Question

what is Enthalpy Values? Please also give me an example problem. this is not an assignment i just want to understand it better.

Answer 1

Step 1 - What is Enthalpy?

Enthalpy is, by definition, the heat involved in a chemical reaction. We usually represent it by the letter H.

A chemical reaction always involve heat exchanges: it may absorb heat from the surroundings or it may heat the surroundings. If a reaction absorbs heat, we call it endothermic. If it liberates heat, we call it exothermic.

To measure the heat involved in a reaction we use delta H, i.e., the variation of enthalpy:

`\Delta H=H_(products)-H_(reagents)`

There are two possible cases:

I) Endothermic reactions: heat is absorbed, therefore Hproducts > Hreagents, so:

`\Delta H>0`

II) Exothermic reactions: heat is lost, therefore Hproducts < Hreagents, so:

`\Delta H<0`

Step 2 - How do we measure enthalpy variation in an experimental setting?

Enthalpy variation can be measured by means of an equipment called calorimeter. Here's a picture of it:

A calorimeter has a sample chamber in which the reaction is carried on, surrounded by water. The reaction will exchange heat with the water, modifying its temperature, which can be detected by the termometer and the temperature reader.

Since heat can be related to temperature variation by the following formula:

`Q=mc\Delta T`

And since, for a chemical reaction, the heat exchanged is the enthalpy, we obtain that:

`\Delta H=mc\Delta T`

Thus, by knowing the mass of water in the calorimeter (m), its specific heat (c) and the temperature variation (provided by the termometer), we are able

to calculate the enthalpy variation for any reaction (at least theoretically).

Step 3 - An example problem

The following reaction is exothermic or endothermic? Justify.

`2H_(2(g))+O_(2(g))\rightarrow2H_2O_((g))\text{ }\Delta H<0`

Answer: This reaction is a combustion (reaction with O2). We expect of a combustion to liberate a lot of heat (after all, we can feel it ourselves when close to a firepit, for example).

Thus, we expect it to be exothermic. Indeed, we can confirm this by its enthalpy variation: its negative. Lost was thus lost during this reaction.