Module 3 - Enthalpy Changes

Question 1

define enthalpy (H)

Answer 1

-the heat that is stored in a chemical system
(elements and compounds contain enthalpy)

Question 2

define enthalpy change (△H)

Answer 2

-difference in enthalpy between the products and reactants in a reaction

Question 3

define an exothermic reaction

Answer 3

-chemical reaction where heat energy is transferred from the system to the surroundings

Question 4

describe exothermic reactions

Answer 4

-any energy loss by the system is balanced by the energy gain by the surroundings
-the temperature of the surroundings increases so the temperature increases
-the enthalpy change is negative

Question 5

examples of exothermic reactions

Answer 5

-burning/combustion
-neutralisation
-oxidation
-handwarmers
-respiration
-condensation

Question 6

define an endothermic reaction

Answer 6

-chemical reaction where heat energy is absorbed from the surroundings into the system

Question 7

describe endothermic reactions

Answer 7

-any energy gain by the system is balanced by the energy loss of the surroundings
-the temperature of the surroundings decreases, so we see a temperature decrease
-the enthalpy change is positive

Question 8

examples of endothermic reactions

Answer 8

-thermal decomposition
-sports injury pack
-photosynthesis
-evaporation

Question 9

is the enthalpy change for the addition of magnesium to HCl endothermic or exothermic

Answer 9

exothermic

Question 10

is the enthalpy change for the addition of sodium thiosulfate crystals to water endothermic or exothermic

Answer 10

endothermic

Question 11

describe the enthalpy of bonds in exothermic reactions

Answer 11

-the products have less enthalpy than the reactants
-more energy is released from making bonds than breaking bonds
-the enthalpy change is a negative value

Question 12

define activation energy

Answer 12

-the minimum energy required to start a reaction

Question 13

describe the enthalpy of bonds in endothermic reactions

Answer 13

-the products have more enthalpy than the reactants
-more energy is required for breaking bonds than making bonds
-the enthalpy change is a positive value

Question 14

how to calculate activation energy of a backwards reaction for a forwards reaction enthalpy profile or vice versa

Answer 14

-use values already on the graph
-switch sign if arrow points a different way in this reaction
-keep sign the same if arrow points the same way

Question 15

define enthalpy change of reaction

Answer 15

-the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation

Question 16

define enthalpy change of formation

Answer 16

-the enthalpy change when 1 mole of compound is formed from its element under standard conditions

Question 17

define enthalpy change of combustion

Answer 17

-the enthalpy change when 1 mole of substance completely combusts under standard conditions

Question 18

define enthalpy change of neutralisation

Answer 18

-the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

Question 19

symbol for enthalpy change of reaction

Answer 19

▲Hr

Question 20

symbol for enthalpy change of formation

Answer 20

▲Hf

Question 21

symbol for enthalpy change of combustion

Answer 21

▲Hc

Question 22

symbol for enthalpy change of neutralisation

Answer 22

▲Hneut

Question 23

symbol for standard conditions

Answer 23

⦵

Question 24

describe standard conditions

Answer 24

standard pressure = 100kPa
standard temperature = 298K (25c)
standard concentration = 1mol dm-3 (solutions only)

Question 25

describe how to tell when standard conditions are not used

Answer 25

-if the enthalpy change values are different to what is expected

Question 26

method for calculating enthalpy change

Answer 26

1- work out heat energy (q) 2- work out the moles (of the limiting reactant) 3- work out the ▲Hr 3- convert to kJs and add sign

Question 27

symbol equation for working out heat in enthalpy change

Answer 27

q= mc▲t

Question 28

equation in words for working out heat in enthalpy change

Answer 28

heat energy = mass of liquid you measure the temp of x specific heat capacity x change in temp (in degrees or k)

Question 29

g to cm3 conversion

Answer 29

1g= 1cm3

Question 30

describe writing equations for enthalpy change of formation

Answer 30

-always include state symbols -write the equation for the substances from its elements -ensure only 1 mole of product

Question 31

describe writing equations for enthalpy change of combustion

Answer 31

-write down the compound + oxygen -most compounds burn to produce co2 and h2o -always include state symbols

Question 32

describe the method to work out enthalpy change of combustion

Answer 32

1- measure out 100mls of water using a measuring cylinder and put into calorimeter (put on bottom clamp + measure distance from bottom of can - keep the same distance each time) 2- measure the temperature of the water (the thermometer should not touch the bottom of the can) 3- weigh the mass to 2.d.p of the alcohol burner and the lid (do not remove the lid at any point until you are ready to burn to prevent evaporation) 4- remove the lid and light the burner - stir continuously until the temperature has increased by 15 degrees 5- replace the lid and keep stirring 6- record the highest temperature to 1d.p 7- reweigh the burner and lid to 2.d.p

Question 33

describe steps for calculation for enthalpy change of combustion

Answer 33

1- q=mc▲t 2- calculate moles 3- calculate ▲H (q/n) 4- convert to kJmol-1 5- add sign

Question 34

define average bond enthalpy

Answer 34

-the average enthalpy when one mole of gaseous covalent bonds are broken

Question 35

why is average bond enthalpy an average

Answer 35

-the bond enthalpies used in the calculation are averages from different compounds -as the exact bond enthalpy depends on the particular compound in which it is either formed or broken

Question 36

are bond enthalpies determined under standard conditions

Answer 36

no

Question 37

describe steps for calculating average bond enthalpies

Answer 37

1- work out which bonds broken and bonds made 2- work out bonds broken 3- work out bonds made 4- work out bonds broken - bonds made (enthalpy change)

Question 38

define hess's law

Answer 38

-if a reaction can take place by two different routes then the total enthalpy change is the same for both routes

Question 39

which direction should the arrows go when using combustion data for hess's law

Answer 39

-arrows go down

Question 40

bottom of cycle for drawing hess's law cycle for combustion

Answer 40

-products for combustion (balanced)

Question 41

steps for calculating enthalpy change using hess's law

Answer 41

-energy for first arrow -energy for second arrow -enthalpy change (reactants + products)

Question 42

why is the arrow direction in a hess's law cycle important

Answer 42

-sign changes if the route goes against the arrow

Question 43

why is oxygens value 0 when working out enthalpy changes with hess's law using combustion data

Answer 43

-oxygen is forming itself

Question 44

which direction should the arrows go when using formation data for hess's law

Answer 44

-arrows go up

Question 45

bottom of cycle for drawing hess's law cycle for formation

Answer 45

-the elements the compound is formed of

Question 46

is bond breaking exothermic or endothermic

Answer 46

endothermic

Question 47

is bond making exothermic or endothermic

Answer 47

exothermic

Question 48

exothermic energy graph

Answer 48

to be learned

Question 49

endothermic energy graph

Answer 49

to be learned