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 released to make bonds —> exothermic reaction

Question 3

how can overall enthalpy change be calculated ?

Answer 3

• energy to break bonds + energy to make bonds

Question 4

what’s some uses of thermochemistry ?

Answer 4

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

Question 5

what’s the difference between exothermic and endothermic reaction ?

Answer 5

• exo = release heat energy and the enthalpy change is negative
• endo = reactions absorb heat energy and enthalpy change is positive .
• units are always kJ per mole

Question 6

what are the standard conditions ? and what does “in standard state” mean?

Answer 6

• 100kPa/ 1 atm pressure
• 298K / 25 degrees temp
• the state an element / compound exists at in standard conditions .

Question 7

give two examples of exothermic reactions

Answer 7

• combustion of fuels
• neutralisation

Question 8

give example of endothermic reaction

Answer 8

• thermal decomposition

Question 9

define enthalpy change .

Answer 9

• an amount of energy that is transferred (absorbed/released) during a chemical reaction at constant pressure
• represented as (triangle) H

Question 10

define standard enthalpy of formation.

Answer 10

• the enthalpy change when mole of a compound is formed from its constituent elements in standard conditions , with reactants and products in their standard states .

Question 11

define standard enthalpy of combustion .

Answer 11

• the enthalpy change when one mole of a substance is burnt completely in excess oxygen in standard conditions , with reactants and products in their standard states .

Question 12

what’s difference between heat and temp?

Answer 12

• heat = sum of all particles energy , thus it’s affected by amount of substance
• temp = related to mean kinetic energy of particles in a system, so independent of number of particles present

Question 13

how can u calculate enthalpy change from experimental data?

Answer 13

• Q = mc ∆T
• m is mass is substance being heated (usually water)
• c = specific heat capacity (water SHC = 4.18g J^-1K^-1
• ∆T = change in temp

Question 14

what’s meant by calorimetry .

Answer 14

• experimental method for finding enthalpy change by measuring temperature over time .
• data can be extrapolated to give accurate value for change in temp .

Question 15

what’s flame calorimeter and how if differ to simple calorimeter ?

Answer 15

• reduced heat lost to surrounding to give accurate results - has spiral chimney made of copper , an enclosed flame and fuel is burnt in pure oxygen not air

Question 16

how would u measure enthalpy change for reaction occurring in (aq)?

Answer 16

• use expanded polystyrene cup as a calorimeter (good insulator —> reduce heat loss)
• heat generated in solution: measure the is temp change. take heat capacity of solution to be 4.18 and density solution -> 1gcm^-3

Question 17

define molar enthalpy of combustion .

Answer 17

• amount of energy released by complete combustion of one mole of a substance .

Question 18

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

Answer 18

• as energy easily lost from system through conduction / convention
• cooling curves help more accurate

Question 19

what is Hess’s law ?

Answer 19

• states that the enthalpy change for a reaction is the same regardless of the route taken .
• if chemical change occur by more than one route - overall enthalpy for each route must be same providing starting + finishing conditions the same

Question 20

define bond dissociation enthalpy ?

Answer 20

• the enthalpy change required to break a covalent bond, with all species in gaseous state : differs for same bond type in diff molecules

Question 21

what’s bond enthalpy

Answer 21

• amount of energy absorbed when a bond is broken
• equals amount of energy released when same bond made
• measured as energy in kilojoules per mole of bonds

Question 22

define mean bond enthalpy

Answer 22

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

Question 23

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

Answer 23

• heat lost to surroundings
• not in standard conditions
• reactions may not go to completion.

Question 24

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

Answer 24

• bond enthalpies are a mean for the same bond across different molecules : standard enthalpy of combustion + formation apply just to that molecules thus more accurate

Question 25

define standard enthalpy change of neutralisation.

Answer 25

• enthalpy change that accompanies reaction of an acid and an alkali to form one mole of water under standard conditions .

Question 26

what do reaction profiles show ?

Answer 26

• the amount of energy that a substance has at start and end of a reaction.
• during exo = reactants lose energy as they form products
• difference in energy between reactants + products is enthalpy change which is negative and arrow points downs
• initial increase in energy = activation energy (minimum amount energy need to get reaction going)

Question 27

what do endothermic reaction graph show?

Answer 27

• products have more energy than reactants
• as energy been absorbed from surrounding - enthalpy change is positive and arrow points upwards .