energectics

Question 1

define exothermic

Answer 1

chemical reaction where energy is realised to the surroundings
temperature of surrounding increases

Question 2

define endothermic

Answer 2

energy is taken by the chemicals from the surroundings
temperature of surrounding would decrease

Question 3

define enthalpy change

Answer 3

change in heat energy at a constant pressure

Question 4

what is sign on delta H for exothermic process

Answer 4

negative

Question 5

what is sign on delta H for an endothermic process

Answer 5

positive

Question 6

what are the standard conditions

Answer 6

100kpa pressure
298K temperature
1moldm^-3 concentration for all solutions

Question 7

breaking bonds is an ______ process

Answer 7

endothermic

Question 8

making bonds is an ________ proccess

Answer 8

exothermic

Question 9

to start an a chemical reaction an intial input of energy is required what is the name of this

Answer 9

activation energy

Question 10

define activation energy

Answer 10

the minimum needed energy to start a reaction

Question 11

define mean bond enthalpy

Answer 11

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

Question 12

define standard enthalpy of formation

Answer 12

the enthalpy charge when one mole of a substance is formed
from its constitute elements under standard conditions
with all reactants and products being in their standard states

Question 13

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

Answer 13

Na is an element in its standard state

Question 14

define standard enthalpy of combustion

Answer 14

the enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions all reactants and products being in their standard states

Question 15

why may enthalpy of combustion of a compound may be difficult to measure

Answer 15

incomplete combustion can occur which is highly exothermic so difficult to monitor

Question 16

what is hess law

Answer 16

the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products

Question 17

what do you use as mass and mol when using a calorimeter in Q=mc^T AND ^H=Q/n

Answer 17

mass of water
mol of combusting fuel

Question 18

what do you use as mass and mol when reacting two solutions in Q=mc^T AND ^H=Q/n

Answer 18

total volume of solution for mass
limiting substance for moles

Question 19

what do you use as mass and mol when using a solid and liquid in Q=mc^T AND ^H=Q/n

Answer 19

mass of liquid and mol of liquid

Question 20

what assumption is made in calorimetery

Answer 20

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

Question 21

sources of error in combustion calorimetery

Answer 21

heat loss to surroundings
incomplete combustion of the fuel
some fuel evaporates

Question 22

sources of error in solution calorimetery

Answer 22

heat loss to surroundings

Question 23

improvements to minimise sources of error in combustion calorimetery

Answer 23

add a lot to insulate top of beaker
insulate sides of beaker to reduce heat loss (NOT BOTTOM)
reduce distance between flame and calorimeter -reduce heat loss
put a sleeve around flame to protect it from drafts

Question 24

improvements to mimise sources of error in solution colorimetry

Answer 24

add a lid and insulate the beaker

Question 25

steps to measure an enthalpy change using a cooling curve (6marks)

Answer 25

record the temp for a suitable time (3 minutes) before adding reactants together
to establish an accurate intial temperature
mix reactants in at minute 4 then record temperature every minute until a trend is seen (for at least 8-10 minutes)
plot a graph against time
extrapolate the cooling curve back to point of addition
establish theortical temperature change accounting for heat loss

improvements
insulate
add a lid
do they follow method exactly