Module 3.2 Flashcards
vEnthalpy refers to what ?
all the heat energy that is stored in a chemical system .
“Chemical system” means all the chemicals present in the reaction .
At A-level , we dont tend to refer to ENERGY , when talking about reactions .
Insted we ue the term enthalpy
-Enthalpy has the symbol H .
What is the problem with enthalpy
Enthalpy is very difficult to determine .
-Insted , scientists determine the change in enthalpy .
Just imagine ite - 2 beakers
-one contains an acid dissolved in water .
-the other beaker contains an alkali dissolved in water .
-If we were to mix these two beakers together
NEUTRALISION WOULD OCCUR .
-NEUTRALISATION –> RELEASES het eneergy . So , if we were to mesure the temperature of our solutions before an after we mix them .
-Then the CHANGE in temperature can be used to calculate the enthalpy change .
equation for
enthalpy change
-products enthalpy - reactants enthalpy-Delt triangle thing , check sheet means change in .
-We can represent the enthalpy changes on a profile like this .
-look at equation is HCL .
-Enthlalpy profile represents the reaction shown t the top , The nuetralisation reaction described before .
describe the energy profile diagram .
-as youc an see , the enthalpy of the products is LESS than the reaction Meaning the chnge in enthalpy is NEGATIVE .
-+We show this with a downwards point arrow .
-Telling us heat energy was rleased from the cemial system to the SURROUDNINGS , known as EXOTHERMIC RREACTIONS .
-iN THIS REACTIOn (hcl ) , the chemicals are dissolved in water . So the surroudnings are the water that the reaction tkes place in .
-Becuase , heat eneergy has passed form the chemical system to the surroudnings , the temperture of the surroudnings will increase .
You will notice that the enthalpy of the chemical system icnreases before it decreases . This increse is called the ctivation energy .
Ea rrow is the activation eneergy .
-In order for any reaction to take place , we start by BREAKING the chemicla bonds in the reactant moelcules .
-Breaking the chemical bonds , REQUIRES ENERGY and thooshown in the ctivation eenergy of the reaction .
What is the activation energy ?
Minimum amount of energy needed for a reaction to take plac e.
-Activation energy always shown as an UPWARD arrow . This arrow runs from the enthalpy of the reactnts to the peak of the curve .
Things to rememeber with energy profile diagrams .
When drawing enthalpy diagrams , it is important to label the enthalpy chnges and activation eenrgy .
-Arrrow heds are the correctway aorund .
-Aslo need to write the formulas and state symbol for the reactants and porducts and these need to be balanced .
example of an energy profile diagram fo rendothermic reaction .
Int his reaction , we can see the enthal of the porducts is GREATER than the enthalpy of the reactants . meaning positive enthaly chne ,s o arrow head points UPWARDS .
-Thiss tells us the chemicl system hs taken in het energy from the surroudnigns .
-So in the case of endothermic reacions , temperature of surroudnings decrease .
-ACTIVTION ENERGY points upwards and it runs from the enthaly of the reactions to the PEAK OF THE CURVE .
kEY ; ALAWAYS DETERMINE ENTHALY CHANGES UNDEER STANDARD CONDITIONS.
-stndard conditions ,
-standard enthalpy changes is what we call them .
What is temperature ?
298 K =25 degrees .
Standard pressure ?
100KP
What is standard concentration ?
1moldm-3
All chemicals msut be in their standard states under standard codnitoins
(AKA - liquids , solid , gas _
4 different standard enthalpyc hanges that we need to udnerstand
need to learnn
1.Standard enthalpy change of combustion Check symbol for this and learn .
-The standard enthalpy of comustion is the enthalpy change ,w hen one mole of a substnce reacts completely with oxygen , under standrd conditions and all of the chemicls are in their standard state .
-Check sheet for the standard enthalpy change for the combustion of hydrocarbon ethane .
Check sheet for the standard enthalpy change for the combustion of hydrocarbon ethane . KEY POINTS (1)
-We KNOW this is complete comustion , as we have amde CO2 and H20 .
-All the chemicals are in standard states 298K and 100kPA pressure .
Check sheet for the standard enthalpy change for the combustion of hydrocarbon ethane . KEY POITNS (2)
-Lastly , the equation shows we are combusting to one mole of ethane .
