Enthalpy Chabges Flashcards
What’s enthalpy
H
Measure of the heat energy in a chemical system (atoms molecules or ions making up chemicals)
Energy stored within bonds
it can’t be measured
Whats the enthalpy change
Differnce in enthalpies
🔼H=H(prudcts)-H(reactants)
It can be posiitve or negative
What’s the law of conservation of energy
Energy can’t be created or destroyed
When a chemical reaction involves an enthalpy change takes place,heat energy transferred between the system and the surroundings
System=reactants n products
Surroundings -apparatus,lab everything but chemical system
Universe=system n surroundings
How can enthalpy change be determined ?🔼H
By measuring energy transfer between system n surroundings
Energy transfer can be in:
system-surroundings=exothermic change
Surroundings to system=endothermic change
What shows the relative enthalpies of reactants n products and enthalpy change
Enthalpy profile diagrams
What’s an exothermic change
Chemical system releases heat to surroundings
Energy loss from chemical system is balanced by energy gain by surroundings
exothermic enthalpy change = -
Temp of surroundings inc as they gain energy
Draw enthalpy profile diagram of an exothermic change
What’s an endothermic change
Chemical system takes in heat from surroundings
Energy gain from chemical system balanced by energy loss by surroundings
Endothermic enthalpy change = +
Temp of surroundings dec as they lose energy
What’s the enthalpy profile diagram for endothermic change
What is the activation energy
Minimum amount of energy required for a reaction to take place.
Reactions w small activation energies happen quick as energy needed to break bonds is readily available from surroundings. Reactions w larger AE becomes such a large energy barrier it slows down reaction
What’s a standard enthalpy change units for it and what’s the standard conditions of a reaction
Enthalpy change under standard conditions
Units =Kjmol-1
Pressure=100KPa
Temp=298K/25•c
Conc=1moldm-3
What’s the standard state
Most physical state of a substance under standard conditions
100Kpa and 298K
What’s the standard enthalpy change of reaction
🔼rHø
Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions w all reactions n products in standard states.
One mole of water is formed from hydrogen and oxygen releasing 286 kJ
H2 (g) + ½O2 (g) → H2O (l) ΔHꝊr= -286 kJ mol-1
Calculate ΔHr for the reaction below:
2H2 (g) + O2 (g) → 2H2O (l)
Answer:
Since two moles of water molecules are formed in the question above, the energy released is simply:
ΔHr = 2 mol x (-286 kJ mol-1)
= - 572 kJ
What’s the standard enthalpy change of formation
🔼fH ø
Enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
Why do we use fractional equation for 🔼fHø
If we used whole numbers it wouldn’t match the definition for fhø which requires formation of 1 mole of mgO
Why is 🔼fhø of elements 0
From definition of fh
It’s the formation of one mole of an element from an element so there’s no change
What’s the standard enthalpy change of combustion
🔼cHø is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions
With all reactants n products in standard states.
What’s the standard enthalpy change of neutralisation
🔼neutHø
The energy change that accompanies the reaction of an acid by a base to form one mole of H20 under standard conditions with all reactants n products in their standard states.
It involved reaction of H+(aq) and Oh-(aq) to form one mole of H20. Value of 🔼neutH is same for all neutralisation reactions=-57kjmol
can 273k and 0 degrees mean the same thing
yeh recording temp w thermometer graduated in c,value of temp change is exactly the same in C and K
what’s specific heat capacity=c
the energy needed to raise the temp of 1g of a substance by 1k.good conductors of heat have small c vice versa.
what’s the c for water=4.18jg-1K-1
how to calc an energy change
q=mcTRIANGLEt
q=jeat energy
how to work out enthalpy change of combustion of methanol
q=mct/1000
mass/rfm
big/small
how accurate is the enthalpy change of combustion value
When compared w book value ours is lower so more heat lost cuh=
coulda been botched due to:
heat loss to surroundings other than water. Includes beaker but mainly air surrounding flame
incomplete combustion of methanol=w co and c being produced than co2(u can see black layer of soot on beaker)
evaporation of methanol from wick.burner must be weighed as soon as possible after extinguishing the flame or some methanol can evaporate from wick.(spirit burners have a cover to reduce this error)
non standard conditions used=hard to get it identical to standard conditioned
leading to a value of ch being less exothermic than expected.
