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Exam Review Notes

Equilibrium Shifts and Temperature

  • If increasing the temperature causes the product concentration to decrease, it means more reactants are being produced.
  • The equilibrium shifts to the left, favoring the reverse reaction.
  • If increasing temperature favors the reverse reaction, that reaction is endothermic.

Exothermic Reactions

  • Burning hydrogen with oxygen to produce water is an exothermic reaction, releasing heat energy.
  • Combustion reactions are exothermic.
  • Increasing temperature shifts the equilibrium towards the endothermic direction.

Rates of Reaction and Graphs

  • Steeper gradients in reaction graphs indicate faster reaction rates.
  • In a reaction graph:
    • The concentration of reactants decreases.
    • The concentration of products increases.

Concentration and Volume

  • Concentration is the number of particles within a given volume.
  • When comparing concentrations, ensure the volume remains the same.
  • If one mole has one black circle, two moles should have two black circles, maintaining the ratio.

Factors Speeding Up Reaction Rates

  • Concentration affects reaction rates.
  • It's crucial to use precise language, focusing on the frequency of successful collisions.
  • Particles can collide without causing a reaction; they might just bounce off.
  • In higher concentrations, particles are closer together.
  • Increased concentration leads to a higher frequency of successful collisions.
  • Particles at higher concentrations possess the same kinetic energy as those at lower concentrations; it's about the number of particles, not the energy of collision.

Moles and Mass

  • The total number of moles may change during a reaction, but the mass remains the same (conservation of mass).
  • For example, 7, 8, 9 moles can become 10 moles, but the mass stays constant.

Molar Mass and Avogadro's Constant

  • Avogadro's constant is used to determine the number of particles.
  • First, calculate the number of moles, then multiply by Avogadro's constant to find the number of particles.
  • Molar mass is the mass of one mole of a substance, and it varies because each particle has a different mass.
  • One mole of any substance contains the same number of particles (Avogadro's number), but their masses differ.

Dependent, Independent, and Controlled Variables

  • Independent variable: what you change (e.g., temperature of coffee).
  • Dependent variable: what you measure (e.g., temperature change).
  • Controlled variables: factors kept constant (e.g., starting temperature, equipment used).
  • If the independent variable is the volume of cold cream (3ml, 6ml, 9ml) added to coffee, the temperature of coffee would be dependent variable.
  • When using temperature as a control make sure to call out the starting temperature.
  • Use the same equipment to minimize heat loss due to surface area.
  • Keep constant anything known to affect reaction speed, except the independent variable.

Equilibrium and Le Chatelier's Principle

  • Changing concentration, pressure, or adding a catalyst will shift the equilibrium to keep the equilibrium constant (k) value the same.
  • Increasing the concentration of a reactant will cause the equilibrium to shift to the right, making more product.
  • Reducing the concentration of a product also shifts the equilibrium to the right, as the reaction tries to replenish the removed product.
  • To maximize product formation, maintain a constant supply of reactants and continuously remove the product.

Catalysts

  • Catalysts reduce the activation energy needed to start a reaction.

Maxwell-Boltzmann Distribution

  • Particles exceeding the activation energy cause a reaction.

  • The Maxwell-Boltzmann curve illustrates the distribution of particle energies.

  • Adding a catalyst lowers the activation energy, allowing more particles to react.

  • The number of particles doesn't change; the required energy threshold is reduced.

  • Increasing temperature shifts the graph to the right while maintaining the same total number of particles.

    • Increased temperature increases the number of particles exceeding the activation energy.

  • Decreasing temperature reduces the number of particles exceeding the activation energy.

  • Most Probable Energy: The energy at the peak of the curve; the energy that the most particles possess.

  • Average energy is typically to the right of this peak.

  • These diagrams are qualitative.

  • Adding a catalyst lowers the activation energy.

  • The number of reacting particles is always a small fraction of the total particles.

  • Temperature changes the shape of the energy distribution curve, while catalysts lower the activation energy.

Ionic Equations

  • Ionic equations… (topic mentioned but not elaborated)

Spectrometry

  • Spectroscopy (infrared, mass, NMR) are methods used to detect substances.

Mass Spectrometry Principles

  • Mass spectrometry can be used to identify substances.

Process:

  1. Ionization: Convert a molecule into an ion (typically by knocking off an electron to create a positive charge).
    • Methods include electrospray ionization.
  2. Acceleration: Accelerate the ions in a machine.
  3. Deflection: Ions deflect based on their mass; lighter ions deflect more, heavier ions deflect less.
  4. Detection: Calculate the speed at which ions reach a detector to estimate their mass.

Output:

  • The output is a graph showing the mass of the substance.
  • Molecular Ion: Represents the mass of the intact molecule, which usually has the highest mass.
  • Fragmentation: Breaking the molecule into smaller pieces to determine its components.

Fragmentation Analysis:

  • Fragments indicate the components of the molecule (e.g., CH3^+, CH2^+, COOH^+.

  • For example, fragments with masses of 15, 14, and 45 may appear, corresponding to these fragments.

  • Fragmentation helps identify functional groups; for example, a fragment with mass 45 suggests the presence of C=O-H.

  • By piecing together the fragments, you can deduce the structure of the unknown substance.

  • Mass spectrometry allows for the identification of unknown substances through analysis of their fragmentation patterns.

  • The molecular ion is very important as it tells you the molecular mass.

  • Given fragments and the elements composing the compound, one can deduce its structure.

  • The kinetic energy equation (KE = frac{1}{2}mv^2) is utilized.

  • Each particle receives the same kinetic energy but has different velocities based on its mass, allowing for mass calculation.

  • The velocity can be used to calculate the mass of the iron.

  • Avogadro's constant is then used to calculate the mass of individual particles