1.4- energetics- exothermic and endothermic reactions

Question 1

enthalpy change (ΔH) definition

Answer 1

heat energy change measured under constant pressure

Question 2

what is the ΔH value for endothermic reactions

Answer 2

+ΔH
-heat absorbed from surroundings

Question 3

what is the ΔH value for exothermic reactions

Answer 3

-ΔH
-heat lost to surroundings

Question 4

why are there different enthalpy changes in different processes

Answer 4

-depends on temperature, pressure and physical state of substances in the process

Question 5

conditions for standard enthalpy changes (ΔH°298) (3)

Answer 5

standard conditions:
-pressure= 100 kPa
-temperature= 298K
-substances in their normal physical states under standard conditions

Question 6

standard enthalpy change of formation (ΔHf°) definition

Answer 6

-enthalpy change when one mole of a substance is formed from its elements under standard conditions with all substances in their standard states

Question 7

standard enthalpy change of combustion (ΔHc∘) definition

Answer 7

enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions with all substances in their standard states

Question 8

things to remember for bond dissociation enthalpy (2)

Answer 8

-all species must be in the gaseous state
-bond dissociation enthalpies are positive (requires energy to break bonds)

Question 9

what is the mean bond enthalpy

Answer 9

-the enthalpy change when 1 mole of covalent bonds is broken
-averaged over a range of different compounds

Question 10

general equation for bond dissociation enthalpy

Answer 10

A-B (g) –> A(g) + B(g)

Question 11

features of exothermic reactions (3)

Answer 11

-heat given out by reaction to surroundings
-energy profile diagram: reactants above products. Ea from reactants to peak of curve
-heat energy lost from system (reactants and products) to surroundings- negative ΔH

Question 12

features of products in exothermic reactions (2)

Answer 12

-products are energetically more stable than reactants
-new bonds formed in products are stronger than bonds broken in reactants

Question 13

features of endothermic reactions (3)

Answer 13

-heat taken in by reaction from surroundings
-energy profile diagram: products above reactants. Ea from reactants to peak of curve
-heat energy gained by system from surroundings- positive ΔH

Question 14

features of products in endothermic reactions (2)

Answer 14

-products energetically less stable than reactants
-new bonds formed in products weaker than bonds broken in reactants