Chapter 5: Thermochemistry

Conversion factor of Latm to J

1 Latm = 101.3 J

Kilojoule

1000 joules, often used to model energy changes associated with chemical reactions

Bomb calorimeter

A device for measuring the heat evolved in the combustion of a substance under constant-volume conditions

Calorimeter

A device that measures heat flow

Chemical energy

A form of potential energy that is stored in chemical bonds between atoms.

System

A part of the universe on which you focus your attention to study

Exothermic

A process in which a system loses heat to its surroundings. An example would be combustion of gasoline where heat exits or flows out of a system into the surroundings.

Endothermic

A process in which a systems absorbs heat from its surroundings. For example, when ice melts, heat flows into the system from the surroundings.

State function

A property of a system that is determined by specifying the system's condition, or state. The value of a state function depends only on the present state of the system, not the path the system took to reach that state. Internal energy is a state function so it only depends on initial and final conditions of the system.

Combustion

A rapid reaction between oxygen and fuel that results in fire. A reaction in which a compound reacts completely with excess oxygen.

Open system

A system in which matter and energy can be exchanged with the surroundings. For example, an uncovered pot of boiling water on a stove so that steam is released to the surroundings.

Isolated system

A system in which neither energy nor matter can be exchanged with the surroundings. For instance, an insulated thermos.

Joule

A unit of work equal to one newton-meter, SI unit for energy

Calorie

Amount of energy needed to raise temperature 1 gram of water 1 degree C. One calorie is equal to 4.184 Joules.

Force

Any push or pull exerted on an object

Thermochemical equations

Balanced chemical equations that show the associated enthalpy change

What are two ways by which a closed system may interact with its surroundings?

By means of energy exchange through work and heat

What is the formula for internal energy change?

Change in E = q + w where q = heat and w = work

What happens when fuel is burned?

Chemical energy is converted into thermal energy. The increase in thermal energy arises from the increased molecular motion and hence increased kinetic energy at the molecular level.

Negative value of the change in internal energy

Efinal < Einitial such that the system lost energy to its surroundings

Positive value of the change in internal energy

Efinal > Einitial such that the system gained energy from its surroundings

First Law of Thermodynamics

Energy can be transferred and transformed, but it cannot be created or destroyed. Any energy lost by a system must be gained by the surroundings and vice versa.

Standard enthalpy of formation

Enthalpy change if 1 mole of compound in standard state were formed directly from its elements in kJ/mol

What is a Calorie?

Equal to 1000 calories or 1 kcal

Surroundings

Everything outside the system

How is potential energy converted into kinetic energy?

If a force acts upon a stationary object, the object will move. Energy will be conserved such that potential energy may be converted into kinetic energy. As potential energy decreases, kinetic energy increases.

What are the signs for electrostatic potential energy?

If two particles have the same charge and repel each other, E el is positive so that potential energy decreases as the particles move farther apart. If two particles have opposite charges, the potential energy increases since they move closer together and E el is negative.

What happens when heat is added to a system or work is done on a system?

Internal energy increases

How is internal energy related to matter?

Internal energy of a system is proportional to the total quantity of matter in the system because energy is an extensive property.

How can Cs be determined experimentally?

Measuring temperature change that a known mass of the substance undergoes when it gains or loses a specific quantity of heat, q. Cs = q/mΔT in J/gC or J/gK

What does a positive change in internal energy mean?

Net gain of energy by a system

What does a negative change in internal energy mean?

Net loss of energy by a system

What are the three major types of systems?

Open, closed, and isolated

Standard enthalpy state

Pressure is at 1 atm and temperature is at 298 K or 25 degrees Celsius

In what way are states of matter valuable for enthalpy change?

States of matter determine enthalpy values. For instance, liquid water does not have the same enthalpy as water vapor.

Potential energy

Stored energy that results from the position or shape of an object. U = mgh.

What does positive q mean?

System gains heat

What does negative q mean?

System loses heat

Closed systems

Systems that can exchange energy but not matter with their surroundings.

What conditions influence internal energy of a system?

Temperature and pressure

Enthalpy of vaporization

The amount of energy absorbed as heat when a specified amount of a substance vaporizes at constant pressure

Energy

The capacity to do work or transfer heat.

Electrostatic potential energy

The energy a charged particle has because of its position relative to another charged particle; it is directly proportional to the product of the charges of the particles and inversely proportional to the distance between them; also called coulombic attraction. The formula is E el = kQ1Q2/d, where k is the constant 8.99 x 10^9 Jm/C^2. The zero of electrostatic potential energy is infinite separation between charged particles.

Why is q-solution = -q-reaction?

