This video covers how to calculate enthalpy changes in neutralization and combustion reactions. Link to practice worksheet: drive.google.com/file/d/0B3Sp...
Hi sir, Thank you! I was wondering, I noticed a question where there was a line drawn from the max temperature towards the y-axis (extrapolation/interpolation or something) - do we need to know about that for the current syllabus + what does it do? Thank you, Mr Jones
This can be used to find the maximum temperature difference without any heat loss. Yes, it is worth knowing this as it has come up in an exam question before.
The enthalpy of neutralisation is for the formation of one mol of water and is the enthalpy of combustion is the for the combustion of one mol of methanol.
Is there any reason that fo the second worked example you rounded from 192280 Jmol to 190kJ mol? Would it not be rounded to 192kJ mol? Also, I simply do (initial - final) for ΔT because it gives you the sign in the front of the calculation instantly and just wanted to make sure it works (gave the same final answer in both examples disregarding the rounding situation in Example B)
+David Safi The change in mass is given to 2 significant figures therefore the final answer is also given to 2 S.F. There are many different ways to calculate the enthalpy change, as long as you get the same answer you can use any method you like.
Exothermic reactions involve the increase in the temperature of the solution (if carried out in aqueous solutions). Basically it is heat being released from system to surroundings.
Sir please reply plzzz When we say the enthalpy change of a reaction,are we saying the heat change for that particular reaction no matter the moles or we mean 1 mole?I mean when we just say the enthaloy of reaction not form combustion like that?
@@MSJChem I mean sir when a chemical reaction no matter is it a combustion or neutralization is given and qn says find the enthalpy of the given reaction?In that case do we take the heat change for the given moles or stick to 1 mole?
@@MSJChem simply if something says what is the enthalpy of the given reaction then is it that we take the heat change for that particular moles of reactants and products?I am talking about the general case.
You divide the heat released by mol of limiting reactant to find the enthalpy change per mole. If it is neutralization, it is per mole of water formed, for combustion, it is burning one mole of substance.
@@MSJChem Sir I mean lets say only 0.5 mole of a substance a reacts with substance 0.5 mol b to give a product.If someone says what is the enthalpy of this reaction we say for 0.5 mol only right if it doesn't say neutralization or others?
When dealing with gases, use kelvin. When calculating enthalpy changes you can use either because an increase in 10 degrees Celsius is the same as an increase in 10 kelvin.
do u add the mass of the two reactants together since the mole rations are 1:1 and therefore the mass of the products will be the same? im a bit confused since u added the mass of the reactants but used the specific heat capacity of water and the only way it made sense to me was what i wrote at the start of the comment
why do we assume that all the heat generated from the neutalization reaction is transferred to water and not also the NaCl? Does it have something to with the states, I'm not sure?
I used the specific heat capacity of water which is 4.18. Of course, it might be different for aqueous solutions, but this is an assumption that we make in this kind of calculation.
@@MSJChem is the specific heat of other solutions found in the data booklet? Or do we have to calculate that ourselves? Thanks for the reply by the way.
No, there are no specific heat capacities given in the data booklet. You are not required to calculate the specific heat capacities yourself. We can use the value for water, even though the actual value of the solution may be different.
Yes - it can be the combined masses of the solutions or the mass of the solid reactant plus the mass of the solutions. In a bomb calorimeter it is the mass of water inside the calorimeter to which the heat is transferred.
The m in the equation is the mass of the substance that changed temperature. In this example the temperature of the water increased so that is used as the mass.
+Busola Olabiran Also how do you know which reactions are endo/exothermic based on the question? Because the questions don't give information about energy release or absorption, Thank you!
+Busola Olabiran It depends on the data that you are given in the question. In the first example, the volume, concentration and temperature are all given to 3 SF, so the answer is also given to 3 SF. The second example could also be given to 3 SF looking at the data given in the question (the temperature of the water).
+Busola Olabiran A negative value for the enthalpy change means the reaction is exothermic. A positive value for the enthalpy change means the reaction is endothermic. Neutralisation and combustion reactions are exothermic, they have a negative enthalpy change.
