaction where two or more molecules react to form a single larger molecule

Question 21

what kind of reaction is ethene + bromine

Answer 21

addition reaction
electrophilic addition

Question 22

how do you conduct the test for alkenes

Answer 22

react bromine water with the alkene, bromine water should go from brown to colourless

Question 23

why do alkenes react with HBr at room temperature

Answer 23

• because it is a polar solvent

Question 24

how do alkenes react with water, what is produced

Answer 24

- high temperature - high pressure - in the presence of a catalyst - alcohols are produced

Question 25

how are alkenes converted into ethanol

Answer 25

- addition of concentrated sulfuric acid - then diluting with water

Question 26

how do alkenes react with hydrogen

Answer 26

- catalyst is needed to break the strong H-H bonds and form hydrogen atoms that can react wi8th an alkene - platinum catalyst is used - process takes place in standard laboratory - if nickel is used instead of platinum , it must be finely powdered and gas must be 150 degrees with 5 atm