The energy gained or lost by the reaction has the opposite effect on the surroundings. For instance, an exothermic reaction means that heat is lost to the surroundings so q for the system would be negative but q for the surroundings would be positive.

Bond enthalpy

The energy needed to break one mole of bonds in gaseous molecules under standard conditions

Kinetic energy

The energy of motion that depends on the mass and speed of an object. KE = 0.5mv^2

Specific heat capacity

The energy required to raise the temperature of one gram of a substance by one degree Celsius, Cs.

Molar heat capacity

The energy required to raise the temperature of one mole of a substance by one degree Celsius, Cm

Heat

The energy transferred from a hotter object to a colder one.

Work

The energy transferred when a force moves an object.

Work

The energy used to cause an object to move against a force.

Heat

The energy used to cause the temperature of an object to increase.

Enthalpy of formation

The enthalpy change that accompanies the formation of a substance from the most stable forms of its component elements

Enthalpy of combustion

The enthalpy change that occurs during the complete combustion of one mole of a substance

Enthalpy of fusion

The enthalpy change that occurs to melt a solid at its melting point

Standard enthalpy change

The enthalpy change when all reactants and products are in their standard states

What is the magnitude of work equal to?

The force times the distance the object moves F x d

What is the relationship between heat capacity and raising temperature?

The greater the heat capacity, the greater the heat required to produce a given increase in temperature.

What is the change in enthalpy equal to?

The heat (qp) either gained or lost from the surroundings by the system at a constant pressure

Enthalpy

The heat content of a system at constant pressure. The sum of the internal energy plus the produce of the pressure and the volume of the system. It is a state function. H = E + PV

What is valuable about enthalpy being an extensive property?

The magnitude of the change in enthalpy is proportional to the amount of reactant consumed in the process.

Heat capacity

The number of heat units needed to raise the temperature of a body by one degree Celsius or K. Denoted as C.

Hess's Law

The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process. If a reaction is carried out in a series of steps, the change in enthalpy for the overall reaction equals the sum of the enthalpy changes for the individual steps.

Calorimetry

The precise measurement of heat flow out of a system for chemical and physical processes

Enthalpy of reaction

The quantity of energy transferred as heat during a chemical reaction, also known as the heat of reaction

What constitutes the system and what constitutes the surroundings in a chemical reaction?

The reactants and products make up the system and the container and all that is beyond it make up the surroundings

In a chemical reaction, what do the reactants and products refer to in terms of the state of a system?

The reactants are the initial state and the products are the final state

In what way is enthalpy change reversible?

The sign of the enthalpy change is the opposite of the forward reaction for the reverse reaction

Thermodynamics

The study of energy transformations that occur in a collection of matter.

Thermochemistry

The study of the relationships between chemical reactions and energy changes that involve heat.

How does a bomb calorimeter work?

The substance to be studied is placed in the bomb, which is an insulated sealed vessel and can withstand great pressure. The bomb has an inlet valve where oxygen and electrical leads for initiating the reaction. After the sample is placed in the bomb, the bomb is sealed and pressurized with oxygen. The bomb is then placed in the calorimeter and submerged in water such that an electric current passes through the fine wire and ignites the sample. The heat released when combustion occurs increases the temperature of the water and this is measured.

Internal energy

The sum of all kinetic and potential energies of the components of the system.

What does a positive change in enthalpy mean?

The system has gained heat from the surroundings, meaning the process is endothermic

What does a negative change in enthalpy mean?

The system has released heat back into the surroundings, meaning the process is exothermic.

What is C cal?

The total heat capacity of the calorimeter that results from combusting a sample that releases a known quantity of heat.

Pressure-volume work

The work involved in the expansion or compression of gases. When pressure is constant, the formula for work is: w = -PV Units of Latm

What decreases internal energy?

When work is done by the system and heat is lost

What does negative w mean?

Work is done by the system

What does positive w mean?

Work is done on the system

What is the standard enthalpy of formation of the most stable form of any element?

Zero since there is no reaction at standard state

Exothermic examples

freezing, condensation, deposition, combustion

Endothermic examples

melting, vaporization, sublimation

What is the formula for q-reaction for the bomb calorimeter?

q = -C cal x ΔT

What is the formula for q?

q = Cs x m x ΔT

How is the q-value of a reaction related to the q-value of the solution?

q-solution = -q-reaction

Change in internal energy

ΔE = Efinal - Einitial

How can the change in enthalpy be modeled for a reaction?

ΔH = H products - H reactants

Derive the equation for change in enthalpy

ΔH = ΔE + PΔV If ΔE = q + w and w = -PΔV, substitution gives ΔH = q -PΔV + PΔV ΔH = q, at a constant pressure