Both combustion and neutralisation are exothermic reactions and have a negative enthalpy change. Exothermic reactions release heat. Watch my previous video on enthalpy changes for more details: ua-cam.com/video/ubzch4uBPTU/v-deo.html
When the temperature rises,It means heat came out of the system hence the negative sign...in an instance where temperature drops it means heat gets into the system hence positive value..
I have a CT coming up so before doing thermochemistry should I first do stoichiometry
Hello is the hemoglobin levels included in this or in a different cell
Well that was helpful thank you 🙃
6:11 should we not convert the mass in kg?
Hi sir,
Thank you! I was wondering, I noticed a question where there was a line drawn from the max temperature towards the y-axis (extrapolation/interpolation or something) - do we need to know about that for the current syllabus + what does it do?
Thank you, Mr Jones
This can be used to find the maximum temperature difference without any heat loss. Yes, it is worth knowing this as it has come up in an exam question before.
Okay, thank you sir!
How is the reaction exothermic at 3:55 if the system is gaining heat??????????
When calculating q for the enthalpy of neutralization, is m the mass of NaCl solution or that of H2O?
It’s the mass of the solution.
5:57 how did you get 32.05 as the molar mass of methanol, how do you calculate it?
Added up the molar masses of carbon (12.01), oxygen (16.00) and 4 hydrogens (4.04).
very epic video indeed, just as good as Gangnam style :)
Why do you divide by mols of water for the neutralisation but divide by mols of methanol for the combustion to find the delta H? Thanks
The enthalpy of neutralisation is for the formation of one mol of water and is the enthalpy of combustion is the for the combustion of one mol of methanol.
Is there any reason that fo the second worked example you rounded from 192280 Jmol to 190kJ mol?
Would it not be rounded to 192kJ mol?
Also, I simply do (initial - final) for ΔT because it gives you the sign in the front of the calculation instantly and just wanted to make sure it works (gave the same final answer in both examples disregarding the rounding situation in Example B)
+David Safi The change in mass is given to 2 significant figures therefore the final answer is also given to 2 S.F. There are many different ways to calculate the enthalpy change, as long as you get the same answer you can use any method you like.
Ah alright, thanks for the fast reply I see what you mean now
Isn't number of moles (n) = mass/molar mass? The first example used n=CV, and would both give different values for n. Why?
For solutions, you use n=CV. If you have a solid reactant, you use n=m/M.
How do we know if the reaction is exothermic or not if the question does not include the + and - signs?
Exothermic reactions involve the increase in the temperature of the solution (if carried out in aqueous solutions). Basically it is heat being released from system to surroundings.
@@MSJChem Thank you!
Sir please reply plzzz
When we say the enthalpy change of a reaction,are we saying the heat change for that particular reaction no matter the moles or we mean 1 mole?I mean when we just say the enthaloy of reaction not form combustion like that?
Enathlpy of combustion and formation is for one mole as per the definition.
@@MSJChem I mean sir when a chemical reaction no matter is it a combustion or neutralization is given and qn says find the enthalpy of the given reaction?In that case do we take the heat change for the given moles or stick to 1 mole?
@@MSJChem simply if something says what is the enthalpy of the given reaction then is it that we take the heat change for that particular moles of reactants and products?I am talking about the general case.
You divide the heat released by mol of limiting reactant to find the enthalpy change per mole. If it is neutralization, it is per mole of water formed, for combustion, it is burning one mole of substance.
@@MSJChem Sir I mean lets say only 0.5 mole of a substance a reacts with substance 0.5 mol b to give a product.If someone says what is the enthalpy of this reaction we say for 0.5 mol only right if it doesn't say neutralization or others?
If at 2.45 there was a limiting reactant, do we use the moles of that limiting reactant when calculating delta H?
yes
Why does dividing your final change in enthalpy by the number of moles of water correct for the assumption you made about the mass of water produced?
Dividing by the amount in mol of water allows use to calculate the enthalpy change in kJ/mol of water produced.
at 2:42 why do we predict them to be 1cm3 if they already gave us a volume?
