topic 8: energetics 1

Question 1

enthalpy change defintion

Answer 1

heat energy change measured at constant pressure

Question 2

standard conditions

Answer 2

• 100 kPa
• 298 K

Question 3

exothermic delta h

Answer 3

negative

Question 4

endothermic delta h

Answer 4

positive

Question 5

standard enthalpy change of formation defintion

Answer 5

-enthalpy change when 1 mole of the compound
-is formed from its elements under standard conditions
- under standard conditions 100 kPa and 298 K
e.g Mg(s) + Cl2(g) -> MgCl2(s)

Question 6

enthalpy of formation of an element

Answer 6

0

Question 7

standard enthalpy change of combustion

Answer 7

-enthalpy change when 1 mole of a substance is completely combusted in oxygen
-under standard conditions 100 kPa and 298 K

Question 8

compare exothermicity of incomplete vs compete combustion

Answer 8

incomplete is LESS EXOTHERMIC than complete

Question 9

enthakpy change of reaction defintion

Answer 9

enthalpy change when the number of moles of reactants as specified in the balanced equation react together

Question 10

enthalpy change of neutralisation defintion

Answer 10

enthalpy change when acid and alkali react togetehru nder standard conditions to produce 1 mole of water

Question 11

enthalpy changes of neutralisation are …

Answer 11

EXOTHERMIC

Question 12

equation for energy transferred (Joules)

Answer 12

Q = m * c* delta T

Question 13

in calrometry, state effect if you: double volume of excess solution

Answer 13

no effect

Question 14

in calrometry, state effect if you: do the experiment in a glass beaker

Answer 14

• heat loss to surroundings
• smaller delta T
• delta h is LESS EXOTHERMIC

Question 15

in calrometry, state effect if you: USE LARGE CHUNKS OF SOLID

Answer 15

• slower reaction rate so more time for heat loss to surroundings
• q smaller
• delta h less exothermic
  TO IMPROVE USE AN EXTRAPOLATION CURVE

Question 15

in calrometry, state effect if you: double mass of solid

Answer 15

• q is doubled
• n is double
• no effect on delta h

Question 16

in calrometry, state effect if you: initial temperature of solution is higher

Answer 16

• more heat loss at a faster rate
• smaller delta t, q, delta H

Question 17

in calrometry, state effect if you: use more concentrated solution but in excess

Answer 17

• no effect

Question 18

in calrometry, state effect if you: use a fine powder

Answer 18

• larger SA
• faster rate of reaction so less heat loss
• larger values

Question 19

largest source of error in calorimetry and how to fix?

Answer 19

• heat loss to surroundings
• do a temperature correction curve w/ extrapolation

Question 20

why not use a larger volume of acid in calorimetry is its in excess

Answer 20

• already in excess so NO NEED
• temperature change becomes smaller , so higher percentage error

Question 21

mean bond enthalpy

Answer 21

enthalpy needed to break the covalent bond in gaseous atoms, averaged over different molecules

Question 22

bond enthalpies are always …

Answer 22

POSITIVE
- as bond breaking is endothermic

Question 23

problem with mean bond enthalpies

Answer 23

• average so not for specific bond
• assumed for GAS, if a liquid or solid more energy required to change state

Question 24

why is experimental values of bond enthalpy less acurate?

Answer 24

• heat loss to surroundings
• incomplete combustion so less energy released

Question 25

why use a hess cycle?

Answer 25

• hard to measure temperature of a solid
• FOR ENDO: hard to add heat and measure temperature change at the same time

Question 26

mass in calrometry

Answer 26

• mass of SOLUTIONS
• for combustion, mass of WATER

Question 27

MEAN BOND ENTHALPY

Answer 27

reactants - products

Question 28

error calrometry COMBUSTION

Answer 28

• heat loss to surroundings
• EVAPORATION OF SOLUTIOM
• incomplete combustion
• specific heat capacity of apparatus ignored

Question 29

delta H from BONDS

Answer 29

reactants minus products

Question 30

delta H from anything bUT bonds

Answer 30

products minus reactants

Question 31

effect of smoky flame on experimental combustio value

Answer 31

• incomplete combustion
• value is LESS EXOTHERMIC