DF 4, 5 Flashcards
what is a bond enthalpy
the energy needed to break one mole of bond into separate atoms all in a gaseous state
why are bond enthalpies positive values
because breaking a bond is an endothermic process that requires energy
what does a bond enthalpy indicate
how strong bonds are
how does a bond form between atoms
-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
what does a short bond length mean
- the stronger the attraction between atoms, the higher the bond enthalpy
what is chemical data of bond enthalpies deduced from, and why
- they are average bond enthalpies because the exact value of a bond enthalpy depends on the compound its in
what is chemical data of bond enthalpies deduced from, and why
- they are average bond enthalpies because the exact value of a bond enthalpy depends on the compound its in
why is it hard to measure bond enthalpies
- 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
define cracking
any reaction which a larger molecule is made into smaller molecules
how is petrol formed
the cracking if heavy oils in the presence of a catalyst
define catalyst
a substance that speeds up the rate of reaction that can be recovered without being chemically unchanged at the end
what is homogeneous catalysis
when reactants and catalysts are inthe same physical state e.g. both are in aqueous solution
what is heterogeneous catalysis and how does it work
- 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
what is an important feature in heterogeneous catalysis
- 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
what is catalyst poisoning and how does it work in heterogeneous catalysis
- 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
why is it not possible to replace expensive metals used in catalytic converters with cheap metals
- 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
how is catalyst poisoning a problem in the Haber process
- 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
how is catalytic poisoning fixed in the cracking of long-chain hydrocarbons
- 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
define electrophile
- 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
describe the bonding in alkenes
- 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
define addition reaction
a reaction where two or more molecules react to form a single larger molecule
what kind of reaction is ethene + bromine
addition reaction
electrophilic addition
how do you conduct the test for alkenes
react bromine water with the alkene, bromine water should go from brown to colourless
why do alkenes react with HBr at room temperature
- because it is a polar solvent
