Energetics Flashcards
all pages from energetics chemrevise except the experiment page
standard conditions
100 kPa and 298 K and standard states
Bond enthalpy
the amount of energy required to break one mole of the stated bond at gaseous state
Mean bond enthalpy
When does this definition only apply?
the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
when the substances start and end in the gaseous state.
Standard enthalpy of reaction definition and symbol
🔺rH (circle with line through it meaning standard conditions)
Enthalpy change for the reaction to occur completely under standard conditions and at the molar quantities stated in the equation
Standard enthalpy of formation definition and symbol
🔺fH (circle with line through it meaning standard conditions)
Enthalpy formation of one mol of the product, made from its elements, with all substances under standard conditions. ONE MOLE OF THE PRODUCT IS FORMED
Standard enthalpy of combustion definition and symbol
🔺cH (circle with line through it meaning standard conditions)
Energy change when one mole of a substance is burnt completely in excess oxygen with all substances under standard conditions.
ONE MOLE OF SUBSTANCE IS COMPLETELY BURNT (reactant has to be one mole)
Enthalpy of neutralisation definition and symbol
🔺neutH (circle with line through it meaning standard conditions)
Enthalpy change when an acid is neutralised by an alkali to produce one mole of water under standard conditions.
What to remember about enthalpy of combustion and moles?
Energy change calculated during a combustion reaction will be according to however many moles you reacted so if they ask you for the enthalpy of combustion it won’t always be the value you got in your experiment as you have to make sure it’s the value obtained when burning one mole.
Therefore say you got enthalpy of 100Kj for 2 moles then the enthalpy of combustion would be 50Kj as that would be per one mole.
Bonds broken - bonds made is the same as…
Reactant bonds - product bonds
What are the four types of reaction and explain an enthalpy of reaction value?
Every reaction has a standard enthalpy of reaction value it’s just that their are different types of reaction such as formation, combustion and neutralisation. According to whatever type of reaction it is the enthalpy of reaction value will be the same. Eg. The standard enthalpy of reaction for Mg + O2 —> CO2 + H2O might be 500Kj however this is also a combustion reaction so the enthalpy of combustion is also 500Kj.
Why might the experiment bond enthalpy be different to the data book bond enthalpy?
The experiment bond enthalpy is enthalpy of the bond for a different state than to what is in the book
What is the state standard bond enthalpy quoted in data books?
Gas
Write an equation to represent the bond enthalpy of O—H in water?
H2O (g)—> HO(g)+ H(g)
Write an equation to represent the mean bond enthalpy of O—H in water?
H20 —> OH + H
OH —> O + H
Each of these have separate bond enthalpies you add them up and divide by 2.
H2O + OH —> OH + H + O + H
Cancel out OH on both sides
H2O—> 2H + O
Simplify
1/2H2O(g) —> H(g) + 1/2O(g)
Describe directions for measuring standard enthalpy of two substances when you put them in a beaker and stir them.
Pipette
Initial temp
Add other substance and stir
Measure final temp
State Hess’ Law
total enthalpy change for a reaction is independent of the route by which the chemical change takes place
If it gives you an enthalpy of formation value what does this suggest
eg. enthalpy of formation of C2H6 is 100kj mol
you need to write an equation at the TOP which is for enthalpy formation (so the product is the thing it has given the value for)
eg. so you would write 2C + 3H2 –> C2H6 with 100kj mol over the top
if it asks you to work out a bond energy using a hess cycle then what usually goes on the top and bottom
and which way do arrows point and why?
top: they usually give you an equation involving the species with the bond it is asking you to find an enthalpy of so use this equation on top
bottom: the singular gaseous atoms of the elements required to make the substance that has the bond it is asking you to find the enthalpy of
arrows point DOWNWARDS because you are breaking the bonds into their singular atoms to get the enthalpy
hess cycles what to always include
STATE SYMBOLS
If you are doing a hess cycle to work out a bond enthalpy and at the bottom are the single atoms which make up the thing that has the bond you are trying to find, what state should these singular atoms be in
GAS
What goes on top of a hess cycle
the equation you want to find!!!
if you are trying to find the enthalpy associated with one arrow which direction should you go
base of the arrow you want to its tip (sometimes means taking the longest version of this route)
bond enthalpy is always
POSITIVE
because it is always ENDOTHERMIC since breaking a bond requires energy absorption
If it gives you an equaton showing a molecular liquid going to an atomic gas what should you be aware of
somewhere in your calculations you need to factor in these two separate enthalpy changes (enthalpy of going from a liquid to a gas [breaking intermolecular forces] AND of going from a molecule to an atom [breaking intramolecular forces])
how do you set up the hess cycle if they give you enthalpy of formation but you are working out enthalpy of combustion
top: enthalpy of combustion (because that is what you want to work out)
bottom: elements
diagonal arrow from elements upwards
how do you set up the hess cycle if they give you enthalpy of combustion but you are working out enthalpy of formation
top: enthalpy of formation (because that is what you want to work out)
bottom: products of combustion
diagonal arrows downwards to products of combustion
all enthalpies of combustion are always
NEGATIVE
enthalpies of formation can be
negative or positive
negative means the formation of the compound is exothermic
If you see enthalpy of formation what should you think?
