energetics

Question 1

system

Answer 1

the chemicals being studied (reactants +products)
- we measure the energy change of the system

Question 2

surrounding

Answer 2

everything around th esystem (container, air, water)
- we cant measure the temp of the system - so we measure the surrounding temp

Question 3

exothermic

Answer 3

energy of system decreases
- heat lost to surroundings
temp of surroundings increases
-ΔH

Question 4

endothermic

Answer 4

energy of system increases
- heat taken in from surroundings
temp of surroundings decreases
+ΔH

Question 5

energy level diagram

Answer 5

energy on y axis
reaction progress on x
reactants +/-ΔH then products

Question 6

reaction profile diagram

Answer 6

energy level diagram with activation evergy

Question 7

why reaction cant react when acid is solid

Answer 7

acids need to be aq to react so ions can move and can release H+ ions

Question 8

why use steel wool not a nail

Answer 8

larger suraface area increases rate of reaction

Question 9

what factors affect temperature change measured

Answer 9

• heat loss to surroundings
• stir - removes hot spots
• distance between flame and calorimeter
• vol of solution - less = faster = less heat loss
• initial water temp - hot = more heat loss
• incomplete combustion

Question 10

calorimeter

Answer 10

any apparatus used to measure ΔT

Question 11

Q (heat energy transferred )equation

Answer 11

Q = mcΔT
(mass) x (specific heat capacity) x (temp chnage)

Question 12

specific heat capacity

Answer 12

4.18
amount of heat energy needed to raise the temp of 1g of a substance by 1degree

Question 13

assumptions in calorimetry

Answer 13

• all heat is absorbed/released by the water present
• any aq mixture has a c = to water
• any aq mixture has a density = to water (vol = mass)

Question 14

mass in calorimetry

Answer 14

vol = mass
add mass of 2 solutions
ignore mass of solids

Question 15

enthalpy change

Answer 15

molar energy change (ΔH)

Question 16

ΔH equation

Answer 16

ΔH = (Q/1000)/n
(energy transfered KJ)/ (amount of LIMITING reactant)

Question 17

ΔH sign

Answer 17

-ΔH = exo
+ΔH = endo

Question 18

%error between experimental and true values

Answer 18

= (difference between experimental +true /true) x 100

Question 19

calculated vs true values

Answer 19

ΔH was less exo/endothermic than true values - lots of heat loss to surroundings/not transfered to calorimeter

Question 20

sources of error in experiment

Answer 20

• incomplete combustion - not as much energy release lower ΔT
• heat loss to surroundings lower ΔT
• lid not put on quickly - fuel evaporates - larger mass and n so less exo ΔH
• assumption all heat is absorbed by water not equiptment - lower ΔT

Question 21

modifications to make final values more acurate

Answer 21

• add lid - insulation
• polestyrene cup - insulation
• insulation to sides /draught sheild
• stir - minimise hotspots
• make flame closer to water
• add O2 supply to base to encourage complete combustion

Question 22

neutralisation reaction

Answer 22

EXOTHERMIC
reaction in solution
eg HCl +NaOH -> NaCl + H2O
- record intial temp of both solutions
- add base
- add acid
- stir and record max temp reached

Question 23

displacment reaction

Answer 23

EXOTHERMIC
reaction in solution
eg Zn + CuSO4 -> ZnSO4 +Cu
- add CuSO4 solution to polestyrene cup
- record intial temp
- add Zn
- stir and record max temp reached

Question 24

combustion reaction

Answer 24

EXOTHERMIC - ethanol
- put water in calorimeter and record initial temp
- record initial mass of fuel
- light fuel under water
- record max temp reached
- record final mass of fuel

Question 25

salts dissolving in water

Answer 25

• record initial temp of water and salt
• put water in cup
• put salt in water
• record max temp reached

Question 26

see pics for calorimeters, energy level diagrams

Answer 26

-

Question 27

bond breaking is

Answer 27

endothermic - takes in energy from surroundings

Question 28

bond making is

Answer 28

exothermic - gives out energy to surroundings

Question 29

so if bond breaking energy is more then bond making energy

Answer 29

if the energy taken in to break bonds is more than the energy given out when making bonds it is endothermic

Question 30

so if bond breaking energy is less then bond making energy

Answer 30

if the energy taken in to break bonds is less than the energy given out when making bonds it is exothermic

Question 31

activation energy

Answer 31

the minimum energy needed to start a reaction
used to break bonds so a reaction can happen

Question 32

change in H in terms of bond making/ breaking

Answer 32

bonds broken - bonds made

Question 33

mean bond energy

Answer 33

the average amount of energy needed to break 1 mole of a particular type of covalent bond

Question 34

why is mean bond energy sometimes inaccurate

Answer 34

bond energy is an average from a range of compounds - strength of bonds can vary depending on atoms in the molecule