-To balance this equation , for one mole o ethane , we need 3.5 moles of oxygen , can be tempting toDOULR BUT THIS IS WRONG , as we would then end up with 2 MOLES OF ETHANE .
-so we need to rememeber , standard enthalpy , change of comustion , always refer to the combustion of one moele of sustance .
2.Standard enthalpy change of netralisation
-check sheet for the symbol for t his .
-This is the enthalp change hwen one mole of water is formed , in a neutralisation reaction under standrd condiions .
-explanation for this reaction with HCL AND NAOH
KE ; in this REACTION ,T HE KEY PUTS THE HDYROGEN ION FROM THE ACID + THE HYDROXIDE ION FROM THE ALKALI .
this means STANDRD ENTHALLPYC HANGE OF NEURALISAITON S always THE SAME , bwtween anys trong acid and strong alkai = vluae is -57kjmol
3.Standard enthalpy change of formation
-check sheet for the symbol for it
-the enthalpy change , when 1 mole of a compound is formed from it elements , under standard condiions .
-All tge cgenucaks are ub tgere stdard states . Shwoing stans enthalpy change of formation of BUTANE .
keyy of stndrd enthalpy change of formation
-is to be accurta with the elements . We have are reactiong 4 moles of carbon with 5 moles of hydrogen .
-As an element , h dyrogen exists as the moelucle H2
-REMEMEBER - standrd enthaly change of formation for ONE MOLE OF A PORUCT .
see one more equation for th is
-check equation for for ammonia
-As you can see , in this equation , we are making 2 moles of ammonia .
-so for standard enthalpy of formation , we need to HALVE THE EQUATIOn (check sheet of rthis _
- Standard enthalpy change of reaction
-the entHALPY CHANGE WHEN A REACTION TAKES PLACE IN THE MOLAR QUANTITIES SHOWNB THE by the balanced chemical equations under standard conditions .
-All of the chemicals must be in their standard states.
-cHECK SHEET FOR THE SYMBOL.
-standard enthalpy change of reaction is used for any chemical reaction which does not fit into the previous 3 categories . check equation on sheet .
RECAP - of the last thing we looked at
we saw that standard enthalpy change of combustion is the enthalpy change .
-going to ,look at determining the standard enthalpy change of combustion for a liquid for example ethanol .
Step one
combust the fuel , and then we use the thermal energy released to heat a known mass of water .
-By measuring the temperature change of the water . We can determine the standard enthalpy change of combustion .
Check the sheet for the apparatus for combustion
we have ethanol in a spirit burner .
-Above this , we have a metal can , containing , a fixed mass of water .
-Scientists call this metal can a - calorimeter .
-We are using a thermometer to measure the temperature of the water .
step one of combustion of ethanol
We use a balance , to measure the starting mass of the spirit burner and fuel .
step two of combustion of ethanol
We then , use a thermometer to measure the start temperature of the water .
step three of combustion of ethanol
Next , we remove the cap from the spirit burner and immediately light the wick
step four of combustion of ethanol
Thermal energy released , now causes , the temperature of water to increase .
-we need to stir the water ,with the thermometer to make sure the thermal energy is what ?
step five of combustion of ethanol
After several minutes ,w e extinguish the flame by replacing the cap on the burner .
-Then we use a balance to measure the final mass from the starting mass .
-We can calculate the mass of the fuel that is combusted .
Sample results
Mass of water = 400g
Temperature of water = 36.5 degrees
Mass of fuel combusted = 3.75g
Now we need to calculate the standard enthalpy change of combustion
- Calculate thermal energy that transferred into the water .
-To do that we use this equation .
Check sheet for the equation .
-For this equation we can treat kelvin as equal to degrees .
… m is the mass of the water . a common mistake that students make is they use the mass of the fuel when they should use the mass of the water .
q= 4004.1836.5
q=61028J
CONVERT TO KJ BY /1000 Q=61.028Kj
-We can use this to calculate the standard enthalpy of combustion of ethanol .
-Units of standard enthalpy of combustion - KJMOL-1
“per m0l” refers to the amount of mols in fuels that was combusted .
