Enthalpy Changes

Question 1

the energy of a system

Answer 1

is the sum of the kinetic energy and potential energy of the particles in the system

Question 2

what is enthalpy

Answer 2

measure of the potential energy of particles

Question 3

what do reactions usually involve

Answer 3

enthalpy changes and so heat energy is taken in or given out to balance the enthalpy change

Question 4

exothermic reactions

Answer 4

• give out energy to surroundings
• system loses energy so is deltaH negative
• products more stable than reactants
• often spontaneous reaction or small energy input needed

Question 5

examples of exothermic reactions

Answer 5

combustion or respiration

Question 6

endothermic reactions

Answer 6

• take in energy from surroundings
• system gains energy so deltaH positive
• reaction vessel feels cool

Question 7

examples of endothermic reactions

Answer 7

photosynthesis or thermal decomposition

Question 8

what does deltaH =

Answer 8

Hproducts - Hreactants

Question 9

what is the activation energy

Answer 9

the minimum energy required for a reaction to take place

Question 10

definition of standard enthalpy change of a reaction ΔrH θ

Answer 10

enthalpy change associated with a stated equation reacting under standard conditions

Question 11

standard enthalpy change of combustion ΔcH θ

Answer 11

enthalpy change when 1 mike of a substance is burnt completely in o2 under standard conditions

Question 12

standard enthalpy change of formation ΔfH θ

Answer 12

enthalpy change when 1 mole of a substance is formed when its constituent elements in their standard states under standard conditions

Question 13

ΔfH θ for elements

Answer 13

zero

Question 14

standard enthalpy change of atomisation ΔatH θ

Answer 14

enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard states under standard conditions

Question 15

standard enthalpy of neutralisation ΔneutH θ

Answer 15

enthalpy change when 1 mole of water is formed according to the following equation under standard conditions
ΔneutH θ = H+ (aq) + OH- (aq) -> H2O (l)

Question 16

standard state

Answer 16

usual physical state under standard conditions eg magnesium mg(s)

Question 17

standard conditions

Answer 17

100kPa (1atm)
298k (25°c)
1 moldm-3 solution

Question 18

thermochemical equation

Answer 18

a chemical equation with a ΔH value after it

Question 19

for weak acids that are partially dissociated

Answer 19

ΔneutH θ is smaller as some energy is used up in breaking the O-H bond in order for some of the H+ ions to be available for neutralisation

Question 20

calorimetry

Answer 20

measures heat produced or absorbed by a chemical equation

Question 21

calorimetry equation

Answer 21

q=mC ΔT
where: q= the energy given out or taken in in joules
m= mass of whatever is being heated in g
c= specific heat capacity and is 4.18Jg-1k-1

Question 22

hess’ law

Answer 22

the overall enthalpy change for converting given reactants into given products is the same whatever path is taken, as long as the initial conditions are the same and the final conditions are the same

Question 23

average bond enthalpy

Answer 23

enthalpy change on breaking 1 mole of bonds in 1 mole of gaseous molecules forming uncharged products
always endothermic

Question 24

why are bond enthalpies never completely accurate

Answer 24

• bond enthalpies quoted in data booklets are averaged for many different compounds so will not be precise for any particular compound
• bond energy is defined for gaseous compounds so won’t be accurate for solids or liquids

Question 25

factors that affect bond enthalpy

Answer 25

• bond length - shorter bonds are stronger because of the shared e-’s closer to the positive nuclei hence cl-cl > br-br > i-i