Chem 1 Thermochemistry

Chapter 7

Conservation of Energy

Total energy remains constant, it is neither created or lost. In an isolated system, only transformed from one form to another.

Work

A force acting through a distance, resulting in energy transfer or change. In thermochemistry work is restricted to pressure-volume (PV) work.

Heat (q)

Energy can be tranferred between bodies at differnt temperatures in a form of heat. Materialistic substance that cound not be “created” or “destroyed” (conserved).

Factors that Affect Heat Capacity

• Mass

• Temperature change of an object (Change of Temp = T final - T initial)

q = 0 if Change of Temp = 0

• Nature of the object (different heat capacities)

heat capacity (C = Mass of Substance • Specific Heat Capacity)includes mass, temperature change, and material properties.

Specific Heat Capacity (c)

Heat required to raise the temprature of one gram of material by one degree

Heat Capacity (C)

Heat required to raise the temperature of an object by one degree K (degrees celcius)

Molar Heat Capactiy (C)

Heat required to raise the temperature of 1 mole of a substance by one degree

Caloric Theory

Antonie Lavosier 1783

Changes in temperature occured due to transfer of an imponderable fluid, invisible and weightless (caloric)

Jouele 1842

demonstrated that heat and work were interchangeable.

Clausius 1850

demonstrated that caloric theory could be reconciled with kinetic theory provided that the conservation of energy was employed rather than the movement of a substance (1st Law of Thermodynamics)

Heat in Modern Terms

type of energy transffered dye to a temperature difference

Joules Experiment

Heat is the energy transferred between a system and its surroundings as a reuslt of temperature difference. It is a manifestation of energy transfer and is measured in joules.

How many calories of heat does it take to increase the temperature of 1 gram of water by 1 degrees celcius (K)

One calorie (1 cal) is the amount of heat (caloric) required

How much work is required to raise the temperature of 1 gram of water by 1 degree C

4.18J

Internal Energy (U)

The total energy within a system (potential & kinetic). Including:

• trasnlational kinetic energy of molecules

• energy associated with molecular rotations/vibrations and electrons in atoms

• energy stored in chemical bonds

• intermolecular attractions

• does not include energy in the form of work (w) or heat (q)

State Function

property of a system that depends only on its current state and not on the path taken (independent) to reach that current state. the value is the same regardless of how it got there. only thing that is important is where it is now.

Examples of State Function

Gravitational Potential Energy (PE = mgh)

Internal energy corresponds to a specific state (pressure, temp, voume, amount of substance)

Kinetic Molecular Theory of Gases

Work performed by the gas is larger in two step expansion compared to a one step (going from state 1 to 2, State 1→ State 2)

1st Law of Thermo Dynamics

• Energy of an isolated system is constant ΔU (isolated system) = 0

• Closed System ΔU = q + w

Law of Energy Conservation

Heat (q) and work (w) only exist during the change in a system. The relationship between heat, work, and changes in internal energy is dictated by the law of conservation of energy

What are the Thermodynamic Systems?

Open, closed, and isolated

Open System

Can exchange matter and energy (heat/work) with its surroundings

Closed System

can exchange energy (heat/work) but not matter with its surroundings

Isolated System

does not interact with its surroundings

System

Whatever we want to study

Surrounding

Everything external to the system

Boundary

Separation between system and surrounding

Work and Heat

• Means within the system that exchanges energy with its surroundings

• Associated with a change in internal energy (delta U)

Internal Energy with Work and Heat

leads to changes in the energy of the system & surrounding (more concerned with change in U that accompanies a change in the system)

State Functions in Systems

• Indicate temp, pressure, kind of substance present

• anything that has a unique value for a specified state (density H₂O at 20 degrees Celcius = 0.99820g/mL)

• Heat and Work NOT state functions

Heats of Reaction

• Reactants (Uinitial) → Products (Ufinal)

• If reaction happens in a sealed vessel (volume doesnt change) then Change of U = q_rxn = q_v

(q_v designates a heat of reaction performed at constant volume)

Bomb Calorimeter

Reactants are ignited in the steel bomb container under high pressure oxygen atmosphere. The reaction occurs at a constant volume. The heat of reaction (q_rxn) is measured as a quantity of heat exchanged between a system and surroundings

Mole Energy Relationships

Exothermic Reaction

A chemical reaction that gives off heat to the surroundings.

