Ch.7 Chemical Energy

Energy

is the capacity to do work (displace or move matter) or

to produce heat

Law of conservation of energy

energy can be converted from

one form to another but can be neither created nor destroyed

Total energy content of the universe is constant

Kinetic Energy

KE = ½mυ2

m = mass, kg

υ = velocity, ms–1

Units are

kg  m2 s–2  J

Potential Energy

in a gravitational field, but also the composition of matter

PE = mgh

m = mass, kg

g = gravity, ms–2

h = height, m

Units are

kg  m2 s–2  J

Work

is defined as the transfer of energy that occurs when a force is

applied to an object, causing it to move.

Heat

is the transfer of energy between a system and its surroundings

due to a difference in temperature. Heat flows from hotter to colder

objects and is not the same as temperature — it is energy in transit,

not energy stored.

Elastic Collision

objects bounce off

each other with no loss in total kinetic

energy, for example collisions

between gas molecules.

Inelastic Collision

some kinetic

energy is converted into other forms

(heat, sound, deformation). Example:

a car crash or a ball of clay hitting the

ground and sticking.

Higher energy systems are less stable

than the lower energy ones.

Heat

is energy transfer between a system and its surroundings,

caused by the temperature difference.

Exothermic Reaction

reaction gives off heat

❖ In an isolated system, system T increases

❖ In a non-isolated system, heat is given off to the surroundings,

i.e., qrxn < 0 (negative)

• Reactants have more energy, products have lower energy

Endothermic Reaction

reaction absorbs heat

❖ In an isolated system, system T decreases

❖ In a non-isolated system, heat is absorbed from the

surroundings, i.e., qrxn > 0 (positive)

• Reactants have lower energy, products have higher energy

Unlike heat, work is caused by a

force moving through a

distance (heat is caused by a temperature difference)

A negative quantity of work

signifies that the system loses

energy

A positive quantity of work

signifies that the system gains

energy

Chemical energy

energy due to chemical bonds and intermolecular forces

Thermal energy

translation, vibration, and rotation of molecules/atoms.

Heat (q):

• Transfer of thermal energy due to a temperature difference.

Enthalpy (H):

• A thermodynamic quantity related to heat at constant pressure:
  ΔH=ΔE+PΔV

qrxn

is the quantity of heat exchanged between

a reaction system and its surroundings.

PV work (w)

Work (w) = − P ΔV

Enthalpy

is a state function—E, P, and V are all state functions;

therefore H must be a state function also

Positive values indicate

endothermic reactions - N2(g ) + O2(g ) ⎯→ 2NO(g ) ΔH = +180 kJ

Negative values indicate

exothermic reactions

State function (or state property)

is a property of the system that depends only on the current state of the system, any change in its value is independent of how the change in state was brought about

Work and heat are

NOT state functions

q < 0, w < 0

• Energy leaving a system

  carries a negative sign If heat given off by the

system

• If work is done by the system,

Energy entering a system

carries

a positive sign:

If heat absorbed by the

system

q > 0

If work done on the

system

w > 0

Heat capacity (C)

of a substance is the quantity of heat required

to change the temperature of the substance by 1 ℃

C = q/ΔT (units are J/℃ or J/K)

Molar heat capacity, 𝐶𝑚

energy required to raise the

temperature of one mole of pure substance by 1℃

Specific heat capacity, 𝐶𝑠

• energy required to raise the temperature of one gram of pure substance (or solution) by 1℃

• 𝐶𝑠 = C/m = q/(m× ΔT)

Constant-Pressure Calorimetry

−𝑞𝑟𝑥𝑛 = q calorimeter

= mass  specific heat  ΔT

𝑞𝑟𝑥𝑛 = −𝐶𝑐𝑎𝑙∆𝑇

Constant–Volume Calorimetry

• − q rxn = q calorim = q V = ΔE

• q calorim = C × ΔT

Petroleum & Natural Gas

• Fossil fuels like gasoline and methane provide energy via combustion.

• Nonrenewable resources contribute to CO₂ emissions and climate change.

·       Air pollution: Burning fossil fuels releases CO₂, NOₓ, and particulates, contributing to smog and respiratory issues.

·       Water pollution: Oil spills and wastewater from fracking contaminate ecosystems.

·       Greenhouse gases: CO₂ and methane emissions from drilling and combustion contribute to climate change.

·       Nonrenewable: These resources are finite, with extraction causing land degradation.

 

Effects of Carbon Dioxide on Climate

• CO₂ traps infrared radiation, leading to global warming.

  ·       Effects include melting ice caps, rising sea levels, and extreme weather patterns.

  ·       Increased atmospheric CO₂ levels correlate with rising global temperatures over the past century.

Wind Energy

·       Clean and renewable: No emissions during operation.

·       Land use: farms can coexist with agriculture.

·       Wildlife concern: Improperly placed turbines can impact bird and bat populations.

Hydrogen as a Fuel

• Hydrogen + O₂ → electricity + water (via fuel cells).

• Zero emissions at the point of use (only water vapor released).

·       Production method matters: If made from natural gas ("gray hydrogen"), it still emits CO₂.

·       Storage and transport: Require high energy and infrastructure.

Other Energy Alternatives

·       Solar: No emissions during use, but panel production involves mining and energy use.

·       Biofuels: Renewable but can compete with food supply and require land use changes.

·       Geothermal: Minimal emissions, but can release trace gases and require water.

Coal

· High energy content but also high pollutant output (SO₂, mercury, CO₂).

·  Environmental issues: acid rain, smog, and mining damage.

High CO₂ emissions: Coal has the highest carbon footprint among fossil fuels.

·       Mining damage: Strip mining and mountaintop removal devastate landscapes and wildlife habitats.

·       Acid rain: Sulfur dioxide (SO₂) released during burning reacts in the atmosphere to form acid rain, harming forests and aquatic systems.

·       Ash waste: Disposal of coal ash can lead to contamination of water supplies.

Green Chemistry Principles:

o   Minimize energy use (favor reactions that occur at room temperature and pressure).

o   Reduce waste and avoid toxic by-products.

o   Design processes that maximize atom economy and use renewable feedstocks.