General Chemistry 1 - Chapter 5 - Thermochemistry

Energy

The ability to do work or transfer heat.

Thermodynamics

Study of energy and its transformations.

Thermochemistry

Study of chemical reactions and the energy changes that involve heat.

Electrostatic Potential Energy

The most important form of potential energy in molecules (E_el = (_kQ₁Q₂)/d).

Energy is _____ when chemical bonds are formed.

Energy is _____ when chemical bonds are broken.

Released, consumed

Electrostatic Potential Energy (E_el) is between ______ charged ions.

Oppositely

First Law of Thermodynamics

Energy can be converted from one form to another, but it is neither created nor destroyed.

Explain the relationship between the system and the surroundings.

The system is the portion of the universe we single out to study, while the surroundings are everything else.

System + Surroundings = The Universe.

Where the universe can be as big or as small as we make it.

Open System

Exchanges heat and mass with its surroundings.

Closed System

Exchanges ONLY heat with its surroundings.

Isolated System

CANNOT exchange heat or mass with its surrounds.

Internal Energy (E)

The sum of all kinetic and potential energy of a system.

When you have /\ (delta) of anything, it is equal to the final minus the initial. So, /\X = X_final - X_initial. This can also be written as /\X = X_products - X_reactants.

• /\E = E_final - E_initial

• /\H = H_products - H_reactants

And so on….

A positive /\E (/\E > 0, E_final > E_initial) results when the system _____ energy from the surroundings.

A negative /\E (/\E < 0, E_final < E_initial) results when the system _____ energy to the surroundings.

gains, loses/releases

What are q and w in the equation, /\E = q + w?

q is the term for heat, and w is the term for work.

In the equation /\E = q + w,

1.) If q is positive, the system….

2.) If q is negative, the system…

1.) GAINS heat

2.) LOSES heat

In the equation /\E = q + w,

1.) If w is positive, work is…

2.) If w is negative, work is…

1.) done ON the system

2.) done BY the system

In the equation /\E = q + w,

1.) If /\E is positive, the system….

2.) If /\E is negative, the system…

1.) Has a net GAIN of energy

2.) Has a net LOSS of energy

Because energy is always conserved, the numerical value of q for the system and surroundings is always…

The same, just with different signs (±).

Endothermic Exchange of Heat

When the system absorbs heat.

• q_sys > 0

• /\H > 0

Exothermic Exchange of Heat

When the system loses heat.

• q_sys < 0

• /\H < 0

State functions

A function that depends only on initial and final states (like internal energy, E), rather than the path by which the system arrived at that state.

Path Functions

A function that depends on the path by which the system arrived at that state, (like heat, q, and work, w), rather than just the initial and final states.

Formula for work?

W = -P*/\V,

Where /\V = V_final - V_initial

Usually, the work done by a chemical or physical change is the mechanical work associated with a change in volume of ___.

gas

Define enthalpy.

The internal energy plus the product of pressure and volume.

H = E + P*V

When a system changes at constant pressure, enthalpy can be written as /\H = /\E + P*/\V. By substituting the values of /\E and w, you can simplify this equation to be….

/\H = q

Or the change in enthalpy is the heat gained or lost

REMEMBER THIS IS ONLY VALID AT CONSTANT PRESSURE

Is enthalpy an extensive or intensive property?

Extensive, because it depends on the sample size.

What is the relationship of the enthalpy change between a reaction and its reverse reaction?

The reactions are equal in magnitude, but opposite in sign (±).

True or false: The enthalpy change for a reaction depends on the states of the reactants and products.

True.