energetics

Question 1

def: enthalpy change

Answer 1

the change in heat energy at constant pressure

Question 2

in order for a reaction to happen…

Answer 2

all the reactants need to break all their bonds,
all bonds in the products need to form

Question 3

breaking bonds is an …. process

Answer 3

endothermic
(requires energy)

Question 4

making bonds is an …. process

Answer 4

exothermic
releases energy

Question 5

def: activation energy

Answer 5

the minimum amount of energy needed to start a reaction,
(amount of energy required to break the reactant bonds)

Question 6

enthalpy diagram: exothermic

Answer 6

reactants

                     products 

difference is -ve

Question 7

enthalpy diagram: endothermic

Answer 7

products

reactants

difference is +ve

Question 8

def: mean bond enthalpy

Answer 8

the energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds

Question 9

the more positive the bond enthalpy…

Answer 9

the larger the amount of energy needed to break the bond,
so the stronger the bond

Question 10

state why the enthalpy of formation of Na(s) is zero

Answer 10

Na is an element

Question 11

state why the enthalpy of formation of liquid Na is not zero

Answer 11

Na(l) is not the standard state of Na

Question 12

what substances cannot be combusted

Answer 12

water, CO2, most other oxides,
have a zero enthalpy of combustion

Question 13

why may the enthalpy of combustion of a compound be difficult to?

Answer 13

incomplete combustion can occur + highly exothermic so difficult to monitor

Question 14

why are some types of enthalpy change very difficult to measure experimentally?

Answer 14

reaction may have a very high activation energy,
reaction rate may be too slow,
possibility of competing reactions

Question 15

give one reason why the bond enthalpy that you calculated above is different from the mean bond enthalpy quoted in a data book

Answer 15

the data book value is averaged over a range of compounds.

Question 16

what do we use calorimetry for?

Answer 16

to measure the change in temperature of the surroundings

Question 17

Q=mc delta t

Answer 17

Q = energy released or taken in during a reaction (J)
m = masss of solution or liquid that is heated up (g)
c = specific heat capacity
delta t = change in temp (K)

Question 18

enthalpy change equation:

Answer 18

Q (kJ) / n (mol) = deltaH (kJmol-1)

Question 19

assumption made in calorimetry:

Answer 19

the energy transferred to the water from the reacting chemicals is equal to the energy released by the reaction

Question 20

sources of error in combustion calorimetry:

Answer 20

• heat lost to surroundings
• incomplete combustion of the fuel
• heat energy transferred to the metal calorimeter
• some fuel evaporates

Question 21

improvments to minimise sources of error in combustion calorimetry:

Answer 21

• add a lid - reduces heat loss
• insulate sides of calorimeter - reduces heat loss
• reduce distance between flame and beaker - reduces heat loss
• put sleeve around the flame to protect it from draughts

Question 22

sources of error in solution calorimetry:

Answer 22

• heat loss to surroundings

Question 23

improvments to minimise sources of error in solution calorimetry:

Answer 23

• add a lid - reduces heat loss
• insulate sides of calorimeter - reduces heat loss

Question 24

steps to measure an enthalpy change using a cooling curve:

Answer 24

• record the temperature for a suitable time (3 mins) before adding reactants together
• to establish an accurate initial temperature
• mix reactants at minute 4 then record the temperature every minute until a trend is seen (for at least 8-10 mins)
• plot a graph of temperature against time
• extrapolate the cooling curve back to the point of addition
• to establish the theoretical temperature change accounting for heat loss
• to reduce heat loss, use a polystyrene cup with a lid

Question 25

a 50g sample of water was used in this experiment.
explain how you could measure out this mass of water without using a balance

Answer 25

water has a known density of 1.0gcm-3
a volume of 50cm3 could be measured out

Question 26

suggest reasons why using powders increases the rate of reaction:

Answer 26

increases the surface area of reactants,
increases collision frequency/ rate of collisions
increase contact between particles

Question 27

suggest reasons why extracting strontium is expensive (by heating a mixture of powdered oxide and powdered aluminium):

Answer 27

• reaction/process requires heat
• it is endothermic