LT#3: Hess's Law

Question 1

Hess’s Law states…

Answer 1

The enthalpy change of a physical or chemical process depends only on the enthalpy of the original reactants, and the final products.

It is independent of the pathway or number of steps in the process.

Question 2

When would you use Hess’s Law?

Answer 2

1. When you can’t perform the reaction you want due to safety or equipment limitations.
2. The reaction gives off energy in a form other than heat.
3. You don’t want to bother doing the reaction at all.

Question 3

Describe the algebraic method of Hess’s Law

Answer 3

You manipulate and combine chemical reactions with known ∆H values so that they add together and equal the equation that you want (target).

Question 4

While performing the algebraic method of Hess’s Law, if you multiply an equation, what do you do to the ∆H value?

Answer 4

Multiply it by the same factor.

Question 5

While performing the algebraic method of Hess’s Law, if you write an equation backwards, what do you do to the ∆H value?

Answer 5

Flip the sign

Question 6

What is the equation for Hess’s Law Enthalpy of Formation Method?

Answer 6

∆H˚ = ∑[n∆H˚f Products] - ∑[n∆H˚f Reactants]

Question 7

Define standard enthalpy of formation (∆H˚f)

Answer 7

The enthalpy change when 1 mole of a substance is formed directly from its elements in their standard states.

Question 8

How do you determine the standard state for an element?

Answer 8

Its most stable form at SATP.

Question 9

What is the ∆H˚f of an element in its standard state?

Answer 9

0 KJ/mol