• Change of H < 0

• H_reactants > H_products (heat released)

• Change of H = Negative

Endothermic Reaction

A chemical reaction that absorbs heat from its surroundings

• Change of H > 0

• H_products > H_reactants

• Change of H = Positive

Enthalpy, H

When U, P, and V are all state functions. Same with q_p. It is a state function where internal energy is a state function and pressure and volume are state variables. Depends on the temperature (usually specified, 25 degrees C)

Absolute values of Enthalpy cannot be determines, but Enthalpy changes can be measured

Standard State

Defined for compounds.

• The standard pressure is 1 bar = 10⁵ Pa

  • Change of H is an extensive property (standard values refer to specific amount of substance, 1 mol)

Standard Enthalpies of Formation of Selected Elements

Many elements have several allotropes (distinctly diff pure forms of the element), most stavle form of the element is assigned Change of H_f of 0

Hess’ Law

allows for an indirect determination of change of H for reactions (based on being a state function)

• If a process occurs, or is considered to occur, in stages/steps, the enthalpy change for the overall process is the sum of the enthalpy changes for individual steps

Change of H is extensive and directly proportional to the amount of substance in a system (it changes sign when reaction is reversed)

How to Use Hess’ Law

1. Reversing a reaction changes the sign on its Change in Enthalpy

2. Multiplying a reaction by a common factor means multiplying the Change in Enthalpy by the same factor

3. If a component is on the opposite side in a given reaction from the requested reaction, then reversal is suggested

4. If the coefficients are different for a component, the multiplication is suggested

Standard Enthalpies of Formation

The formation of any substance is the enthalpy changes that occurs in forming 1 Mole of the substance in the standard state (1 bar, 298.15K)

Standard Enthalpy formation of a pure element in its reference form is 0.

Allotropy

The existence of an element in two or more molecular forms

ex: graphite and diamond (both carbon)

Formation Reactions

A chemical reaction where one mole of a compound is produced form its constituent elements in their standard states.

ex: formation reaction for liquid water

H₂ (g) + 1/2O₂ (g) → H₂O(l)

Types of Change in Enthalpy

• Heat of chemical reaction (bonds are formed and broken)

  • Heat of Compound Formation, Combustion

• Heat of Solution (dissolving)

• Heat of Vaporization (liquid→gas)

• Heat of Fusion (solid→liquid)

• Heat of Sublimation (solid→gas)

Fuels as Energy Sources

• Fossil Fuels are petroleum, natural gas, coal

  • Combustion = exothermic

  • Nonrenewable resource

  • Environmental impact (Sulfur and Nitrogen oxides formed in combustion cause air pollution and Greenhouse gasses cause global warming)

Typical Heat of Combustion

• Glucose

• Typical Carbohydrate

• Pinewood

• Peat

• Coal

• Isooctance

• Natural Gas

• Hydrogen

• Typical Fat

Photosynthesis

Coal Formation

When plants decompose by bacteria without O₂, Oxygen and Hydrogen atoms are remoced and Carbon content is increased forming and organic rock ____

Petroleum and Natural Gas

Marine biomasses like zooplankton and algae that have fallen onto the ocean floor gets covered by sand and exposed to high pressure and temperature. They are then transformed into ______

Burning Coal

Not always convenient

Glasification of Coal

Happens by passing steam through heated coal

Liquefacation of Coal

Converts coal gas into long chain hydrocarbons using catalysis

Methanol

prepared from coal or thermal decomposition (pyrolysis) of various organic materials such as manure, wool, waste

Ethanol

Mostly from ethylene but also by fermentation

Biofuels

Biodiesel is composed of methyl esters of fatty acids

Hydrogen

Excellent fuel (clean/efficient) currently made from petroleum and natural gas

Problems of Using Fossil Fuels

• Non renewable resource

• environmental impacts of

  • CO₂ (global warming)

  • Sulfur & Nitrogen Oxides (combustion/air pollution)

Spontaneous Process

A process that occurs naturally wihtouth any external energy input, meaning it happens on its own under given conditions

Non Spontaneous Process

Process that requires a continuous supply of energy from an external source to proceed

Take Aways