Volume is converted to mass, therefore, 100 cm3 = 100 g, assuming that the solution has same density as water.
When do you know to convert to K for temperature or to use degrees celsius
When dealing with gases, use kelvin. When calculating enthalpy changes you can use either because an increase in 10 degrees Celsius is the same as an increase in 10 kelvin.
@@MSJChem thank you
do u add the mass of the two reactants together since the mole rations are 1:1 and therefore the mass of the products will be the same? im a bit confused since u added the mass of the reactants but used the specific heat capacity of water and the only way it made sense to me was what i wrote at the start of the comment
nvm i see u made an assumption so im assuming that unless m and c are given on the question we can do the same?
m is mass so you need to know the total mass of the solution (or water).
why do we assume that all the heat generated from the neutalization reaction is transferred to water and not also the NaCl? Does it have something to with the states, I'm not sure?
basically, the heat is transferred from the system to the surroundings - the solution is the surroundings so we assume the heat is transferred there.
@@MSJChem Don't the measurements of the solution include the reactants (system ) as well.?
How did you get a specific heat of 4.18?
I used the specific heat capacity of water which is 4.18. Of course, it might be different for aqueous solutions, but this is an assumption that we make in this kind of calculation.
@@MSJChem is the specific heat of other solutions found in the data booklet? Or do we have to calculate that ourselves? Thanks for the reply by the way.
No, there are no specific heat capacities given in the data booklet. You are not required to calculate the specific heat capacities yourself. We can use the value for water, even though the actual value of the solution may be different.
@@MSJChem alright, thank you!
in q=mct, do we consider m to be mass of the reactants??
Yes - it can be the combined masses of the solutions or the mass of the solid reactant plus the mass of the solutions. In a bomb calorimeter it is the mass of water inside the calorimeter to which the heat is transferred.
@@MSJChem ok thank you sir
in the question at 5:02 why do u multiply with the mass of water and not methanol?
The m in the equation is the mass of the substance that changed temperature. In this example the temperature of the water increased so that is used as the mass.
Sir, how do we find the limiting reactant?
Basically you divide the amount in mol of each reactant by its coefficient in the balanced equation. The lowest value is the limiting reactant.
What if you have the heat capacity of the cup
The IB doesn’t require students to consider the heat absorbed by the cup.
thankyou !
how to tell if reaction is exothermic or endothermic?
The sign of the enthalpy change - if it is negative the reaction is exothermic and if it’s positive the reaction is endothermic.
How do we know how many significant figures to put answers to? You did one to 3sf and one to 2sf
+Busola Olabiran Also how do you know which reactions are endo/exothermic based on the question? Because the questions don't give information about energy release or absorption, Thank you!
+Busola Olabiran It depends on the data that you are given in the question. In the first example, the volume, concentration and temperature are all given to 3 SF, so the answer is also given to 3 SF.
The second example could also be given to 3 SF looking at the data given in the question (the temperature of the water).
Ah okay, thank you!
+Busola Olabiran A negative value for the enthalpy change means the reaction is exothermic. A positive value for the enthalpy change means the reaction is endothermic. Neutralisation and combustion reactions are exothermic, they have a negative enthalpy change.
Thank you so much
how did you get the mass?
In these calculations, we assume that the solution has the same density as water (1g/cm3), therefore 1 cm3 of solution is equal to 1g.
Thanks! How do you know the reaction is exothermic?
Both combustion and neutralisation are exothermic reactions and have a negative enthalpy change. Exothermic reactions release heat.
Watch my previous video on enthalpy changes for more details:
ua-cam.com/video/ubzch4uBPTU/v-deo.html
How'd you get the answer in positive but u changed it to negative??
The temperature of the solution increased which means the reaction is exothermic. Exothermic reactions have a negative enthalpy change.
When the temperature rises,It means heat came out of the system hence the negative sign...in an instance where temperature drops it means heat gets into the system hence positive value..
At 2:00 you divide 50 by 1000, where is the 1000 coming from?
To convert from cm3 to dm3
doesn’t make any fucking sense