How Do You Calculate the Molar Enthalpy of Formic Acid?

[JameB]

Homework Statement

I'm doing report for a Thermochemistry lab and I need to calculate the molar enthalpy of formic acid, ΔH°₂.

Two reactions are taking place

1) H₃O + OH <-> 2H₂O with ΔH°₁ = -58400J/mol and some q1
2) HCOOH + OH <-> H₂O + HCOO and we need to calculate ΔH°₂.

I also know the pH is 1.96 before the neutralization took place.

Homework Equations

q = nΔH°
q_system = q₁ + q₂ = -q_surroundings = -(C_calΔT_cal + C_vV_acidΔT_acid +C_vV_baseΔT_base)...[2]

The Attempt at a Solution

So my general approach is:::

First calcualate what q₁ is because I know the pH.

[H3O] = 10^-1.96
[H3O] = 0.011 M
=> moles of H3O = Vacid*[H3O] = 0.05L * 0.011M = 0.00055 mol H3O {Is this right?}

Then, q = nΔH° = 0.00055mol * 58400 J/mol = 32.12 {does this make sense?}

Now I calculate q₂ using equation [2]
Ccal = 76 J/°C
ΔTcal = 6.2°C

Cv = 4.16
Vacid = 50mL
ΔTacid = 6.2°C

Vbase = 50mL
ΔTbase = 5.2°C

q₂ = 2842.4J {correct?} Now, ΔH°₂ = q₂/moles of HCOOHHow do I find the moles of HCOOH? I'm told that "some of the formic acid dissociated and some of the OH reacted with H3O, so you need to determine the mole sof HCOOH left over after the reaction of H3O and OH"

How do I do that? and am I right so far?

 

[Borek]

It won't hurt if you will tell us what you really did, what are q₁ and q₂, and what are you expected to determine. We don't know how to read your mind, and what you wrote so far is not enough, it is chaotic and leaves a lot ambiguity. Some things you did are wrong for sure, no matter what you were trying to calculate.

 

[JameB]

Ignore that whole question, I have a whole new question:

if you have a galvanic cell set up like:

Pt | 0.1M Fe³⁺, 0.1M Fe²⁺ || 0.1M AgNO₃ | Ag

What does it mean to write a balanced cell reaction equation and how do I do it?

I know that the + electrode was Ag_(s) so that means that the cathode was Ag meaning Ag was reduced from Ag²⁺. So is the answer Pt_(s) + Ag²⁺_(aq) -> Pt²⁺_(aq) + Ag_(s)?

 

[Borek]

Please don't pot new questions in old threads, start a new one.

Pt is just an electrode material, it doesn't take part in the reaction.

 

[DeeNos]

Your overall approach is correct, but there are a few things to consider in your calculations:

1. The first reaction given is an equilibrium reaction, so it is important to take into account the concentrations of both H3O+ and OH- when determining the moles of H3O+ that reacted. You have correctly calculated the concentration of H3O+ at the start (before the neutralization reaction), but you also need to consider the concentration of OH- at the start since it will also react with H3O+.

2. In the second reaction, the molar enthalpy of formation for formic acid (ΔH°f) is -393.5 kJ/mol. This means that the reaction as written is an exothermic reaction, and you need to make sure your final value for ΔH°2 is a negative value.

3. To determine the moles of HCOOH left over after the reaction, you will need to use the stoichiometry of the reaction. From the given reactions, you can see that for every 2 moles of H3O+ that react, 1 mole of HCOOH is formed. So, you can use the moles of H3O+ that you calculated earlier to determine the moles of HCOOH that reacted, and then subtract that from the total moles of HCOOH that were initially present.

4. It is also important to consider the volume changes in the system when calculating q2. The volume of the solution will increase when H3O+ and OH- react to form H2O (since H2O is a liquid and takes up more volume than the reactants), so this will affect the final temperature and therefore the value of q2.

Overall, your approach is correct, but make sure to take into account the points mentioned above to get a more accurate and complete calculation.