3.75 g was combusted
mr of ethanol = 46.0 mol
mass/mr = 3.75g / 46gmp;
the number of mols of ethanol combusted is = 0.0815mol
calculate the standard enthalpy change of combustion
check sheet for the equation
61.028/0.0815
= 748kjmol
However , the published value for this is actually a lot higher at
-1367mol
there are several reasons for this .
Reason 1
If w e leave the unlit spirit burner unoccupied , then the fuel will evaporate .
-Making it appear , we burned more fuel than we actually did .
-DUE TO THIS , our results will show the reaction being less exothermic than it actually is .
Reason2
A lot of heat energy , released by the fuel , does not pass into the water .
-Some of the heat energy is transferred into the metal calorimeter and a great deal of heat energy is simply transferred to the air /
reason 3
also possible , that not al the fuel underwent complete combustion .
-If incomplete combustion occurs , then this releases less thermal energy than complete combustion .
reason 4
experiment , may not have been carried out under standard conditions
-check video for practice questions for this .
What is standard enthalpy change
When a reaction , takes place in the molar quantities , shown by the balanced chemical equation , under standard conditions with all of the chemicals in their standard state .
check sheet for the equation
-because Mg is more reactive , it can displace the copper form copper sulphate solution .
-This reaction , is exothermic so heat energy passes from the chemical system to the surroundings .
-
Why would the temperature of the surroundings increase ?
Temperature of the solution will increase , because we are adding a solid to a solution , so in this case , the surroundings are the solution .
CUS04 , = 100cm3 + 1moldm3 – now carry out the experiment .
step one , weigh out the magnesium powder .
-copper sulphate solution has a concentration of 1 moldm3 .
-100 cm3 of this solution contains 0.1mols of copper sulphate .
-we want the copper sulphate to be in excess .
-equation shows the magnesium reacts with the copper sulphate is 1:1 molar ratio.
If it is a 1:1 ratio , what does this mean ?
-This means , we need to add less than 0.1 mols of magnesium .
-If we want the copper sulphate to be in excess .
-We are going to add 0.05 mols of magnesium . The molar mass of magnesium is 24.3 mol , so we need to add 1.22 g .
step two of cup
-Next we place the polystyrene cup inside a glass beaker , to reduce the risk of tipping over .
-Now we place a thermometer in our solution , we need to make certain that this solution is the same temperature as the room .
-So remember to take readings every 30 seconds .
step three of cup
-now we add our pre weighed magnesium powder and stir with the thermometer .
-take temperature readings every 30 seconds for 5 minutes .
check the graph star one
magnesium powder was added at one minute 30 .
-as you can see , the temperature rises due to thermal energy released by the reaction .
graph , star two
once the temperature reaches a maximum , it can then decrease .
-Could take the maximum temperature and use this to calculate the standard enthalpy change of reaction .
What is the PROBLEM
-As we have seen , the temperature of the reaction increases and then decreases .
-This decrease in temperature is due to the thermal energy being lost to the surroundings for example the air .
Have to bear in mind , these heat losses will have taken places throughout the reaction .
-So , we need to correct for these , to get an accurate value for temperature increase .
-To do that , we draw a vertical line , from the start of the reaction . Then draw the line back from the cooling part oft he curve .
-Wherever the lines meet , is the point the temperature would have met if there was no cooling . (79 degrees )
start temp was 21 degrees , 79-2 = 58 degrees which is the change in temperature .
…
1.calculate the heat energy released by the reaction - use this equation on the sheet .
-as it is an aqueous solution , we can assume the density of this solution is close the the density of the water . aka 1g/cm3 .
-so the mass of this solution would be 100g.
We can now calculate , the specific capacity of water
2. standard enthalpy of the reaction = heat energy / mol
q= 1004.1858 /1000 = 22.299
2.22.22/0.05 mol = 48.988 kjmol
Check sheet for reaction in which chemical A is converted to Chemical B
-Chemical B is converted into chemical C
…
What does Hess’s Law state
That if a reaction can be carried out by two different pathways . Then the total enthalpy change for these two pathways will be the same .
-Provided that the starting and final conditions are the same for both pathways .
in the reaction n the sheet , there are two different pathways to make product c .
