Chemistry enthalphy change question
In a past paper this is the question:
In a different experiment 50.0 cm3 of 0.500 mol dm–3 aqueous hydrochloric acid are
reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3
The markscheme has this as the answer:
n(HCl) or n(NaOH) = 50 x 0.500 / 1000 = 0.025 moles
q = –ΔH x n = 57.1 x 0.025 = 1.4275 kJ
ΔT = q/mc
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
Final Temperature = 18.5 + 3.4 = 21.9 °C
Which I get up until
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
... why do they times by 1000? Does the volume need to be in dm^3 or something?
In a different experiment 50.0 cm3 of 0.500 mol dm–3 aqueous hydrochloric acid are
reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3
The markscheme has this as the answer:
n(HCl) or n(NaOH) = 50 x 0.500 / 1000 = 0.025 moles
q = –ΔH x n = 57.1 x 0.025 = 1.4275 kJ
ΔT = q/mc
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
Final Temperature = 18.5 + 3.4 = 21.9 °C
Which I get up until
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
... why do they times by 1000? Does the volume need to be in dm^3 or something?
Original post by bl0bf1sh
q = 1.4275 is in kilojoules
for the ΔT = q/mc you want q to be in J
for the ΔT = q/mc you want q to be in J
wow I feel dumb, I guess the exam stress is getting to me
thank you and sorry you had to witness that
Original post by KookieCrumble
wow I feel dumb, I guess the exam stress is getting to me
thank you and sorry you had to witness that
thank you and sorry you had to witness that
no worries, it happens to everyone!
good luck in your exams
try here it may help a bit
https://science-revision.co.uk/A-level_energetics_enthalpyy_in_solution.html
https://science-revision.co.uk/A-level_energetics_enthalpyy_in_solution.html
Original post by bl0bf1sh
q = 1.4275 is in kilojoules
for the ΔT = q/mc you want q to be in J
for the ΔT = q/mc you want q to be in J
Why did they multiply 4.18 by 100
Original post by Mathswarrior
Why did they multiply 4.18 by 100
ΔT = q ÷ mc = (1.4275 x 1000) ÷ (100 x 4.18)
4.18 J K–1 g–1 is the specific heat capacity (c) of the reaction mixture
100 g is the mass (m) of the mixture (they are aqueous solutions. The mass of 1 cm3 of water is 1 g, so assume that 50 cm3 + 50 cm3 (the two solutions) is about 100 g)
Original post by bl0bf1sh
ΔT = q ÷ mc = (1.4275 x 1000) ÷ (100 x 4.18)
4.18 J K–1 g–1 is the specific heat capacity (c) of the reaction mixture
100 g is the mass (m) of the mixture (they are aqueous solutions. The mass of 1 cm3 of water is 1 g, so assume that 50 cm3 + 50 cm3 (the two solutions) is about 100 g)
4.18 J K–1 g–1 is the specific heat capacity (c) of the reaction mixture
100 g is the mass (m) of the mixture (they are aqueous solutions. The mass of 1 cm3 of water is 1 g, so assume that 50 cm3 + 50 cm3 (the two solutions) is about 100 g)
Ohh, thanks so much. I got confused but it's good now.
why is q= negative of delta H ????????????
Original post by Ajsjdhdhdh
why is q= negative of delta H ????????????
It's just showing that it's an exothermic reaction but when you use the delta H value in your calculation you don't use the negative
Original post by bl0bf1sh
q = 1.4275 is in kilojoules
for the ΔT = q/mc you want q to be in J
for the ΔT = q/mc you want q to be in J
why are we multiplying 4.18 by 100 then?
Original post by mahii786
why are we multiplying 4.18 by 100 then?
100 g is the mass, which you multiply by the specific heat capacity
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