Energetics

Question 1

what energy change is breaking bonds associated with?

Answer 1

Energy is taken in to break bonds - endothermic reaction

Question 2

what energy change is making bonds associated with?

Answer 2

Energy is released to make bonds - exothermic reaction

Question 3

what are some uses of thermochemistry?

Answer 3

• measuring and comparing energy values of fuels
  -calculating the energy requirement for industrial processes
  -working out the theoretical amount of energy released/taken in, in a reaction
  -predicting if a reaction will take place or not

Question 4

what is an endothermic reaction?

Answer 4

• positive enthalpy change
  energy in breaking bonds > energy in making bonds

Question 5

what is an exothermic reaction?

Answer 5

• negative enthalpy change
  energy in making bonds> energy in breaking bonds

Question 6

if a reversible reaction is endothermic in one way, what type of reaction is the other way?

Answer 6

exothermic

Question 7

give two examples if exothermic reactions?

Answer 7

combustion of fuels
neutralisation

Question 8

give an example of endothermic reaction?

Answer 8

thermal decomposition

Question 9

define enthalpy change? what symbol is used to represent it?

Answer 9

energy change in system at constant pressure
△H

Question 10

what are the standard conditions?

Answer 10

pressure - 1 atm
temperature - 25° / 298K

Question 11

what does ‘in standard state mean’ ?

Answer 11

State an element or compound exist in standard conditions. 1atm or 298K

Question 12

draw an enthalpy change diagram for an endothermic and exothermic reaction

Answer 12

diagram - labelled w activation energy and change in enthalpy

Question 13

define standard enthalpy formation?

Answer 13

The enthalpy change when 1 mole of compound is formed from its constituent elements, in standard conditions (100Kpa, 298K), with reactant and products in their standard states.

Question 14

give an example of an equation which represents standard enthalpy of formation?

Answer 14

H2(g) +0.5O2(g) –> H2O(l)

Question 15

define standard enthalpy of combustion?

Answer 15

The enthalpy change when 1 mole of substance is burnt completely in oxygen in standard conditions (100Kpa,298K) with reactant and products in their standard states.

Question 16

What is the difference between heat and temperature?

Answer 16

Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance.

Temperature is related to the mean kinetic energy of the particles in a system, so is independant of the number of particles present.

Question 17

How can you calculate enthalpy change from experimental data?

Answer 17

Q= mc△t
m = mass
c= SHC
water SHC = 4.18gJK
△t - change in temperature

Question 18

draw a simple calorimeter?

Answer 18

diagram

Question 19

what is a flame calorimeter? How does it differ from a simple calorimeter?

Answer 19

reduces heat lost to the surroundings to give more accurate results - has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.

Question 20

How would you measure the change in enthalpy for a reaction occurring in (aq)?

Answer 20

• use a expanded polystyrene cup as a calorimeter = it is a good insulator and reduced heat lost
• heat is generated in solution
• measure the change in temperature
• take SCH of solution = 4.18 and density as = 1 g cm-3

Question 21

what is Hess’s Law?

Answer 21

states the enthalpy change of the reaction is the same regardless of the route taken

Question 22

what is the enthalpy of an element?

Answer 22

the enthalpy of of all elements in their standard states
100KPa 298K

Question 23

define bond dissociation enthalpy?

Answer 23

the enthalpy change required to break a covalent bond, with all species in their gaseous state. Differs for the same bond in different types of molecule.

Question 24

define mean bond enthalpy?

Answer 24

average value for the bond dissociation enthalpy of a given bond

Question 25

why may experimental methods for enthalpy determination may not be very accurate?

Answer 25

• Heat is lost to the surroundings
• Not in standard conditions
• Reaction may not go to completion

Question 26

why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/fermentation?

Answer 26

Bond enthalpies are the mean for one bond across many different molecules. Standard enthalpy of combustion/ formation apply to just one molecule, therefore they are more accurate.