Homework Help: chemistry

Posted by Anonymous on Friday, July 18, 2008 at 6:02pm.

I originally used 1 gram of NaOH. What if I used 2 grams of NaOH. Would that change the heat evolved, Q and change in ethalpy, (delta)H?

Through direct calculation, I see that the Q change by only about 0.1 kcal, but for (delta)H, I see that the value halved(which is expected since the mole of NaOH would be doubled).

However, this is based on the assumption that the change in temperature remains the same. I'm not sure if the temperature will be smaller or larger with an increase in temperature.

Can you help me with this problem? Thanks!

chemistry - DrBob222, Friday, July 18, 2008 at 6:24pm
You need to clarify your question. I assume you have done something such as adding NaOH to water and measuring the temperature. Adding twice the mass of NaOH WILL change Q.
chemistry (for DrBob222) - Anonymous, Saturday, July 19, 2008 at 1:13am
Yes, NaOH is added to the water. How much will adding twice the mass of NaOH will change Q? I am asked what would have been the heat evolved based on my actual observations and what effect would this have on my calculation of (delta)H.

Would saying "based on the assumption that the change in temperature remains the same, the Q change by only about 0.1 kcal, but for (delta)H, I see that the value halved(which is expected since the mole of NaOH would be doubled)" be sufficient? I am not sure how much the temperature will change by adding twice the mass.

Thanks for the help!
- chemistry (for DrBob222) - DrBob222, Saturday, July 19, 2008 at 12:00pm
  Adding twice the amount of NaOH to the same amount of water will double Q but delta H per gram or delta H per mole will not change. If you ASSUME that the temperature will not change (a false assumption) then delta H will be halved. From a practical standpoint, does it make sense that if 1 g NaOH in 100 mL water changes the temperature of the water by 10 degrees that adding 2 grams would also change the temperature of the same water by 10 degrees. Doesn't sound reliable to me.

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