Enthalpy Level Diagrams

Enthalpy Level Diagrams

What is an enthalpy level diagram?

A diagram showing the relative enthalpies of the reactants and products in a reaction.

What is shown on the y-axis of an enthalpy level diagram?

Enthalpy, H.

How is enthalpy change calculated?

ΔH = H(products) − H(reactants).

Where are the products on an exothermic enthalpy level diagram?

At a lower enthalpy level than the reactants.

Where are the products on an endothermic enthalpy level diagram?

At a higher enthalpy level than the reactants.

Why are the products lower than the reactants in an exothermic reaction?

Energy is transferred from the system to the surroundings.

Why are the products higher than the reactants in an endothermic reaction?

Energy is transferred from the surroundings to the system.

What is the sign of ΔH for an exothermic reaction?

Negative.

What is the sign of ΔH for an endothermic reaction?

Positive.

What does ΔH < 0 indicate?

An exothermic reaction.

What does ΔH > 0 indicate?

An endothermic reaction.

For an exothermic reaction, how do the enthalpies of reactants and products compare?

H(reactants) > H(products).

For an endothermic reaction, how do the enthalpies of reactants and products compare?

H(reactants) < H(products).

Which direction should the ΔH arrow point on an enthalpy level diagram?

Always from the reactants towards the products.

Which direction does the ΔH arrow point for an exothermic reaction?

Downwards, from the higher reactant level to the lower product level.

Which direction does the ΔH arrow point for an endothermic reaction?

Upwards, from the lower reactant level to the higher product level.

What two things must be correct when labelling ΔH on an enthalpy level diagram?

The sign of ΔH and the direction of the ΔH arrow.

How do you draw an exothermic enthalpy level diagram?

Draw the reactants at a higher enthalpy level, products at a lower level, and a downward ΔH arrow.

How do you draw an endothermic enthalpy level diagram?

Draw the reactants at a lower enthalpy level, products at a higher level, and an upward ΔH arrow.

What does the vertical distance between the reactant and product levels represent?

The magnitude of the enthalpy change, ΔH.

If ΔH = −394 kJ mol⁻¹, is the reaction exothermic or endothermic?

Exothermic.

If ΔH = +394 kJ mol⁻¹, is the reaction exothermic or endothermic?

Endothermic.

For C(s) + O₂(g) → CO₂(g), ΔH = −394 kJ mol⁻¹. Which has the higher enthalpy?

The reactants, C(s) + O₂(g).

For C(s) + O₂(g) → CO₂(g), why is the ΔH arrow drawn downwards?

The products have lower enthalpy than the reactants, so ΔH is negative.

How can you tell whether a reaction is exothermic from an enthalpy level diagram?

The products are below the reactants and ΔH is negative.

How can you tell whether a reaction is endothermic from an enthalpy level diagram?

The products are above the reactants and ΔH is positive.