Topic 5: Energetics

Question 1

energy

Answer 1

• measure of the ability to do work

- conserved in chemical reactions

Question 2

enthaply

Answer 2

• measure of the amount of energy contained in a substance
• stored in chemical bonds and intermolecular forces as potential energy
• when substances react, the different in the enthalpy between the reactants and products results in a heat change

Question 3

heat

Answer 3

a mode of energy transfer which occurs as a result of temperature difference

• increases the average kinetic energy of molecules
• its the TOTAL kinetic energy of a system

Question 4

endothermic reactions

Answer 4

• heat absorbed by system
• breaking bonds; separation of particles which are held together by a force of attraction
• enthalpy is positive
• reactants more stable than products

Question 5

exothermic reactions

Answer 5

• heat give out by system
• making bonds; bringing together of particles which haven an attrative force between them
• enthalpy is negative
• products more stable than reactants
  e. g. combustion and neutralization reactions

Question 6

standard conditions for enthalpy changes

Answer 6

• a pressure of 100 kPa
• concentrations of 1 mol dm^-3 for all solutions
• all substances in their standard states

Question 7

standard state of a substances

Answer 7

pure form of the substance under stnadard condtions

Question 8

temperature

Answer 8

• meaure of the average kinetic energy of the particles

- Kelvin scale; measures relationship between kinetic energy and temperature (298K usually given as standard)

Question 9

what does the increase of temperature of an object when heated depend on?

Answer 9

1. mass of the object (shared vaue of kinetic energy among particles)
2. heat added (amount of kinetic energy)
3. nature of the substance

Question 10

specific heat capacity

Answer 10

• property of a substance which gives the heat needed ot icnrease the tempreature of unit mass by 1 k
• depends on number of particules in unit mass sample

Question 11

heat change formula

Answer 11

q=mc@T

q= heat change in joules
m= mass in grams
c= specific heat capacity
@T= change in temperature  (in kelvin)

Question 12

specific heat capacity of water

Answer 12

4.18 J g^-1 K^-1

Question 13

heat capacity formula

Answer 13

heat change (q)/temperature change (@T)

Question 14

standard enthalpy of combustion

Answer 14

enthalpy change for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions

Question 15

some reasons for differences in values of combustion reactions + ib data booklet values?

Answer 15

1. not all heat produced by combustion transferred to water
2. incomplete combustion due to imited oxygen
3. experiment not performed under standard conditions

Question 16

how can the enthalpy change of a solution be measured?

Answer 16

carrying out reaction in insulated system (e.g. polysterene cup) as heat release or absorved by reaction can be measured from the water temperature change

Question 17

hess law

Answer 17

states that the enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and final conditions, and reactants and products are the same

Question 18

what happens when you reverse the direction of a reaction?

Answer 18

you reverse the sign of the enthalpy value

Question 19

standard enthalpy of formation

Answer 19

enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under standard condtions of

1. 298 K (25 C)
2. 1.00 x 10^5 Pa

Question 20

standard enthalpy change of formation in its stable form?

Answer 20

its zero, as no enthalpy change occurs when an element is formed from itself

Question 21

Hf formula

Answer 21

AH of reaction= AH of Products- AH of Reactants

Question 22

bond enthalpy

Answer 22

energy needed to break one mole of bonds in gaseous molecules under standard conditions

Question 23

why do we use average bond enthalpies?

Answer 23

• to take into account bond enthalpies and compare them which exist in different environments
• different enthalpies of atoms bonded in different ways

Question 24

average bond enthalpies

Answer 24

energy needed to break one mole of bonds in gasesous molecules under standard conditions averaged over similar compounds

Question 25

oxygen forms

Answer 25

• O2; double bond
• O3; oxygen to oxygen bond is between a single and double bond
• both protect earths surface by UV light radaition ((form protective screen))

Question 26

oxygen

Answer 26

• double bond in O2 stronger than 1.5 bonds in izone; broken by a radiation of higher energy and shorter wavelengths

Question 27

ozone

Answer 27

• 1.5 bond length

- longer wavelength radiation needed to break bonds

Question 28

Ephoton

Answer 28

plance equation; hv

Question 29

Free radicle

Answer 29

species with an unpaired election

Question 30

natural formation and depletion of ozone

Answer 30

• UV breaks down oxygen and ozone covalent bonds for form free radicles
• oxygen and oxygen radicals react to form radical (exxoterhmic)
• continous reactions of ozone with free radicles and ozygen with free radicals creates Chapman Cycle