topic 8: energetics 1 Flashcards
enthalpy change defintion
heat energy change measured at constant pressure
standard conditions
- 100 kPa
- 298 K
exothermic delta h
negative
endothermic delta h
positive
standard enthalpy change of formation defintion
-enthalpy change when 1 mole of the compound
-is formed from its elements under standard conditions
- under standard conditions 100 kPa and 298 K
e.g Mg(s) + Cl2(g) -> MgCl2(s)
enthalpy of formation of an element
0
standard enthalpy change of combustion
-enthalpy change when 1 mole of a substance is completely combusted in oxygen
-under standard conditions 100 kPa and 298 K
compare exothermicity of incomplete vs compete combustion
incomplete is LESS EXOTHERMIC than complete
enthakpy change of reaction defintion
enthalpy change when the number of moles of reactants as specified in the balanced equation react together
enthalpy change of neutralisation defintion
enthalpy change when acid and alkali react togetehru nder standard conditions to produce 1 mole of water
enthalpy changes of neutralisation are …
EXOTHERMIC
equation for energy transferred (Joules)
Q = m * c* delta T
in calrometry, state effect if you: double volume of excess solution
no effect
in calrometry, state effect if you: do the experiment in a glass beaker
- heat loss to surroundings
- smaller delta T
- delta h is LESS EXOTHERMIC
in calrometry, state effect if you: USE LARGE CHUNKS OF SOLID
- slower reaction rate so more time for heat loss to surroundings
- q smaller
- delta h less exothermic
TO IMPROVE USE AN EXTRAPOLATION CURVE
in calrometry, state effect if you: double mass of solid
- q is doubled
- n is double
- no effect on delta h
in calrometry, state effect if you: initial temperature of solution is higher
- more heat loss at a faster rate
- smaller delta t, q, delta H
in calrometry, state effect if you: use more concentrated solution but in excess
- no effect
in calrometry, state effect if you: use a fine powder
- larger SA
- faster rate of reaction so less heat loss
- larger values
largest source of error in calorimetry and how to fix?
- heat loss to surroundings
- do a temperature correction curve w/ extrapolation
why not use a larger volume of acid in calorimetry is its in excess
- already in excess so NO NEED
- temperature change becomes smaller , so higher percentage error
(mean) bond enthalpy
enthalpy needed to break the covalent bond in gaseous molceuls,( averaged over different molecules)
bond enthalpies are always …
POSITIVE
- as bond breaking is endothermic
problem with mean bond enthalpies
- average so not for specific bond
- assumed for GAS, if a liquid or solid more energy required to change state
why is experimental values of bond enthalpy less acurate?
- heat loss to surroundings
- incomplete combustion so less energy released
why use a hess cycle?
- hard to measure temperature of a solid
- FOR ENDO: hard to add heat and measure temperature change at the same time
mass in calrometry
- mass of SOLUTIONS
- for combustion, mass of WATER
MEAN BOND ENTHALPY
reactants - products
error calrometry COMBUSTION
- heat loss to surroundings
- EVAPORATION OF SOLUTIOM
- incomplete combustion
- specific heat capacity of apparatus ignored
delta H from BONDS
reactants minus products
delta H from anything bUT bonds
products minus reactants
effect of smoky flame on experimental combustio value
- incomplete combustion
- value is LESS EXOTHERMIC
lattice energy
- enthalpy change when an ionic compound is made from its gasous atoms
- at standard conditions
enthalpy change of foramtion makes the product in what state
its norma, state
eg water liwuid
when doing enthalpy change of gas experiment, what has to be kept constant?
PRESSURE
why might a hydration of an ion be more exothrmic than for another ion
it has a SMALLER IONIC RADIUS so harder to hydrate
