Energetics I

Question 1

what is the enthalpy change of a reaction?

Answer 1

heat energy change in a reaction at constant pressure (kJ mol-1)

Question 2

⦵ (standard conditions)

Answer 2

298 K and 100 kPa

Question 3

what is the standard enthalpy change of a reaction?

Answer 3

enthalpy change when reaction occurs in the molar quantities shown in the chemical equation under standard conditions

Question 4

standard enthalpy change of formation
ΔH f⦵

Answer 4

enthalpy change when one mole of a compound is formed from its elements in their standard state under standard conditions

Question 5

enthalpy change of combustion

Answer 5

enthalpy change when one mole of substance is completely burned in oxygen gas under standard conditions

Question 6

what does ⦵ mean

Answer 6

standard conditions-298 K and 100 kPa

Question 7

standard enthalpy change of neutralisation

Answer 7

enthalpy change when an acid and alkali react together under standard conditions to form one mole of water

Question 8

enthalpy changes of neutralisation are always…

Answer 8

exothermic

Question 9

what is the specific heat capacity used of water?

Answer 9

4.18

Question 10

is Ea shown in enthalpy level diagrams?

Answer 10

no, only in reaction profile diagrams

Question 11

why is an extrapolation is required to determine an accurate measure of the maximum temperature change?

Answer 11

the reaction is not instantaneous (it takes time to release heat) so some heat is lost as mixture heats up

Question 12

what is bond enthalpy?

Answer 12

amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase

Question 13

what is mean bond enthalpy?

Answer 13

energy needed to break one mole of bonds in the gas phase averaged over many different compounds

Question 14

why is the value of enthalpy change (ΔH) negative in exothermic reactions?

Answer 14

because exothermic reactions give out heat energy so the temperature rises

Question 15

why is the value of enthalpy change (ΔH) positive in endothermic reactions?

Answer 15

because heat energy is absorbed so the temperature falls

Question 16

what type of reaction is bond making?

Answer 16

exothermic (ΔH is negative)

Question 17

what type of reaction is bond breaking?

Answer 17

endothermic (ΔH is positive)

Question 18

what is calorimetry?

Answer 18

measuring heat changes

Question 19

what is Hess’s law?

Answer 19

total enthalpy change of a reaction is always the same no matter the route taken
route 1=route 2

Question 20

what is the ΔH f⦵ for elements?

Answer 20

0

Question 21

how to calculate enthalpy change?

Answer 21

total energy to break bonds - total energy released forming bonds

Question 22

what is calorimetry used for?

Answer 22

to work out the enthalpy change of combustion

Question 23

what is ‘q’

Answer 23

heat energy lost/gained

Question 24

why are bond enthalpies always positive?

Answer 24

because breaking bonds is an endothermic process

Question 25

in combustion hess cycles what way are the arrows turned?

Answer 25

downwards

Question 26

in formation hess cycles what way are the arrows turned?

Answer 26

upwards

Question 27

what are the limitations of mean bond enthalpy?

Answer 27

in the data book the mean bond enthalpy may be different because its the average for a much bigger range of molecules

Question 28

why is there a slight variation between the enthalpy change of reaction calculated from mean bond enthalpies and the true enthalpy change of reaction?

Answer 28

the specific bond enthalpies of the molecules in a reaction will be slightly different from the average values