Hi Firstly I'd like to thank you for uploading these video's - they are a great revision source and make me feel much more confident going into my chemistry exams. My question is when calculating Enthalpy change of neutralisation, do you divide by the mols of H20. Also, when doing any enthalpy calculation, do you always use the mass of the the reactant thats in soultion when calulating Q? (I'm assuming like in the video if both reactants are in solution, you add the masses) Thanks again!
+kHromAtic99 Hi, thanks for your comment. Yes you divide by the moles of H2O produced as you are calculating per mole of water. To calculate Q it's always the mass of the solution that is being heated eg. if it's 100cm3 of water being heated then 100g goes into the Q calc. Students often mix up the mass of fuel that heats up the water with the mass of the water. Always the mass of the water. And yes if it's two solutions being mixed and the team changes then its the combined mass of the solutions. Hope this helps
Hannah Bee if a reagent is in excess then the moles of products are determined by the moles of the other reagent (aka the limiting reagent) hope that helps
Sir which mass do you use in enthalpy change solutions? In certain questions they use the volume of the solution added which is turned into mass using density. However, in others they use the mass of the solid too? When do you use which?
Questions normally include the phrase 'ignore any increase in mass of solution' Obviously if density of solution info given, they expect a conversion of volume to mass. I agree that some questions don't make it clear. In that case, mark schemes allow both inclusion/omission of solid mass
Hi, thanks for the video. What number of moles do you divide by when calculating enthalpy for the temperature change caused by a reaction? e.g. if you are reacting Zn + CuSO4, which number of moles do you use? I know that one might be in excess, e.g. CuSO4 is in excess, but you can still find the number of moles of CuSO4 that reacted even if it is in excess. So which number of moles would you use?
Hi there! It's because the ratio between both the acid and alkali and water is 1:1. Remember, it's all about the moles of water formed, so if there are 0.05 mol of acid (or 0.05 mol alkali) then 0.05 moles of water will form. Hope that makes sense!
If it’s an alcohol being combusted, it’s the from the mass of that. If it’s a reaction that takes place in the calorimeter itself, it’s the moles of the limiting reagent
In the second example, if the amount of moles were different for the two reactants, what number would you divide by in the final calculation (to work out the enthalpy change)?
MaChemGuy hi, is that only if it’s a 1:1 ratio? If 3 mol of 1 thing reacted with 2 of another, and the second was limiting, would u diving enthalpy change by 2?
+Sorcha Falvey I'm working out the moles of EACH chemical then. You combine the volumes to establish the mass of the solution, which I do around 10:30 :)
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2:32 never seen that type of thermometer
This video helped a lot; totally adding it to my study materials. I appreciate the educational upload!
Hi
Firstly I'd like to thank you for uploading these video's - they are a great revision source and make me feel much more confident going into my chemistry exams.
My question is when calculating Enthalpy change of neutralisation, do you divide by the mols of H20. Also, when doing any enthalpy calculation, do you always use the mass of the the reactant thats in soultion when calulating Q? (I'm assuming like in the video if both reactants are in solution, you add the masses)
Thanks again!
+kHromAtic99 Hi, thanks for your comment. Yes you divide by the moles of H2O produced as you are calculating per mole of water. To calculate Q it's always the mass of the solution that is being heated eg. if it's 100cm3 of water being heated then 100g goes into the Q calc. Students often mix up the mass of fuel that heats up the water with the mass of the water. Always the mass of the water. And yes if it's two solutions being mixed and the team changes then its the combined mass of the solutions. Hope this helps
ples continue making videos... they a re sooo good and helpful! Thnx a bunch!
Hi there Sir,
Just wondering, what is the significanceof a question emphasising that one reagent is in excess?
Thanks in advance.
Hannah Bee if a reagent is in excess then the moles of products are determined by the moles of the other reagent (aka the limiting reagent) hope that helps
MaChemGuy I think I get it. Thank you again🖒
Hi, is the second example of Enthalpy change of Neutralisation?
Sir which mass do you use in enthalpy change solutions? In certain questions they use the volume of the solution added which is turned into mass using density. However, in others they use the mass of the solid too? When do you use which?
Questions normally include the phrase 'ignore any increase in mass of solution' Obviously if density of solution info given, they expect a conversion of volume to mass. I agree that some questions don't make it clear. In that case, mark schemes allow both inclusion/omission of solid mass
Hi, thanks for the video.
What number of moles do you divide by when calculating enthalpy for the temperature change caused by a reaction?
e.g. if you are reacting Zn + CuSO4, which number of moles do you use?
I know that one might be in excess, e.g. CuSO4 is in excess, but you can still find the number of moles of CuSO4 that reacted even if it is in excess. So which number of moles would you use?
You divide by the moles of the limiting reagent
@@MaChemGuy thank you. Why is this? Because you could still determine how many moles of the excess reactant that were involved in the reaction
@@meh3080 Yes but they are directly related to the moles of limiting reagent.
Hi sir, why didn't you double the number of moles? 2 x 0.05 mol = 0.1 mol?
Hi there! It's because the ratio between both the acid and alkali and water is 1:1. Remember, it's all about the moles of water formed, so if there are 0.05 mol of acid (or 0.05 mol alkali) then 0.05 moles of water will form. Hope that makes sense!
@@MaChemGuy Ahh ok thanks so much MaChemGuy! I appreciated your very swift and amazing reply!
Thank you for your videos, love them!!
6:18 how do we know this is an exothermic reaction and when calcualting using q=mcAT is value of enthlapy always negative. Thank you sir
The temperature increases
Hi can I please ask, when using delta H = Q/n how do you know what to work out the moles of?
If it’s an alcohol being combusted, it’s the from the mass of that. If it’s a reaction that takes place in the calorimeter itself, it’s the moles of the limiting reagent
In the second example, if the amount of moles were different for the two reactants, what number would you divide by in the final calculation (to work out the enthalpy change)?
The smallest number of moles = the limiting reagent
Ah I understand, thank you :)
MaChemGuy hi, is that only if it’s a 1:1 ratio? If 3 mol of 1 thing reacted with 2 of another, and the second was limiting, would u diving enthalpy change by 2?
helpful video!
why do you do it in degrees c when it is per kelvin?
Temperature differences are the same whether in K or celcius
8:43 wouldn't you double the volume because there is a combined 0.05 Dm-3 of solution?
+Sorcha Falvey I'm working out the moles of EACH chemical then. You combine the volumes to establish the mass of the solution, which I do around 10:30 :)
+MaChemGuy thank you that makes sense , these videos are really fantastic!
q should be lower case!
Both are accepted
Not even a real thermometer, disliked.
Thanks for the view!
@@MaChemGuy (I didn’t actually dislike idk who did that 😔)