-What is the significance of making product c ?
As both pathways start with chemical A and end with chemical C , it means Hess;s law applies .
check sheet for reaction for my making reaction1 , what is this called ?
scientists call this an enthalpy . or a thermochemical cycle .
Why are enthalpy cycles useful ?
As some reactions simply do not take place under normal conditions .
-If we can find an alternative pathway going from reactants to products , then we can measure the enthalpy changes of these reactions and then apply Hess’s law to workout the enthalpy change that we need .
For questions requiring Hess’s law - data will be provided in the question .
-What is the most common type of data that will be given ?
-Standard enthalpy change of formation .
-Standard enthalpy change of combustion .
check sheet for typical question on standard enthalpy change of formation
methane + chlorine –> chloromethane + hcl .
-going to calculate the standard enthalpy of reaction using the standard enthalpy of formation products .
One thing to notice about this reaction
The standard enthalpy change of formation of chlorine is zero .
This is because chlorine is an element , not a compound and standard enthalpy change of formation only applies to compounds .
step one for first q
-write down all the elements involved
-one atom of carbon , standard state of carbon is solid .
h2 (g) standard state but there is 4 atoms so 2h2
cl2(g) as there are two atoms of chlorine at cl2 is the standard conditions .
-we can use all these elements to make both the reactants and the products for the reaction we need . What way do the arrows point ?
key : enthalpy cycles like this , when using standard enthalpy of formation , the arrows point UPWARDS .
-now fill in the data (check sheet )
-next step
now consider the directions of the arrow , we want to go from reactants to to the products .
-However , the lhs arrow is pointing in the wrong direction so remember to change the sign for the standard enthalpy change of formation of methane .
calculation for q1
total rhs - -176kjmol-1
+74.9kjmol-1 +(-176kjmol-1)
=-101.1kjmolo-1
check the next video for more practice questions
Standard enthalpy change of combustion data using ethane .
-The enthalpy change for this reaction is standard enthalpy change of formation .
check the sheet for the combustion data .
-What is key about all the combustion data .
We can combust all the reactants and products in the equation shown .
-In all of these cases , we are carrying out the complete combustion in the presence of oxygen .
-However , we don’t need to show oxygen in the equation .
What way do the arrows point for standard enthalpy of combustion ?
they point down
step one for combustion equation
write in the standard enthalpy changes of combustion , but take into account the number of moles reaction .
(check sheet )
step 2 for combusting
consider directions of arrows .
-we want to go from reactants to products .
-howeveer ,r hs the arrows are ponitng wrong so change sign
soc heck sheet
bond enthalpy equation
all chemicals involved in a reaction contain chemical bonds .
KEY ; during a chemical reaction , we first need to break all the chemical bonds in the reactant moleucles .
What do we need to do to break these chemical bonds ?
We need to put energy into the reaction , aka breaking the chemical bonds is an endothermic process .
-Once we have done that , we can form the chemical bonds in the product molecules .
-When we make chemical bonds , energy is released from the reaction .
AKA making chemical bonds is an exothermic reaction .
scientists have determined the energy required to break the chemical bonds , these values are called average bond enthalpy .
What is the definition for average bond enthalpy ?
Average bond enthalpy is the energy needed to break one mole of a specific bond of a molecule in the GASEOUS state .
-Energy required to break a specific bond ,w ill have the same values as the energy released when that bond s formed .
Why are they called average bond enthalpies >
-Check sheet for molecules .
-All of these molecules contain carbon to hydrogen bonds .
-The bond enthalpy of a specific bond depends on the molecules we find that bond in.
the c-h bond in all of these molecules is different , so what do scientists do ?
scientists take an average value for the bond enthalpies
check sheet for bond enthalpy equation
why are state symbols important here ?
h20 is presented as a gas .
-As bond enthalpies are determined for molecules in a gaseous state .
step one for bond enthalpies
show all the bonds
-then add up all the average bond enthalpies for the bonds , we need to break and reactant molecule .
then
on the right hand side as we are breaking bonds , putting energy in so it is an endothermic reaction .
-however for doing bond energies of left hands side , as it is forming bonds , it is an exothermic reaction , so should have a - sign .
then add it all up .