It’s about elements
If you see enthalpy of combustion what should you think?
It’s about oxides
If you see enthalpy of neutralisation what should you think?
It’s about stuff turning into salts + water + carbon dioxide
If the hess cycle is about dissolving, what goes at the bottom and where do the arrows go?
aq at the bottom
arrows going towards aq
Why do we use Hess cycles?
We cannot measure delta H either because
-we can’t do that exact reaction without loads of stuff in the way
or
- because it is endothermic so no heat to measure
What law is Hess’ law related to?
The first law of thermodynamics; energy is always conserved.
When do we use Hess’ law?
when the enthalpy change for a reaction cannot be measured directly by experiments.
Give an example of when we might use Hess’ law, involving dissolving, to work out an enthalpy change.
to work out the enthalpy change to form a hydrated salt from an anhydrous salt.
Why can working out enthalpy change to form a hydrated salt from an anhydrous salt not be done experimentally?
because it is impossible to add exactly the right amount of water to the anhydrous salt
because it is hard to measure temperature change of a solid
What would you do to find enthalpy change to form a hydrated salt from an anhydrous salt using Hess?
dissolve both salts in excess water (forms a solution which is the bottom part of the Hess cycle), use their temperature changes to work out the enthalpy.
What is hess all about?
basically if you can’t directly react one thing with another thing but you want to find the enthalpy for this reaction, you find a common product which they both make when the same reactant is added to them, measure the enthalpy of these two separate reactions, then use these enthalpies to work around the arrows and calculate what youw ere originally trying to find out
enthalpy of reaction =
enthalpy of the top equation
sum of enthalpy of formation of products - sum of enthalpy of formation of reactants
enthalpy of combustion =
(enthalpy of the top equation)
?????? check later
=sum of enthalpy of combustion of reactants - sum of enthalpy of combustion of products
(it is reactants - products this time because the arrows go downwards)
heat capacity units
J g^-1 K^-1
I think this is the same as J/g/K
Q = mC delta T units
Q - joules (J)
m - mass - g
C - heat capacity - J g^-1 K^-1
delta T - temperature - Kelvin (K)
what is enthalpy essentially
energy change per mole
What to assume about the density of solutions when working with Q=mCdeltaT?
assume that the solutions have the density of water,
which is 1g cm-3. (Eg 25cm3 will weigh 25 g)
How to calculate enthalpy change of a reaction (from experimental data)
General method
- Using Q=mCdeltaT, calculate energy change for quantities used
- Q/ moles (of reactant not in excess)
- Add a sign and unit (delta H and Kj/mol)
How to convert from Jmol-1 to kJmol-1
divide by 1000
In a neutralisation reaction, Q=mCdeltaT, what makes up the mass?
the mass of the acid + mass of the alkali
When calculating enthalpy change of a reaction, what do you do if you aren’t told which substance is excess?
need to work out the moles of both reactants and work out using the balanced equation which one is in excess.
In a combustion reaction, Q=mCdeltaT, what makes up the mass?
mass of water (NOT alcohol)
because you are measuring the temp change of the water
6 potential errors with the Q=mCdeltaT combustion calorimetry reaction
• Heat capacity of calorimeter not included
- Energy losses from calorimeter
• Evaporation of fuel after weighing
• Incomplete combustion of fuel
• Incomplete transfer of energy
• Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment
Why do we use MEAN bond enthalpies?
because every single bond in a compound has a slightly different bond energy
Equation to show the value for the bond enthalpy for the C-H bond in methane
¼ CH4 (g) ——-> C (g) + H (g)
What is enthalpy change?
the heat energy change (per mole I think) measured at constant
pressure
Two types of AS energetics reaction diagrams
- enthalpy level diagrams
- reaction profile diagram
Difference between energy level and reaction profile diagrams?
Reaction profile diagrams include the activation energy, energy level diagrams DO NOT.
Describe the heats of combustion for successive members of a homologous series
there is a constant rise in the size of the heats of combustion as the number of carbon atoms increases
If you were to plot a graph of enthalpy of combustion as hydrocrabon chain increases, having worked out the data experimentally then what will the results be like?
and how would they if you calculated the enthalpy of combustion instead?
the experimental results will be much lower than the calculated one due to inevitable significant heat loss (or incomplete combustion leading to less energy being released)
*imagine a line going up to the top right corner of the graph, this is the plots when using calculated results. a line below this going to the middle right side is with experimental results)
When doing Q/moles to work out enthalpy (enthalpy of the reaction) what should you remember
that it should be the moles of THE REACTANT IN EXCESS
where is activation energy always labelled on reaction profile diagrams?
from the top of the hump to the reactant line
in reaction profiles what does the line between the reactants and the products indicate
overall energy change
which way does the arrow in REACTION PROFILES point for exothermic and endothermic recations?
exothermic - downwards
endothermic - upwards
are energy level diagrams or reaction profiles more useful and why?
reaction profiles
because the describes how the energy of the chemicals change during the reaction
SEE WORD DOCUMENT FOR ALL TYPES OF REACTION PROFILE IMAGE EXAMPLES
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