Chapter 6: Thermochemistry (Chemical Energy)

Three helpful guidelines

1. Enthalpy is an extensive property. The magnitude of ΔH is directly proportional to amount of reactant consumed in the process. 2. The enthalpy change for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction. 3. The enthalpy change for a reaction depends on the physical state of the reactants and products

The heat of solution can be regarded as the sum of the enthalpy changes of three intermediate steps:

1. The breaking of bonds within the solute, such as the electrostatic attraction between two ions (endothermic) 2. The breaking of intermolecular attractive forces within the solvent, such as hydrogen bonds (endothermic) 3. The formation of new attractive solute-solvent bonds in solution (exothermic).

Energy

Energy is defined as the capacity to do work, or transfer heat.

The First Law of Thermodynamics

Energy is neither created nor destroyed. Energy is conserved. Any energy lost by the system is gained by the surroundings.

There are some standard enthalpy changes associated with certain processes:

Enthalpies of vaporization, ΔHvap (liquid to gas) • Enthalpies of fusion, ΔHfus (solid to liquid) • Enthalpies of combustion • Enthalpy of formation, ΔHf , (or heat of formation), the formation of a compound from its constituent elements.

increases the internal energy

Heat (q) added to the system, and/or (w) work done on the system

Enthalpy

Heat (q) transferred under constant-pressure conditions is called enthalpy, H state function

Hess's Law (Indirect Method)

Hess's law states that if a reaction is carried out in a series of steps, ΔH for the reaction will be equal to the sum of the enthalpy changes for the individual steps. Hess pointed out that the heat absorbed (or evolved) in a given chemical reaction is the same whether the process takes one step or several steps. This is useful when a reaction cannot be measured directly

There are many different types of Energy:

Radiant Energy Thermal Energy Chemical Energy Nuclear Energy Kinetic Energy Potential Energy

Constant Pressure Calorimetry

Since it is open to the atmosphere, it is constant Pressure. But we approximate this as an isolated system. Heat is exchanged between the reaction and the solution (water). The thermometer measures the change in temperature. The heat of the reaction, -qp = qsolution = C Δt = s m Δt

The Thermodynamic Standard State

The standard state of a material (pure substance, mixture or solution) is the reference point used to calculate its properties under different conditions. It is that of a pure substance in a specified state (its most stable form and physical state), at 1 atm pressure and at 25 °C; and 1 M concentration for all substances in solution. It is indicated by the superscript °

For expansion, since Vfinal > Vinitial, then

V must be positive. So expansion work is negative

Using Enthalpies of Formation to Calculate Enthalpies of Reaction (Direct Method)

We can calculate the standard enthalpy change for any reaction for which we know the ΔH°f values for all reactants and products. ΔH°rxn = Σ n ΔH°f (products) − Σ m ΔH°f (reactants) (m and n are the equation coefficients)

Mechanical work

can also be expressed by expansion or compression of a gas

closed

can exchange energy but not matter with surroundings.

open

can exchange mass (matter) and energy with surroundings (most common).

Enthalpy of Reaction

change in enthalpy going from reactants to products.

There are two common forms of Calorimeter

constant pressure and constant volume Constant Pressure Calorimetry is the simplest.

isolated

does not allow the transfer of either mass or energy. (E.g. sealed, vacuum coated thermos flask)

Work (w)

force (F) applied through a distance The SI unit of work is the Joule

Adiabatic Process

is defined as a process that occurs in an isolated system, meaning where neither energy nor matter crosses the system/surroundings boundary.

The state of a system

is defined by the values of all relevant macroscopic properties that specify an object's current condition. E.g. composition, energy, temperature, pressure and volume, etc.

Potential Energy

is due to the position relative to other objects. It is "stored energy" that results from attraction or repulsion (e.g. gravity or electrostatic attraction/repulsion).

Radiant Energy

is energy that comes from the sun (heating the Earth's surface and the atmosphere).

chemical

is stored within the structural units of chemical substances. It is determined by the type and arrangement of the atoms of the substance.

heat capacity

is the amount of heat required to raise the temperature of a given quantity of substance by 1 K (or 1 °C).

kenetic

is the energy of motion. Chemists usually relate this to molecular and electronic motion and movement. It depends on the mass (m), and speed (v) of an object. ke =𝟏/𝟐mv^2

nucleur

is the energy stored within the collection of protons and neutrons of an atom.

The heat of hydration

is the enthalpy change associated with the hydration process.

The heat of dilution

is the heat change associated with the dilution process.

The heat of solution, ΔHsoln,

is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.

Calorimetry

is the measurement of heat changes. (Apparatus used is a calorimeter). Understanding calorimetry depends on the concepts of specific heat, and heat capacity

Internal Energy (E)

is the sum of all kinetic and potential energy of all components of the system (translational, rotational, vibrational, electronic, nuclear, etc). It is very difficult to determine or measure the absolute total energy of entities, but it is more convenient to measure the change in Internal Energy (ΔE) for a given process: ΔE = Efinal - Einitial

The value of the overall heat of solution

is the sum of these steps. ΔHsoln = U + ΔHhydr Where U = Lattice energy (the energy required to completely separate one mole of a solid ionic compound into gaseous ions). The gaseous ions then must become hydrated, which is the heat of hydration.

Heat

is the transfer of thermal energy between two bodies that are at different temperatures. (Remember that temperature is related to the average kinetic energy of an object's particles). Thermochemistry is the study of heat change in chemical reactions.

bomb calorimeter

is used for constant volume reactions. A known amount of compound is placed the bomb. The sturdy bomb does not change its volume Gas is added to achieve the desired pressure. The bomb is immersed in a known amount of water. Ignition causes the reaction, and the heat released raises the temperature of the water. The bomb and the water create an isolated system.

A state function

m is defined by the values of all relevant macroscopic properties that specify an object's current condition. E.g. composition, energy, temperature, pressure and volume, etc.

Larger specific heats

mean it takes more energy to heat them up; and conversely, they release a larger amount of heat as they cool. Notice the large value for water - great for a cooling system like your body; or a heating system like radiators; also explains maritime climates.

Thermodynamic quantities have three parts

number, unit and sign (+ or -).

Pressure-volume work (or PV work)

occurs when the volume (V) of a system changes: So at constant pressure (such as atmospheric): Work, w = - PV P =opposing pressure against which the system (e.g. a piston) pushes. V = change in volume of gas during expansion = Vfinal - Vinitial.

The standard enthalpy of formation

of the most stable form of any element is zero.

three types of systems

open, closed and isolated

specific heat

or specific heat capacity is the heat capacity of one gram of substance. Molar heat capacity is the heat capacity of 1 mol of substance

A state function not depend

particular history how the change occurs the path by which the system arrived at its present state.

A state function depend

present condition only on the initial and final states of the system A state function describes the equilibrium state of a system

heat gained or lost (q) is directly

proportional to the temperature change (Δt). q = C Δt where C is the heat capacity, with units of J/oC, or J/K. C = s m where m = mass in grams, s has units of J/g oC. So combining these: q = s m Δt (Heat exchanged = Specific Heat × mass × temperature change)

force (F)

push or pull

The heat of solution

represents the difference between the enthalpy of the final solution and the enthalpies of its original components (i.e. the solute and the solvent).

thermal

the energy associated with the random motion of atoms and molecules.

Surroundings

the rest of the universe outside of our system (e.g. the room where the experiment is performed).

Endothermic

the system absorbs heat. Heat flows into system from surroundings. (+ΔE)

Exothermic

the system evolves heat. Heat flows out of system and into surroundings. (-ΔE)

Law of Conservation of energy

the total quantity of energy in the universe is assumed constant.

System

what we study (e.g. a beaker or flask).

Relating ΔE to Heat (q) and Work (w)

ΔE = q + w

constant volume

ΔE = q - PΔV = qv −qrxn = qcalorimter = CcalΔt where Ccal = heat capacity of the calorimeter (determined by known samples, calibration).

The change in enthalpy,

ΔH, equals the heat, qp gained or lost by the system when the process occurs under constant pressure (indicated by the subscript p). ΔH = Hfinal - Hinitial = qp If ΔH is positive the system has gained heat from the surroundings, and is an endothermic process. • If ΔH is negative, the system has released heat to the surroundings, and is an exothermic process.

The standard enthalpy of formation of a compound

ΔHdegreef is the change in enthalpy for the reaction that forms 1 mol of the compound from its elements, with all substances in their standard states.