Experiment: Heat of Reaction help

[legking]

Okay, to begin with, I'll set the lab up for you. Apparently this is a pretty common lab, so I imagine most of you are familiar with it - I think I've heard it called a coffee-cup calorimeter lab before. I am to perform three reactions:

Reaction 1: Dissolve 1 tbsp (5.5) solid sodium hydroxide in 200 mL water; record temperature change.

Reaction 2: Mix 100 mL aqueous 1.0 mol/L NaOH with 100 mL aqueous 1.0 mol/L hydrochloric acid; record temperature change.

Reaction 3: Dissolve 1 tbsp (5.5 g) solid sodium hydroxide in 200 mL aqueous 1.0 mol/L hydrochloric acid; record temperature change.

Then I am to determine ΔH for each reaction, being the heat lost or gained by each reaction.

There are a series of questions I am to answer - some I've already answered but would like to verify, others I'm really lost on. I'll post them one or two at a time...

QUESTION 1: Write the net ionic equation for each reaction, and note the value of ΔH for each reaction.

MY ANSWER:

Now, I don't have any troubles crunching the numbers to determine ΔH, which is m•ΔT•Q. What I would like to know, though, is how I'm supposed to come up with a negative result. I mean, I know ΔH for each reaction is supposed to be negative because all the reactions are exothermic, but m, ΔT and Q are all positive numbers.

For example, ΔH for reaction 1 would be
ΔH = m•ΔT•Q
ΔH = (200 g + 5.5 g)(final temperature - initial temperature)(0.00418 kj/cal)
ΔH = (205.5 g)(41 °C - 26 °C)(0.00418 kj/cal)
ΔH = 13 kJ

I could achieve a negative result by making ΔT initial - final, but I always thought Δ values were final - initial. It may seem trivial, but it's bugging me.

Anyway, as for my net ionic equations:

For reaction 1, ionic equation: NaOH(s) ----- Na+(aq) + OH-(aq)
For reaction 2, ionic equation: OH-(aq) + H+(aq) ----- H2O(l)
For reaction 3, ionic equation: NaOH(s) + H+(aq) ----- H2O(l) + Na+(aq)

Do these look alright? I only ask because of Question 4:

QUESTION 4: Add the net ionic equation for Reaction 1 to the net ionic equation for Reaction 2. How does the result compare with the net ionic equation for Reaction 3?

MY ANSWER

Adding reactions 1 and 2:
NaOH(s) + OH-(aq) + H+(aq)----- Na+(aq) + OH-(aq) + H2O(l)

Compared to reaction 3:
NaOH(s) + H+(aq) ----- H2O(l) + Na+(aq)

The result is identical with the exception of an additional hydroxide ion found on either side of the reaction from the combination of the first two reactions. However, since reaction 3 is basically reactions 1 and 2 combined, shouldn't the two be identical? Did I get my net ionic equations wrong?

Please help! I will reward you all with a link to the funniest video you'll see this week!

 

[wmhcan]

yes, it is right, you just have to canceled out the OH-(aq) on both side of the equation, and for the negative value, the value you got is for the final solution, which means if you reverse your equation, it is positive, if you want to write it like "NaOH(s) ----- Na+(aq) + OH-(aq)", then it is negative.

 

[Blueshift5]

I would like to start by commending you for setting up the lab correctly and following the instructions for the three reactions. It is important to always follow the protocol in an experiment to ensure accurate results.

To address your concern about obtaining a negative result for ΔH, it is important to remember that ΔH is a measure of heat lost or gained by the reaction. In exothermic reactions, heat is released, resulting in a negative value for ΔH. This negative value indicates that energy is being released from the system. In your calculations, you are correct to use the final temperature minus the initial temperature, as this is the change in temperature caused by the reaction. The positive values for m, ΔT, and Q simply represent the magnitude of the change in temperature and the amount of heat released.

Moving on to your net ionic equations, they look accurate for the most part. However, I would like to point out that in reaction 1, the sodium hydroxide is in solid form, so it should be written as NaOH(s) in the ionic equation. Also, in reaction 2, you have written OH-(aq) twice, but it should only appear once as it is a spectator ion. The correct net ionic equation for reaction 2 is H+(aq) + OH-(aq) → H2O(l). In reaction 3, you have correctly combined reactions 1 and 2, but it should be NaOH(aq) instead of NaOH(s) as it is now in aqueous form after being dissolved in the hydrochloric acid solution.

In response to question 4, you are correct in your observation that the result of adding the net ionic equations for reactions 1 and 2 is identical to the net ionic equation for reaction 3. This is because reaction 3 is essentially the combination of reactions 1 and 2. The additional hydroxide ion on both sides of the equation is just a result of combining the two reactions.

I hope this helps clarify your questions and concerns. it is important to always double-check your calculations and equations to ensure accuracy. I look forward to seeing the results of your experiment and the link to the funny video you promised!