energectics Flashcards
define exothermic
chemical reaction where energy is realised to the surroundings
temperature of surrounding increases
define endothermic
energy is taken by the chemicals from the surroundings
temperature of surrounding would decrease
define enthalpy change
change in heat energy at a constant pressure
what is sign on delta H for exothermic process
negative
what is sign on delta H for an endothermic process
positive
what are the standard conditions
100kpa pressure
298K temperature
1moldm^-3 concentration for all solutions
breaking bonds is an ______ process
endothermic
making bonds is an ________ proccess
exothermic
to start an a chemical reaction an intial input of energy is required what is the name of this
activation energy
define activation energy
the minimum needed energy to start a reaction
define mean bond enthalpy
the energy required to break one mole of a covalent bond into gaseous, averaged over a range of compounds
define standard enthalpy of formation
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
state why the enthalpy of formation of Na(s) is zero
Na is an element in its standard state
define standard enthalpy of combustion
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
why may enthalpy of combustion of a compound may be difficult to measure
incomplete combustion can occur which is highly exothermic so difficult to monitor
what is hess law
the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products
what do you use as mass and mol when using a calorimeter in Q=mc^T AND ^H=Q/n
mass of water
mol of combusting fuel
what do you use as mass and mol when reacting two solutions in Q=mc^T AND ^H=Q/n
total volume of solution for mass
limiting substance for moles
what do you use as mass and mol when using a solid and liquid in Q=mc^T AND ^H=Q/n
mass of liquid and mol of liquid
what assumption is made in calorimetery
the energy transferred to the water from the reacting chemicals is equal to the energy realised by the reaction
sources of error in combustion calorimetery
heat loss to surroundings
incomplete combustion of the fuel
some fuel evaporates
sources of error in solution calorimetery
heat loss to surroundings
improvements to minimise sources of error in combustion calorimetery
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
improvements to mimise sources of error in solution colorimetry
add a lid and insulate the beaker
steps to measure an enthalpy change using a cooling curve (6marks)
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
