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.
how to minimise errors from heat loss n incomplete combustions
use cover for spirit burners
use draught screens-shields flame from air currents
input of 02 gas-to undergo complete C
how to work out enthalpy changes of reaction 🔼rH. for a solid n solution
When carrying out reactions between aqueous solutions. n solid.solution itself becomes surroundings whilst chemical particles react n collide w each other n thermometer used to calc temp change of solution
how to work out enthalpy changes of reaction 🔼H. for a solid n solution
worked example:
q=mct/1000
conc x vol
big/small
look at the moles
diff between enthalpy change of neutralisation n enthalpy change of reaction
happens between a solution n solution not solid x
how to determine the enthalpy change of neutralisation worked example:
q=mct usually u add the volumes up for the m
do conc x vol
big/small
look at moles after
combustion of 1.656g of ethanol,c2h5oh raised temp of 150g of water from 22.5c to 74.5
a)write equation to represent enthalpy change of combustion of ethanol
b)calculate enthalpy change of combustion of ethanol to 3 sf.
1)c2h5oh+3o2=2co2=3h20
q=mctrianglet
150g x 52 x 4.18=32604=32.064kj
moles=mass/rfm of c2h5oh=1.656/46=0.036
1 mole of ethanols lost 32.604/0.036
trianglech(ch30h)=-906
what is average bond enthalpy
the energy required to break 1 mole of a specified type of bond in a gaseous molecule.
always endothermic and positive.
limitations of average bond enthalpies
The energy required to break a bond depends on the surroundings of that bond within the molecule.
Eg a C-H bond in methane will have a different strength than a C-H bond in ethanol, because of the influence of nearby atoms or functional groups.
Average bond enthalpies are calculated from many different molecules, so they don’t perfectly reflect these specific differences. + not considered in average bond enthalpy calculations.
Because bond strength varies from molecule to molecule, we take the mean value of that bond across many compounds.
So average bond enthalpies are just approximations, not precise values.
Bond enthalpies don’t account for the specific mechanism of a reaction Bond enthalpy calculations assume a simple break/make process and don’t consider reaction mechanisms or intermediates, which can affect the actual energy changes.
Bond enthalpies are defined and measured for reactions w gaseous reactants + products.
so, not applicable to reactions w solids/ liquids, as energy needed to overcome IM forces in these states is not accounted for.
For eg, if you are trying to calculate the enthalpy change for a reaction where a solid reactant is involved, you would need to account for the energy needed to vaporize the solid first.
what happens bonding wise in chemical reactions
energy is required to break bonds, bond breaking is endothermic
energy is released when bonds form, bond making is exothermic
worked example combustion of propane
PHONE
enthalpy profile diagrams for and breaking in exothermic n endothermic reactions
how to calculate enthalpy change of reaction triangleRH
3E=(bond enthalpies in reactants)-(bond enthalpies in products)
why enthalpy changes from average bond enthalpies not standard enthalpy changes
bond enthalpies are averages across various compounds and they need all reactants n products to be gaseous so standard enthalpy accounts for all physical states of substances and uses standard conditions sometimes in liquid.if it involves liquids/solids average bond enthalpy calc will not be same as standard enthalpy change as they don’t account of that
Why is the enthalpy of combustion of methane calculated using average bond enthalpies different from the data book value?
summary questions p137-average bond enthalpy
The value using bond enthalpies is less exothermic than book value =
Bond enthalpy values assume all substances are in the gaseous state
So H₂O is treated as gaseous, not liquid
The data book ΔHᶜ uses water as a liquid, which releases more energy when formed
Therefore, the bond enthalpy calculation underestimates the true enthalpy change
what allows enthalpy changes to be determined indirectly
hess law
what’s hess law
if a reaction can take place by 2 routes and the starting n finishing conditions are the same the total enthalpy change is the same for each route so A+B=C
providing two values known we can calc third
idea comes from conservation of energy
enthalpy diagram illustrating hess law
worked example hess law enthalpy diagram
worked example of trianglefh of hess law
worked example trianglech of hess law
Why can’t we calculate enthalpy changes of formation values
Because elements would form many compounds alongside the one we’re looking at
To calculate unfamiliar Hess cycles
Label enthalpies given and work out 2 routes to solve for unknown
Equations for Hess law combustion and formation
If your finding reaction and ur given formation=P-R
If your finding formation and ur given
Combustion=R-P
If you’re finding combustion and ur given formation=R-P
