R1.2 Energy cycles in reactions

Question 1

what is a chemical bond

Answer 1

the force of attraction between 2 atoms

Question 2

energy required to break a chemical bond

Answer 2

bond dissociation energy (bond enthalpy/energy)

Question 3

bond breaking

Answer 3

Absorbs energy
Endothermic
Energy IN

Question 4

bond forming

Answer 4

Releases energy
Exothermic
Energy OUT

Question 5

a reaction is exothermic if

Answer 5

more energy is released when new bond are formed, the products are more stable than the reactants

Question 6

a reaction is endothermic if

Answer 6

more energy is required to break bonds than energy released when new bonds are formed, products are less stable than reactants

Question 7

why does bond enthalpy data differ from those measured experimentally

Answer 7

• Bond enthalpy data are average values and differ from those measured experimentally
• Average bond energy = energy needed to break 1 mole of bonds in a gaseous molecule averaged over similar compounds
• Every single bond in a compound has slightly different bond enthalpy

Question 8

1st law of thermodynamics

Answer 8

every cannot be changed or destroyed, it can only change from

Question 9

enthalpy cycle

Answer 9

• Enthalpy cycle give an indirect route to calculate enthalpy change for a reaction using known enthalpy changes of other reactions
• It is hard to measure the enthalpy change of some reactions directly

Question 10

Hess’s Law

Answer 10

• states that the enthalpy change for a reaction is independent of the pathway between the initial and final states
• the enthalpy change of a reaction is the same regardless of whether A and B react directly to form C, or whether the reaction proceeds via intermediate D and E

Question 11

what is a Born-Haber cycle and what is it used to show

Answer 11

• an application of Hess’s Law
• used to show energy changes in the formation of an ionic compound

Question 12

first ionisation energy

Answer 12

ΔH when each atom in 1 mole of gaseous atoms loses 1 electron to form 1 mole of gaseous 1+ ions

Question 13

second ionisation energy

Answer 13

ΔH when each ion in 1 mole of gaseous 1+ ions loses 1 electron to form 1 mole of gaseous 2+ ions

Question 14

enthalpy of atomisation

Answer 14

• (using sublimation and/or bond enthalpies)
• ΔH when 1 mole of gaseous atoms is produced from an element in its standard states, an endothermic process (+ve)

Question 15

first electron affinity

Answer 15

ΔH when each atom in 1 mole of gaseous atom gains 1 electron to form 1 mole of gaseous 1- ions, exothermic for many non-metals

Question 16

second electron affinity

Answer 16

• ΔH when each ion in 1 mole of gaseous 1- ions gain 1 electron to form 1 mole of gaseous 2- ions - endothermic as adding negative electrons to a negative ion

Question 17

enthalpy of formation

Answer 17

• ΔH when 1 mole of a substance is formed from its constituent elements with all substances in their standard states
• exothermic for most substances

Question 18

lattice enthalpy of formation

Answer 18

ΔH when 1 mole of a solid ionic compound is formed from its constituent elements in the gas phase, exothermic (-ve)

Question 19

Greater lattice enthalpies

Answer 19

• smaller ions with smaller ionic radii
• ions with higher charges

Question 20

lattice dissociation enthalpy

Answer 20

• the standard enthalpy change that occurs on the formation of 1 mole of gaseous ions from the solid lattice
• a measure of the strength of the ionic bond in different compounds, influenced by ion radius and charge
• this is endothermic so the enthalpy change will always be positive
• endothermic because energy is always required to break any bonds between the ions in the lattice