Physical 1: Energetics

Question 1

what energy change is breaking bonds associated with?

Answer 1

energy 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 the energy values of fuels
• calculating the energy requirements for industrial processes
• working out the theoretical amount of energy released/taken in a reaction
• predicting if a reaction will take place or not

Question 4

what is an endothermic reaction?

Answer 4

one with an overall positive enthalpy change – energy in breaking bonds > energy in making bonds

Question 5

what is an exothermic reaction?

Answer 5

one with an overall negative enthalpy change – energy in breaking bonds < energy out making bonds

Question 6

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

Answer 6

exothermic

Question 7

give 2 examples of exothermic reactions

Answer 7

• combustion of fuels

- neutralisation

Question 8

give an example of an endothermic reaction

Answer 8

thermal decomposition

Question 9

define enthalpy change?

Answer 9

energy change of a system at a constant pressure

Question 10

what are standard conditions?

Answer 10

100kPa / 1 atm pressure

298K / 25 degrees Celsius temperature

Question 11

what does “in standard state” mean?

Answer 11

the state an element/compound exists at in standard conditions (100kPa , 298K)

Question 12

define standard enthalpy of formation

Answer 12

the enthalpy change when one mole of a compound is formed from it’s constituent elements in standard conditions (100kPa, 298K) with reactants and products in their standard states

Question 13

give an example of an equation which represents standard enthalpy of formation (hydrogen and oxygen)

Answer 13

H2 (g) + 1/2 O2 (g) –> H2O (l)

Question 14

define standard enthalpy of combustion

Answer 14

the enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa, 298K), with reactants and products in their standard states

Question 15

give an example of an equation which represents standard enthalpy of combustion (carbon and oxygen)

Answer 15

C (s) + O2 (g) –> CO2 (g)

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 independent of the number of particles present

Question 17

how can you calculate enthalpy change from experimental data?

Answer 17

use the equation Q = mc∆T
m = mass of substance being heated
c = specific heat capacity of substance
∆T = change in temperature

Question 18

water specific heat capacity

Answer 18

4.18gJ-1K-1

Question 19

what is a flame calorimeter, and how does it differ to a simple calorimeter?

Answer 19

reduced heat lost to the surrounding 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 enthalpy change for a reaction occuring in (aq) ?

Answer 20

use an expanded polystyrene cup as a calorimeter (good insulator –> reduce heat loss)
Heat is generated in the solution; measure this temperature change
take heat capacity of solution to be 4.18 and density of solution = 1gcm-3

Question 21

what can you use to make experimental determination of enthalpy change of reaction more accurate?

Answer 21

cooling curves

Question 22

what is Hess’ Law?

Answer 22

states that the enthalpy change for a reaction is the same regardless of the route taken

Question 23

what is the enthalpy of an element?

Answer 23

the enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0

Question 24

define bond dissociation enthalpy

Answer 24

the enthalpy change is required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules

Question 25

define mean bond enthalpy

Answer 25

average value (across different chemical environments) for the bond dissociation enthalpy of a given bond

Question 26

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

Answer 26

heat is lost to the surroundings
not in standard conditions
reaction may not go to completion

Question 27

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

Answer 27

bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate