Unit 6 Part 1 - Thermochemistry

Covers modules 6.1-6.5 of AP Chemistry :)

What needs to happen for something to melt/boil? What happens ot the energy of the system?

For something to melt/boil, it requires in input of energy (endothermic).

The energy of the system increases.

What needs to happen for something to freeze/condense? What happens ot the energy of the system?

For something to freeze/condense, it requires a release of energy (exothermic).

The energy of the system decreases.

What happens when something goes through a phase change (change of state)?

It causes a change in potential energy, NOT kinetic energy. Therefore, the temperature of the system does not change.

What is ΔH_fusion?

It is the change in enthalpy of melting (+) or freezing (-); typical unit = kJ/mol.

What is ΔH_vaporization? What is the typical unit?

It is the change in enthalpy (boiling (+) or condensing (-)); typical unit = kJ/mol

Notice that BOTH exothermic/endothermic reactions have (the same/a different) magnitude (value) with opposite signs.

the same

What happens when you have a positive “q” value from the heat capacity equation?

It means the energy of the system is increasing and is endothermic.

What happens when you have a negative “q” value from the heat capacity equation?

It means the energy of the system is decreaseing and it is exothermic.

What is the definition of specifc heat?

The amount of energy required to raise the temperature of one gram of a substance by one degree Celsius/Kelvin.

Why is the specific heat different for every substance?

It is because the transfer of energy does not equal the same temperature change of different substances (at the same mass).

What is true about particles in a warm body in terms of kinetic energy?

They have a higher average kinetic energy THAN particles in a cooler body.

What happens when you pour particles from a warmer body into a cooler body?

Those warm particles, which are already moving faster, will make the cold particles move faster, increasing THEIR temperature, but decreasing the WARM particle temperature.

Why does the average kinetic energy of warmer particles DECREASE when added to cooler particles?

It is because the warmer particle’s energy is transferred into the cooler particles.

Energy lost by the system is absorbed by what?

the surroundings!

What happens when things are being heated (endothermic) in terms of a physical change?

Heat goes into the system and therefore increases the temperature CAUSING melting/vaporizing, etc.

What happens when things are being heated (endothermic) in terms of a chemical change?

The energy (heat) going into the system will break the bonds in the reactants, which will then be released by making new bonds in the products.