What does Hess's law say about the enthalpy of a reaction?

Answer 1

According to the law, a reaction's total enthalpy change occurs regardless of how many steps are involved in the process.

Stated differently, if a chemical change occurs through multiple pathways, the total enthalpy change remains constant, irrespective of the pathway used (as long as the starting and ending conditions remain the same).

When a reaction cannot be measured directly, Hess' law allows the enthalpy change (ΔH) to be calculated. This is done by applying previously determined values for the enthalpies of formation to basic algebraic operations based on the chemical equation of reactions.

A net equation, also called an overall equation, is created when chemical equations are added together. The enthalpy change for the net equation is obtained if the enthalpy change for each equation is known.

AN EXAMPLE

Determine the heat of combustion, #ΔH_"c"#, of CS₂, given the following equations.

Resolution

Put down the equation you are aiming for, the target equation.

CO2(g) + 2SO2(g) → CS₂(l) + 2O₂(g)

Commence with equation 3, which includes the target's first compound (CS₂).

To move the CS₂ to the left, we need to flip equation 3 and its ΔH, which yields equation A below.

A. CS₂(l) → C(s) + 2S(s); -#ΔH_"f"# = -87.9 kJ

We now remove C(s) and S(s) one at a time. Since C(s) is present in Equation 1, we express it as Equation B below.

B. C(s) + O₂(g) → CO₂(g); #ΔH_"c"# = -393.5 kJ
We use Equation 2 to eliminate the S(s), but we have to double it to get 2S(s). We also double its #ΔH#. We then get equation C below.
C. 2S(s) + 2O₂(g) → 2SO₂(g); #ΔH_"c"# = -593.6 kJ

The target equation is then obtained by adding equations A, B, and C, and canceling terms that appear on opposite sides of the reaction arrows.

A. CS₂(l) → C(s) + 2S(s); -#ΔH_"f"# = -87.9 kJ B. C(s) + O₂(g) → CO₂(g); #ΔH_"f"# = -393.5 kJ C. 2S(s) + 2O₂(g) → 2SO₂(g); #ΔH_"f"# = -593.6 kJ
CS₂(l) + 3O₂(g) → CO₂(g) + 2SO₂(g); #ΔH_"c"# = -1075.0 kJ
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Answer 2

Hess's law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps taken to reach the final state. In other words, the overall enthalpy change is independent of the pathway and depends only on the initial and final states of the system.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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