[tou]

Hi

I am not sure what is exactly meant with it. I know that |3| is -3 and 3. But how is that applied to chemistry?

Here is what I don't really get:

The absolute value of the enthalpy of the reactants and products cannot be known, but what can be measured is the difference between them.

... or...

Unlike enthalpy, absolute values of entropy can be measured. The standard entropy change for a reaction can be determined by calculating the difference between the entropy of products and reactants.

Can someone please explain this to me? What do they mean with absolute value?

Thanks!

[Astrokel]

In this case, absolute means the exact value. For example, enthalpy change is always a measured of relative value but not of exact value.

But how is that applied to chemistry?

It is a consequence of law of conservation of energy that absolute value of enthalpy cannot be obtained.

[cliverlong]

Hi

I am not sure what is exactly meant with it. I know that |3| is -3 and 3. But how is that applied to chemistry?

Here is what I don't really get:

The absolute value of the enthalpy of the reactants and products cannot be known, but what can be measured is the difference between them.

... or...

Unlike enthalpy, absolute values of entropy can be measured. The standard entropy change for a reaction can be determined by calculating the difference between the entropy of products and reactants.

Can someone please explain this to me? What do they mean with absolute value?

Thanks!

Hello,

May be thinking about temperature scales will help.

Two commonly known and used scales for measuring temperature are Celcius/ Centigrade and Kelvin http://en.wikipedia.org/wiki/Celsius

(Roughly but very nearly) one degree Celcius is one hundredth of the temperature change between freezing water, 0C,  and boiling water, 100C, at standard atmospheric pressure.

Now, one can achieve temperatures below 0C, they are negative temperatures

Therefore, the Celcius scale is a relative scale, relative to the temperature of the freezing point of water.

No temperature can be lower than 0K, so the Kelvin scale is an absolute scale of temperature.

Generally relative scales pick some "easy" to measure, reproducable and stable reference point against which to measure other values on the scale. The reference point in Celcius is freezing point of water.

If it is possible to identify some reference point that defines the "end" of some scale beyond which it is not possible to measure values, such as 0K on the Kelvin scale, then this is an absolute scale. Sometimes these absolute points are theoretical and can never be achieved - such as absolute zero of temperature or gravitational potential or zero mass.

In the case of chemical changes it may be impossible to define an absolute reference point as we are only able to measure in experiments changes in energy or temperature - for example, enthalpy changes.

In the case of electrode potential (and others in chemistry), the term standard is used not relative or absolute. I believe this means that all measurements are relative to some well defined, reproducable and "stable" measurement point. In the case of electrode potential this standard reference point is the hydrogen cell. It turns out that this results in the spread of electrode potentials for other half-cells, relative to the hydrogen cell, being between +3V and -3V.

Is that any help?

Clive