Chemistry Lecture

• The rate law relates the reaction rate to the concentration of reactants.
• What is chemical kinetics?

  The study of the rates at which chemical reactions occur
• The energy needed to initiate a chemical reaction is called activation energy.
• Which of the following terms is used to describe the rate at which a chemical reaction takes place?

  Reaction rate
• The binding energy of an atomic nucleus is the energy required to break it apart into its individual nucleons.

  True
• The time required for half of a radioactive substance to decay is called half-life.
• Which of the following is an example of a nuclear reaction?

  Nuclear fission
• Radioactive decay is a spontaneous process by which unstable atomic nuclei undergo:

  Transformations into more stable nuclei
• What is a nuclear reaction?

  A reaction that involves changes in the atomic nucleus
• The equilibrium constant (Kc) is a ratio of the products to the reactants.

  True
• What is chemical equilibrium?

  A state in which the rates of forward and reverse reactions are equal
• At chemical equilibrium, the concentrations of the reactants and the products remain constant.
• Which of the following is NOT true about chemical equilibrium?

  All reactions reach equilibrium at the same rate
• What is the relationship between the equilibrium constant (Kc) and the reaction quotient (Qc)?

  Kc is calculated at equilibrium, while Qc can be calculated at any point in the reaction
• The study of speeds of reactions and the nanoscale pathways or rearrangements by which atoms and molecules are transformed from reactants to products. =Chemical Kinetics
• Chemical kinetics is also called reaction kinetics.
• Rate = the change in some measurable quantity perunit time
• Reactants decrease with time(Negative sign), Products increase with time(Positive sign).
• A Catalyst is a substance that:• increases reaction rate without being consumed• changes the mechanism for the reaction.• provides a lower Ea in the rate limiting step
• A catalyst does not change the products or their relative proportions.
• In a constant-V reaction, partial pressures change as concentrations change.
• Kc >> 1 Reaction is strongly product favored.• very little reactant remains.• often written as a forward reaction only.• assume reaction goes to completion.

  Kc << 1 Reaction is strongly reactant favored.• very little product forms.• usually written as “no reaction” or NR.

  Kc ≈ 1 Reactants & products present at equilibrium.• use equilibrium methods discussed here.
• Factors affecting the speed of all reactions:

  Properties

  Concentrations

  Temperature

  Catalyst
• Le Chatelier’s Principle

  “If a system is at equilibrium and the conditions are changed so that it is no longer at equilibrium, the system will react to reach a new equilibrium in a way that partially counteracts the change”

  • A system at equilibrium resists change.
  • If “pushed”, it “pushes back”
• Exothermic reactions:

  • Kc decreases as T increases.

  • Are less product favored at higher T.

  Endothermic reactions:

  • Kc increases as T increases.

  • Are more product favored at higher T.
• Antoine Henri Becquerel (1896):

  • U salts emitted rays that “fog” a photographic plate.

  • U metal was a stronger emitter.
• Marie and Pierre Curie:

  • Isolated Po and Ra that did the same.

  • Marie Curie called the phenomenon radioactivity.
• Thomson and Rutherford:

  • Studied the radiation, and found two types: α and β.
• Villard:

  • Discovered y radiation.
• Nuclear Reactions

  Rutherford & Soddy (1902)

  “Radioactivity is the result of a natural change of a radioactive isotope of one element into an isotope of a different element”.
• The Geiger counter measures pulses per unit time.
• Nuclear Fission

  Chain reactions are possible:

  Small amounts of 235U can’t capture all the neutrons.(stays under control).

  Nuclear bombs exceed the critical mass;

  the chain reaction grows explosively.
• Thermal energy from fission is used to generate power in a nuclear reactors.
• Nuclear Fusion

  • Very exothermic (ΔE = -2.5 x 109 kJ/mol ).

  • The energy source for stars.

  An attractive power source:

  • Hydrogen (the fuel) can be extracted from oceans.

  • Waste products are short-lived, low-mass isotopes.
• Food Irradiation

  • g-rays kill bacteria, molds, spores...

  • Food spoils much less rapidly.

  • It does not make food radioactive
• Tracers

  • Chemicals made with radioactive atoms

  • Introduced into plants, animals...

  • Concentrate where used (rapid growth regions)

  • Uptake can be monitored with a Geiger counter.
• Medical Imaging

  y-emitters are often used (e.g. 99mTc)

  ▪ Gamma rays can exit the body

  ▪ Less damaging than α or β.Tracers are used by organs, bones
• PET (positron emission tomography)

  • A β+ emitter is injected.

  • The g-rays emit in opposite directions.

  • Detectors show the origin of the y-rays.
• Chemotherapy = use of radiation to treat cancer.

  • Rapidly dividing cells are more susceptible to radiation than mature cells.

  • Cancerous cells divide and grow more rapidly than normal cells.

  ▪ hair follicles, bone marrow... also affected.

  • Malignant cells are more likely to be killed than normal cells.
• Applications of Radioactivity

  • Chemotherapy
  • PET
  • Medical Imaging
  • Tracers
  • Food Irradiation