How is the standard state of an element defined? Why do
elements ...

Question

How is the standard state of an element defined? Why do
elements always have ∆H°f = 0?

Accepted Answer

hi everyone This problem reads defined the standard state of elements and explain why their standard heat of formation is always equal to zero. To define this, we're going to want to go ahead and write out what we mean by standard state of an element and by standard state of an element we're referring to standard conditions and for standard conditions. So let's go ahead and write that standard conditions. What this means is the most stable form of a substance at one atmosphere pressure and 25°C. That is what we mean by standard state of an element and the standard entropy a formation of the most stable form of an element is equal to zero by definition. So since the entropy change for the formation of an element from itself is equal to zero, what that means is the standard entropy change or the standard? Yes. The standard entropy change of formation is going to equal zero. This is the reference state of other entropy changes to be calculated or measured. So looking at our answer choices, let's take a look at answer choice. One. The standard state of an element in its most stable form is zero degrees Celsius and one atmosphere pressure. This is already incorrect because of the zero degrees Celsius, the temperature needs to be 25 degrees Celsius. Okay, so statement be the standard state of an L. Um In its most stable form at 25°C and one atmosphere of pressure. So yes, this is standard conditions. The standard heat of formation of an element is equal to zero because the element is already in its standard state and the entropy change from itself is equal to zero. This is correct. So let's go ahead and highlight number B as our answer because this the way that it's written by definition is true. But let's go ahead and take a look at C N D. For see the standard state of an element Is in its most stable form at 0°C. In one atmosphere pressure, the temperature is incorrect. So that makes statements see incorrect. And lastly, the standard state of an element is in its most stable form at 25°C and one atmosphere of pressure. That is true. The second sentence, the standard heat of formation of an element is equal to zero because it is set to zero. To make calculations easier. This is not correct. So D is incorrect. Statement B here is going to be the correct answer and that is it for this problem. I hope this was helpful

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