Enthalpy

Question 1

which is more energetically stable?

graphite or diamond?

Answer 1

graphite

Question 2

definition of Hess law

Answer 2

the total enthalpy change of a chemical reaction is independent of the pathway between the initial and final states

Question 3

can combustion continues spontaneously whenever it started?

Answer 3

can.

the energy released provides activation energy to the unreacted chemicals

Question 4

why experimental and actual enthalpy change is different

Answer 4

Not performing the experiment in standard condition
Heat transfer to the surrounding
The heat capacity of the container is neglected.

Question 5

state the standard condition

Answer 5

1atm 298K

in standard state

Question 6

can the standard enthalpy change of formation of NH3 be determined directly from experiment

Answer 6

No
all reactants and products are gas so its hard to measure the temperature change
also it is not a must for the reactant to form NH3

Question 7

why the combustion is exothermic in terms of the breaking and forming of covalent bond

Answer 7

energy released in the bond forming process is larger than the energy absorbed in the bond breaking process

Question 8

what is an exothermic reaction

Answer 8

a reaction which release heat to the surrounding

Question 9

why the enthalpy change of neutralization involving weak acid or base is lower

Answer 9

some heat is absorbed for their ionization

ionization involves breaking bond so its endothermic

Question 10

when will the heat change be considered as enthalpy change

Answer 10

constant pressure

Question 11

species lower in enthalpy is more energetically stable

T/F

Answer 11

T

so exothermic reaction tends to be spontaneous (not a must)

Question 12

define standard enthalpy change of neutralisation

Answer 12

standard enthalpy change of neutralization is the enthalpy change when 1 mole of water is formed from the neutralization of an acid and base under standard condition

Question 13

why less energy will be released if the neutralisation reaction involves a weak acid or a weak alkali

Answer 13

some energy is absorbed for ionisation of the weak acid or weak alkali

Question 14

define standard enthalpy change of combustion

Answer 14

standard enthalpy change of combustion is the enthalpy change when 1 mole of the substance is completely burnt in oxygen under standard conditions

Question 15

define standard enthalpy change of formation

Answer 15

standard enthalpy change of formation is the enthalpy change for the formation of 1 mole of substance from its elements in their standard states under standard conditions

Question 16

how can enthalpy change of a reaction be determined?

Answer 16

by experiments

using Hess’s law

Question 17

why should we use a calorimeter instead of a beaker to carry out the calorimetric experiment

Answer 17

to prevent heat loss to the surrounding

Question 18

why should zinc powder instead of granule be used when determining the heat change in the experiment?

Answer 18

faster reaction rate

reduce the error due to the heat loss to the surrounding

Question 19

suggest why the metal can holding the water should be very thin and light in the experiment of determining the enthalpy change of combustion

Answer 19

to minimize the heat absorbed by the metal can

Question 20

why the enthalpy changes of some reactions cannot be determined directly by experiment under standard condition

Answer 20

explosive reaction (K+HCl)
reaction rates are too slow (rusting)
formation of side products (C+ O2)

Question 21

what is the major apparatus needed for determining the enthalpy change through experiment

Answer 21

A well-insulated expanded polystyrene cup with a lid, a thermometer , a stirrer and an electric balance

Question 22

why is it difficult to determine the enthalpy change of formation of MgO from its element in school laboratory directly

Answer 22

the extent of reaction cannot be controlled
the direct combustion of magnesium can be violent
side reaction may occur

Question 23

compound of Al toxic?

Answer 23

no
it is non-toxic
so it can be used to contain food

Question 24

state one assumption while calculating enthalpy change

Answer 24

the Hess’ law is followed

Question 25

need to rmb wt when drawing enthalpy change cycle

Answer 25

use stateeeeeesssss
rmb to write the O2 add
C ‘s state should be graphite not (s)