Ppq Flashcards
- use a colorimeter to measure colour changes with time
- use a pH meter to measure pH changes with time
- use a gas syringe to measure changes in volume with time
Why cant a reaction be used to measure the enthalpy change for the reaction
Cannot directly measure enthalpy change but u can measure change in temperature
Suggest a reason why bond enthalpies are described as being average bond enthalpies
An average is used since each individual bond will be in a different environment and therefore have a different strength
Describe the bonding in propene and use this to explain its reactivity
- alkenes contain sigma and pi bons
- sigma bonds are formed from end-on orbital overlap
- pi bonds are formed from sideways overlap of p orbitals
- the pi bond gives a region of high electron density
- this is susceptible to electrophilic attack which can lead to addition reactions
What would be the effect on the initial rate of reaction if the halogen in the
halogenoalkane were changed from chlorine to bromine? You should assume that this is the only change made. Explain your answer.
The rate would be faster because the C-X bond needs to be broken and this is weaker in C-Br than in C-Cl. This outweighs the effect of the greater dipole of C-Cl
Explain how the apparatus used in a reflux results in the process of reflux
The liquid evaporates, the vapour then goes into the condenser and cools and goes back to liquid
Why is it sometimes necessary to use a reflux technique?
- the reaction is slow
- allows time for equilibrium to be established
- stops the reactants boiling away
Define average bond enthalpy
average value of the enthalpy
required to break a given type of
covalent bond in the molecules of a
gaseous species.
Define bond enthalpy
Average energy required to break 1 mol of covalent bond all in their gaseous phase
