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