Chemistry questions on Energy changes

anpanman SG

I was wondering that if you conduct an experiment, and the temperature you measured (say a reaction between an acid and another substance) increases, are you measuring the SURROUNDING temperature or the TEMPERATURE OF SOLUTION?

Thank a lot. By the way can someone tell me how to identify an endothermic and exothermic reaction? I know the properties but there are times when I find questions rather confusing. I usually take into consideration whether a particular reaction needs energy. If yes, it will absorb energy from surrounding and thus an endothermic reaction. If no, then it will give out energy to surrounding instead and hence an exorthermic reaction. Is this way of defining the 2 processes a right one or are there better alternatives? Please advise.

Example: Baking bread is considered an endothermic reaction because you need energy for that to happen. Thus heat energy is taken in from surrounding. Whereas, in the condensation of rain from water vapour, there is no need for energy to be absorbed. Thus energy is GIVEN OUT instead, rendering it an exothermic reaction.

And my third question is, during an exothermic reaction, heat energy is given out to surrounding, causing an increase in temperature of surrounding. Does this mean the object loses heat and vice versa for an endothermic reaction?

Please reply to my queries because I really want to have them answered ASAP. Thank you.

d3sT1nY SG

Erm. Let's say you are heating water.

Heat energy is supplied to the water thus there will be increase in temperature. Until the boiling point of the water, the temperature will keep increasing. At 100 degree Celsius, it's where the water starts evaporating into steam. Heat energy is supplied to break the H-bond between the water molecules instead of increasing the temperature. That's when water becomes water vapour in the air.

So if ice --> water is endothermic (think when u put ice water into a beaker, water condenses on the surface of the water), water --> steam is also endothermic.

Water vapour --> water is exothermic and water --> ice is also exothermic. But normally you don't feel the heat energy from the condensation because it's negligible. Neither can you feel it when freezing because of the environment.

Understand? Haha. Sorry that my explanation is a bit screwed. =X

So an exothermic reaction releases heat due to bond formation. You will FEEL the increase in temperature from the reaction. Can't think of any examples now though.

An endothermic reaction absorbs heat from the surrounding to break the bonds. Is either you provide heat to the reaction or the reaction happens spontaneously by itself.

Feel free to PM me then. I'll try my best to explain. Haha.

UltimaOnline SG

d3sT1nY is completely correct, of course. U can liao, anpanman?

anpanman SG

Understood! Thanks!!! (:

anpanman SG

I have another question though.

Which of the following processes are endothermic?

1 Oxidation of carbon to carbon dioxide

2 Dissolving of anhydrous copper(II) sulphate in water

3 Adding of water to ammonium nitrate

4 Formation of carbohydrate and oxygen from carbon dioxide and water

A 1 and 2 only

B 3 and 4 only

C 1, 2, and 3 only

D 2, 3 and 4 only

Isn't option 2 is an endothermic reaction? My Chemistry teacher said the answer is B. But I strongly think it is D. After all it is stated in the textbook that the dissolving of ionic compounds such as copper(II)sulphate crystals is endothermic. I am not sure whether CuSO4 being in crystal or anhydrous form has any effect on reaction being endothermic or exothermic. Because they are both in solid state.

Thanks to those who cleared my doubts!

16/f/lonely SG

When an ionic compound dissolves in water, there are two processes at work. Firstly, the ionic compound must be split up into its two sets of ions, the cation and the anion. These then are attached to the poles of one or more hydrogen ion. The splitting up of the lattice is endothermic (takes in energy) wheras the solvation of the ions produced (forming dipole-dipole bonds with the water molecules) is exothermic (gives out energy).

The implication is:Not all dissolving of ionic compounds are actually endothermic in nature. In fact, the more exothermic the dissolving, the easier the compound dissolves. Given this fact, and the knowledge that copper sulphate dissolves fairly easily, I think you can infer that it is actually exothermic in nature.

UltimaOnline SG

Aqueous ions have "ion-dipole" interactions with polar water solvent molecules, not "dipole-dipole".

anpanman SG

These then are attached to the poles of one or more hydrogen ion. The splitting up of the lattice is endothermic (takes in energy) wheras the solvation of the ions produced (forming dipole-dipole bonds with the water molecules) is exothermic (gives out energy).

The quoted parts, I do not understand. Can somone simplify the explanation please? Thank you very much!

16/f/lonely SG

The quoted parts, I do not understand. Can somone simplify the explanation please? Thank you very much!

H2O has got ends that are positively and negatively charged. O being the more electronegative atom is obviously the negative end, while the 2 H atoms form the postive ends.

What happens when a ionic compound enters water? The Hs in a H2O naturally attracts to the SO4 ions, while the Os are naturally attracted to the Cu ions. The bonds formed in this case are dipole-dipole bonds and causes the ions in the ionic compound to detach from the lattice structure. In other words, the formation of the bonds releases energy which is then used to detach the ions from the lattice structure.

If the energy released during the bond-formation does not exceed that needed to break free from the lattice structure (exo < endo), then external energy would have to be provided if the ionic compound is to be dissolved naturally. If endo < exo, then the compound will dissolve by itself. As is the case with copper sulphate.

16/f/lonely SG

Maybe this is a little beyond your level.....this is beyond O level.

UltimaOnline SG

16/f/lonely is in the army now, so yes, he's explaining from his past 'A' level experience... which is almost entirely correct, except for that fact that aqueous ions have ion-dipole interaction with water solvent molecules, not dipole-dipole.

But 16/f/lonely's explanation does bring up a point : 'O' level chemistry is oversimplified and some (basic) 'A' level concepts are certainly required for 'O' level students to properly understand chemistry matters, as opposed to blindly accepting and memorizing facts ("because the teacher/textbook said so.").

Nonetheless, the MOE/SEAB/UCLES syllabus is pragmatic and based on overall "what works" for the majority of students. 'O' level students will just have to be satisifed with the (over)simplified limited explanations (at 'O' levels). But for students with an innate interest and intellectual curiosity for a deeper, truer, comprehension, they could do their own research (the internet revolution has brought greater power and capabilities to everyone... everyone with an internet connection anyway).

(And/or they could come for my tuition. For the benefit, understanding and enjoyment of my students, I do not unreasonably limit discussions to syllabus contents. My 'O' level students enjoy a gainful glimpse of 'A' level chemistry, and my 'A' level students enjoy a gainful glimpse of University level chemistry. Results have shown students fare better when they understand (depth of which, may require going beyond syllabus requirements, but of course only where still relevant and helpful) rather than blindly accept and memorize. Which is an unfortunately over-common instruction from school teachers, due to a misplaced blind emphasis on paper exam results, leading to a fallacious thinking that understanding be sacrificed to blind memorizing for the end paper results).

So u can liao, anpanman? Cannot also must can. (army phrase). Just kidding. Any more questions, anpanman?

anpanman SG

Fully understood! Thanks a million times!