DF - energetics *03

Question 1

Thermodynamics first law?

Answer 1

energy can be neither created nor destroyed but it can be converted from one form to another

Question 2

What is enthalpy

Answer 2

a measure of the heat content of a substance at constant pressure

enthalpy can’t be measured directly

Question 3

What are the standard conditions?

Answer 3

Pressure = 100kPa (1atm)
Temperature = 298K (25 degrees)
All solutions have a concentration of 1 moldm-3

Question 4

Enthalpy change of reaction

Answer 4

enthalpy change of a stated reaction

Question 5

enthalpy change of combustion

Answer 5

enthalpy change when one mole of a substance undergoes complete combustion

Question 6

enthalpy change of formation

Answer 6

enthalpy change when one mole of a substance is formed from its elements

Question 7

enthalpy of neutralisation

Answer 7

the enthalpy change when one mole of water is formed form the reaction of an acid and alkali

Question 8

what is the enthalpy of elements in their standard conditions

Answer 8

zero

Question 9

bond dissociation enthalpy

Answer 9

energy required to break one mole of gaseous bond to form one mole of gaseous atoms

Question 10

why are mean values used

Answer 10

because bond strength depends on environment

- the stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length.

Question 11

what is the bond enthalpy for diatomic gases

Answer 11

twice the enthalpy of atomisation

Question 12

what does higher bond enthalpy mean?

Answer 12

stronger bonds

Question 13

Hess’s Law

Answer 13

enthalpy change is independent of the path taken

Question 14

what is hess’s law used for

Answer 14

To calculate changes that cant be measured directly.

e.g combustion. formation or lattice enthalpy

Question 15

exothermic reaction

Answer 15

a reaction that releases more energy from making bonds than the energy required (taken in) to break bonds

e. g combustion - CH4 (g) + 2O2 (g) –> CO2 (g) + 2H2O (l)
e. g respiration

Question 16

endothermic reaction

Answer 16

a reaction that requires more energy to break bonds than the energy released when bonds are made

e. g photosynthesis
e. g thermal decomposition - CaCO3 (s) –> CaO (s) + CO2 (g)

Question 17

enthalpy of reaction

formation reactants and products

Answer 17

enthalpy of the formation products - enthalpy of formation of the reactants

Question 18

enthalpy of reaction

combustion of reactants and products

Answer 18

enthalpy of combustion of reactants - enthalpy of combustion of the products

Question 19

enthalpy change of a reaction ( when given average bond enthalpies)

Answer 19

total energy absorbed to break bonds - total energy released in making bonds

Question 20

is bond making exo or endo

Answer 20

exothermic

Question 21

is bond breaking exo or endo

Answer 21

endo

Question 22

enthalpy level diagram of an endothermic reaction

Question 23

enthalpy level diagram of an exothermic reaction

Question 24

techniques and procedures for measuring energy transferred when reactions occur in solution

Answer 24

1. add a known vol. of acid to an insulated container
2. then add known vol. of alkali
3. record temp. rise (stir solution)
4. can work out heat needed to raise temp. using q=mcT
5. assume all solutions have the same density of water

Question 25

techniques and procedures for measuring energy transferred when reactions occur when flammable liquids burn

Answer 25

calorimetry

1. as fuel burns it heats the water
2. the temp. change and SHC of water can be used to work out the heat absorbed by the water (using q=mcT)

Question 26

how do you calculate enthalpy changes from experimental results?

Answer 26

q = m x c x T

q - heat lost or gained in J
m - mass of water in g
c - specific heat capacity of water (4.18 J g-1 K-1)
T - change in temperature of water in Kelvin (+273)

Question 27

PAG 3