Chemistry - Unit 3: Thermodynamics

What is Thermodynamics / Thermochemistry?

It is the study of energy change (Enthalpy)

What are the two parts of the "universe"?

The universe is divided into two parts: system & surroundings.
- The system is the part of the universe that you are studying.
- The surroundings are everything else in the universe.

What is the difference between Enthalpy and the Enthalpy of Reaction?

Enthalpy: The specific heat content of a system.
Enthalpy of Reaction: The CHANGE in the heat content (or enthalpy) during a reaction.

What is the difference between a closed system and a open system?

Closed system: exchange of energy only.
Open system: exchange of matter and energy.

What is the first law of thermodynamics?

Energy cannot be created nor destroyed, it can only be converted from one form of energy to another.
** Energy Gained = Energy Lost

How does heat transfer to objects?

Heat will always transfer from hot to cold.
** Nothing gets cold, it just gets less hot.

How does thermal energy get transferred?

Through heat.

What is the second law of thermodynamics?

When energy is converted from one form to another, some energy is lost within the system.
** Nothing 100% efficient.

What is kinetic energy?

The transfer of thermal / kinetic energy (energy in motion).
** Temperature is kinetic energy.

What is specific heat capacity?

The amount of energy required to heat one gram of substance by one degree celsius.
** A measure of how well a substance preserves its temperature.

What do substances with higher heat capacity require?

They require more energy to increase their temperature. (water)

What do substances with lower heat capacity require?

They require less energy to increase their temperature. (metals)

When should you use Q = mcAT?

When their is a temperature change present. (Thermal energy present)

What is Potential energy in chemistry?

Bond energy.

What does "Break in, and Making Out" refer to?

When energy is absorbed INTO a reaction, bonds are BROKEN. (endo)
When energy is released OUT of a reaction, bonds are MADE. (exo)

What are intermolecular bonds? When are they broken?

They are bonds that are between molecules. They broken during phase change.
** LDF, DD, HB

What are intramolecular bonds? What happens when these bonds are made or broken?

They are between individual atoms/ions. Energy changes occur when these types of bonds are made or broken during a chemical reaction.
** Ionic or Covalent.

What are potential energy diagrams?

These graphs show the change in potential energy for a bond energy change, and the difference in energy between reactants and products.
** Always start diagram at zero

How does potential energy relate to an exothermic reaction? How does this relate to the surroundings?

Potential energy of the reactants is always GREATER than the potential energy of the products. As a result of this, the surroundings feel warmer.

How does potential energy relate to an endothermic reaction? How does this relate to the surroundings?

Potential energy of the reactants are LESSER than the potential energy of the products. As a result of this, the surroundings feel cooler.

What is the molar heat / molar enthalpy?

The amount of energy gained or released when one mol of substance is reacted (kJ/mol)
** Energy change that one mol of substance undergoes

Where does all energy originate from?

The sun.

What is a Calorimeter?

An apparatus used to measure energy change from heat being released or absorbed of a system.

What is significant about the water in a calorimeter?

By knowing how much heat the water is losing / gaining, we are able to find out how much energy the substance is absorbing or releasing.

What is the most important part of a Calorimeter problem that needs to be considered?

Heat lost = Heat gained.
** If the system loses energy (exo), the surroundings (endo) will gain the same amount of energy that was lost. (vice versa)

What type of reaction is it when the water (surroundings) gains energy (heat), and the reaction (system) loses energy (heat)?

The reaction is EXOTHERMIC.

What type of reaction is it when the water (surroundings) loses energy (heat), and the reaction (system) gains energy (heat)?

The reaction is ENDOTHERMIC.

What is the specific heat capacity of water AND dilute aqueous solutions?

4.19 J / g x C
** Can assume that dilute solutions are so dilute that they are essentially water.

What else can 1 g and 1 kg be equal to?

1 g = 1 mL
1 kg = 1 L

Can we use a Calorimeter to solve any enthalpy change? If not, why?

No, because can be different variables involved like the reaction being to slow, or other products being formed.

What is the principle of Hess's Law?

Net changes in some properties of a system are independent of the way the system changes from the initial to the final state.

How do you find the final enthalpy of a reaction using Hess's law?

By adding two or more thermochemical equations together, Their enthalpies can also be added together.

What is Hess's law?

It states that the addition of chemical reactions yeilds a NET chemical equation whose enthalpy is the sum of the individual enthalpy changes.

What happens to the individual enthalpy changes in reactions if the reaction is reversed or the reactions coefficients are multiplied / divided by a number?

Reversed: The individual enthalpy signs will be reversed.
Multiplied / Divided: The individual enthalpies will be multiplied or divided by the same value as the reaction.

What is the heats of formation an indirect measure for?

Thermal stability.

What is thermal stability?

The ability of a compound to resist decomposition when heated.

What is the general rule for thermal stability?

The more exothermic the formation, the more stable the compound (you have to add energy to decompose it)
**The greater the (negative) enthalpy change, the greater the thermal stability.

What remains zero when calculating the enthalpy change in a formation reaction? Why?

Any elements are denoted as having a molar enthalpy of formation as zero because they cannot be directly measured.

What is activation energy?

It is the energy barrier that must be overcome for a chemical reaction to occur.
** Species must reach this minimum energy before they can react.

Why is activation energy needed in order to make chemical reactions?

Some reactions do not react spontaneously when two reactants are mixed together. Some reactions require a "spark" or activation energy in order for the reactions to occur.

What is usually the form that activation energy is in?

Heat, light, or electricity.
** Input energy

How does activation energy change the the overall enthalpy?

It DOES NOT change the overall enthalpy.

What do catalysts do in terms of activation energy?

Catalysts reduce the amount of activation energy required to occur.
** Catalysts INCREASES the rate of the reaction in the forward and reverse reaction.

Are Catalysts consumed in a reaction?

No, catalysts only speed up a reaction.

How does activation energy increase?

Increase of temperature = more successful collisions = increased activation energy.

What is the activated complex? Where does it reside on a Ep diagram?

The activated complex a cluster of chemical species in the region at the top of a Ep diagram.

What are the chemical species that reside in the activated complex? What state are they in?

The chemical species are very unstable. There are no reactants or products that are formed and their are not real bonds. All species are in a transition state.

Can species in the activated complex become reactants or products? why?

Yes, because the activated complex is so unstable the species with in it can either break down to form products, or decompose to re-form reactants.

What happens to the kinetic energy of a substance when the substance goes through a phase change?

The kinetic energy of a substance does not change during a phase change (breaking of intermolecular bonds)