Heating Curves & Hess's Law

What happens to temperature during a phase change?

Temperature remains constant while the substance changes phase.

Why does temperature stay constant during a phase change?

Energy is used to break intermolecular forces instead of increasing kinetic energy.

What are the main phase changes shown on a heating curve?

Melting, freezing, vaporization, and condensation.

What happens during solid warming?

Temperature increases within the solid phase.

What equation is used for warming within a phase?

Heat equals mass times specific heat times change in temperature.

What happens during melting?

A solid is converted into a liquid at constant temperature.

What equation is used for melting?

Heat equals mass times heat of fusion.

What happens during liquid warming?

Temperature increases within the liquid phase.

What happens during vaporization?

A liquid is converted into a gas at constant temperature.

What equation is used for vaporization?

Heat equals mass times heat of vaporization.

What happens during gas warming?

Temperature increases within the gas phase.

What is the heat of fusion?

The energy required to convert a solid into a liquid at its melting point.

What is the heat of vaporization?

The energy required to convert a liquid into a gas at its boiling point.

In a heating curve problem, how is total heat calculated?

By summing the heat for each step including warming and phase changes.

What determines how many steps are needed in a heating curve problem?

The initial and final states and whether phase changes occur between them.

What is enthalpy?

The total heat content of a system at constant pressure.

What does ΔH represent?

The heat change for a reaction at constant pressure.

What is an exothermic reaction?

A reaction that releases heat to the surroundings.

What is an endothermic reaction?

A reaction that absorbs heat from the surroundings.

What is the sign of ΔH for exothermic reactions?

Negative.

What is the sign of ΔH for endothermic reactions?

Positive.

How can reaction enthalpy be determined experimentally?

By using calorimetry.

What are the three main ways to calculate reaction enthalpy?

Using calorimetry, bond enthalpies, or Hess's Law.

What is bond enthalpy?

The energy required to break one mole of a bond in the gas phase.

Is bond breaking endothermic or exothermic?

Endothermic.

Is bond formation endothermic or exothermic?

Exothermic.

What is the formula for reaction enthalpy using bond energies?

Sum of bond energies of bonds broken minus sum of bond energies of bonds formed.

Why are all species in bond enthalpy calculations considered gas phase?

Because bond enthalpies are defined for gaseous atoms.

What is standard enthalpy of formation?

The enthalpy change when one mole of a compound forms from its elements in their standard states.

What are standard conditions?

298 Kelvin and 1 atmosphere.

What is the enthalpy of formation for elements in their standard state?

Zero.

What is the formula for reaction enthalpy using formation values?

Sum of enthalpies of formation of products minus sum for reactants.

What is Hess's Law?

The total enthalpy change of a reaction is the sum of enthalpy changes of individual steps.

Why does Hess's Law work?

Because enthalpy is a state function.

What happens to ΔH when a reaction is reversed?

The sign of ΔH changes.

What happens to ΔH when a reaction is multiplied by a factor?

ΔH is multiplied by the same factor.

What is the goal when applying Hess's Law?

To manipulate given reactions so they add up to the target reaction.