practice questions Flashcards
explain how a catalyst increases the rate of reaction
- lowers the activation energy by providing an alternate pathway
- a greater proportion of molecules have energy greater than or equal to the activation energy
describe and explain the effect of increasing the pressure on the rate of the reaction for the formation of ammonia
- increases the rate of reaction
- because molecules are closer together so there are more collisions per unit time
the reaction for the production of methanol in the presence of the copper catalyst is carried out at 200–300 °C, explain why use of the catalyst reduces energy demand and benefits the environment
- less thermal energy
- reduces carbon dioxide emissions
at room temperature and pressure, the first four members of the alkanes are all gases but the first four alcohols are all liquids, explain this difference in terms of intermolecular forces
- alcohols have hydrogen bonds
- hydrogen bonds in alcohols are stronger than the London forces in alkanes
- more energy is required to overcome the hydrogen bonds
what is meant by the term homologous series?
- a series of compounds with the same functional group
- each successive member differing by CH2
state the general formula for cycloalkanes
CnH2n
state and explain the shape around each carbon atom in cyclohexane
tetrahedral, four bonding pairs repel
state the conditions of temperature and pressure used for standard enthalpy measurements
temperature = 25°C
pressure - 100kPa
At the end of the experiment, the student adds a few drops of aqueous sodium chloride to the reaction mixture in the polystyrene cup to test whether all the aqueous silver
nitrate has reacted, explain how the results would show whether all the aqueous silver nitrate has reacted.
- if a white precipitate forms, it indicates that not all the silver nitrate has reacted
- Ag+(aq) + Cl−(aq) → AgCl(s)
what is the enthalpy change of vaporisation?
the enthalpy change when one mole of a substance changes from a liquid to a gas at its boiling point
suggest whether the enthalpy change of vaporisation of bromine is exothermic or endothermic, explain your answer
- endothermic
- energy is required to overcome the london forces
explain, in terms of bond breaking and bond forming, why a reaction can be exothermic
more energy is released by forming bonds than energy required when breaking bonds
the calculated value of ΔcH for hexane from this experiment is different from the data book value, suggest two reasons for this difference.
- heat released to the surroundings
- incomplete combustion
- non-standard conditions
define standard enthalpy change of combustion
the enthalpy change when one mole of a substance reacts fully with oxygen under standard conditions of 25°C and 100kPa
what is meant by the term average bond enthalpy?
- when one mole of bonds
- of gaseous covalent bonds is broken
