4 Energetics

Question 1

what is thermochemsitry

Answer 1

the study of heat changes during chemical reactions

Question 2

during an endothermic reaction what happens to the heat energy

Answer 2

heat energy is absorbed from the surroundings

Question 3

during an exothermic reaction what happens to the heat energy

Answer 3

heat energy is released to the surroundings

Question 4

what is another phrase for enthalpy change

Answer 4

heat change

Question 5

what are the standard conditions when meausirng enthalpy change

Answer 5

pressure of 100kPa

temperature of 298K

Question 6

is ∆H negative or positive for exothermic reactions and why

Answer 6

the products end up with less heat energy than the starting materials because they have lost energy when they heated up their surroundings therefore ∆H id negative

Question 7

is ∆H negative or positive for endothermic reactions and why

Answer 7

the produvts end up with more energy than the starting materials thefore is ∆H is negative

Question 8

why is it imporntant to have a standard pressure for measuing enthalpy changes

Answer 8

pressure affects the amount of heat energy given out by reactions that involve gases.If a gas is given out some energy is required topush away the atmosphere the greater the atmospheric pressure the more energy is used for this.This kmeans less nergy remains to be given out as heat by the reaction

Question 9

define standard enthalpy of combustion (∆cHƟ)

Answer 9

the enthalpy chnage when one mole of a substance is nurnt completely in oxygen under standard conditions

Question 10

define standard enthalpy of formation (∆fHƟ).

Answer 10

the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions

Question 11

define standard enthalpy of neutralisation (∆nHƟ).

Answer 11

is the enthalpy chnage when one mole of water is fomred in the reaction between an acid and a base

Question 12

what si the equation used to energy transferred in a reaction

Answer 12

energy transferred

=mass of solution x specific heat capacity of water x temp change

Question 13

what is the equation used to calculate enthalpy change

Answer 13

enthalpy change=energy transferred/number of moles

=mass of solution x specific heat capacity of water x temp change /number of moles

Question 14

what assumptions do we make during calorimeter practicals

Answer 14

we assume complete combustion of the fuel
assume no heat is transferred from burner to surroundings
assume heat is evenly distributed to the water in the can

Question 15

what improvements can be made in calorimeter practicals

Answer 15

metal can be used instead of a glass beaker as its a better conductor of heat
if glass beaker is used make sure it is covered using mineral wool as mineral wool isnt flammable

Question 16

what does Hesses law state

Answer 16

states that the energy chnage for a chemical reaction is always the same regardless of the route taken to get from the reactants to the products

Question 17

when do we use the combustion cycle

Answer 17

when given enthalpy combustion data

Question 18

what does the combustion cycle show

Answer 18

arrows pointing down to combustion elements CO2 + H2O

Question 19

how do you calculate ∆rHƟ in the combustion cycle

Answer 19

you flip the direction of the arrow from products combsution element so the arrow points upwards towards the products

Question 20

when do we use the formation cycle

Answer 20

when given enthalpy of formation data

Question 21

what does the formation cycle show

Answer 21

arrows pointing up from the elements to the reactants and products

Question 22

how do you calculate ∆rHƟ in the formation cycle

Answer 22

you flip the direction of the arrow going from elements to reactants so the arrow points from the reactands down towards the elements at the bottom

Question 23

what are the rules in combustion and formation cycles

Answer 23

the main equation is balanced
changind direction of arrow=chnaging the sign
you need the arrows to be rearranged so they follow the reactants to the products to find ∆rHƟ
enthalpy changes of an element is zero

Question 24

define bond dissocaiton enthalpy

Answer 24

wnthalpy chnage required to break a covalent bond with all species in their gaseous state

Question 25

define mean bond enthalpy

Answer 25

enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules

Question 26

what do bond enthalpies do

Answer 26

they give a measure of the strength of bonds and can help predict which bond in in a molecule is most likely to break

Question 27

explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law.

Answer 27

the mean bond enthalpies are averages whereas the answer obtained from hesss cyle are more accurate as the enthalpy of formation values have been obtained from the actual compunds involved.

Question 28

explain why using bond dissociation energies only give you an approximate value for the overall energy change

Answer 28

bond energies are average values and the average value may be different from the bond energy in that particular environment

Question 29

is endothemric reactions breaking or making bonds

Answer 29

breaking bonds

Question 30

is exothermic reactions breaking or making bonds

Answer 30

making bonds