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Reactivity 2.1 Flashcards

How Much, How Fast & How Far. Select a sub-topic below

R2.1 Moles, Stoichiometry & Yield (SL): read all 15 questions and answers
  1. [SL] What is the definition of a mole in chemistry?
    The mole (mol) is the SI unit for amount of substance. It contains exactly 6.02214076 × 10²³ elementary entities (rounded to 6.02 × 10²³ in the IB data booklet).
  2. [SL] What is the Avogadro constant and what are its units?
    The Avogadro constant is approximately 6.02 × 10²³ mol⁻¹.
  3. [SL] Distinguish between the units for relative formula mass and molar mass.
    Relative formula mass (Mr) has no units. Molar mass (M) has the units g mol^-1.
  4. [SL] What formula relates the amount of substance in moles to mass and molar mass?
    n = m / M, where n is moles, m is mass in grams, and M is molar mass in g mol^-1.
  5. [SL] What is the difference between an empirical formula and a molecular formula?
    The empirical formula gives the simplest whole-number ratio of atoms. The molecular formula gives the actual number of atoms of each element in a molecule.
  6. [SL] What do the coefficients in a balanced chemical equation represent?
    They show the stoichiometric mole ratio of the reactants and products involved in the reaction.
  7. [SL] Define the term limiting reactant.
    The limiting reactant is the substance that is completely used up in a chemical reaction and determines the theoretical yield of the product.
  8. [SL] Define the term excess reactant.
    The excess reactant is the substance that is not fully used up and remains in the system after the reaction is complete.
  9. [SL] What is the theoretical yield of a chemical reaction?
    The calculated maximum amount of product that can be formed from a given amount of the limiting reactant, assuming perfect conversion and no losses.
  10. [SL] State the formula for calculating percentage yield.
    Percentage yield = (experimental yield / theoretical yield) x 100%
  11. [SL] List three reasons why experimental yield is usually lower than theoretical yield.
    1. Incomplete reactions. 2. Side reactions producing unwanted byproducts. 3. Loss of product during transfer or purification.
  12. [SL] What is the formula for percentage atom economy?
    % atom economy = [(stoichiometric coefficient x molar mass of desired product) / Sum of (stoichiometric coefficient x molar mass of all reactants)] x 100
  13. [SL] What does atom economy measure in green chemistry?
    Atom economy is a theoretical measure of how efficiently reactant atoms are converted into desired products based on stoichiometric coefficients and total mass, assessing sustainability and waste minimisation.
  14. [SL] Why might a reaction with 100% yield still be inefficient?
    Even with a perfect yield, the reaction may have a low atom economy if a large mass of the reactant atoms ends up as unwanted byproducts rather than the desired product.
  15. [SL] State Avogadros law regarding gas volumes.
    Equal volumes of all gases measured under the same conditions of temperature and pressure contain equal numbers of molecules.
R2.1 Solutions, Gases & Q vs K (HL): read all 5 questions and answers
  1. [SL] How do you calculate the moles of a solute in aqueous solution?
    n = C x V, where n is moles, C is the molar concentration (mol dm^-3), and V is the volume in dm^3.
  2. [SL] State the ideal gas equation.
    PV = nRT, where P is pressure (Pa), V is volume (m^3), n is moles, R is the gas constant (8.31 J K^-1 mol^-1), and T is temperature (K).
  3. [HL] What is the reaction quotient (Q)?
    A value calculated using the equilibrium expression but with non-equilibrium concentrations of the reactants and products at a given moment.
  4. [HL] How does Q help predict the progress of a reaction?
    By comparing Q to K: if Q < K, the reaction proceeds forward (towards products). If Q > K, the reaction proceeds in reverse (towards reactants). If Q = K, the system is at equilibrium.
  5. [HL] How is the equilibrium constant (K) determined?
    K is determined from the stoichiometry of a homogeneous reaction at a state of dynamic equilibrium, using equilibrium concentrations.
R2.2 Collision Theory & Factors (SL): read all 11 questions and answers
  1. [SL] How is the rate of a chemical reaction expressed?
    Rate is expressed as the change in concentration of a particular reactant or product per unit time. Units: mol dm^-3 s^-1.
  2. [SL] What two criteria must be met for a successful collision?
    Particles must collide with: 1. Sufficient kinetic energy (greater than or equal to Ea). 2. Correct spatial orientation to allow new bonds to form.
  3. [SL] Define activation energy (Ea).
    The minimum amount of kinetic energy that colliding particles must possess to have a successful collision that leads to a chemical reaction.
  4. [SL] How does increasing concentration affect the rate of reaction?
    More reactant particles in a given volume increases the frequency of collisions, leading to more successful collisions per second.
  5. [SL] Why does a powdered solid react faster than a large lump?
    A powder has a much greater surface area, exposing more particles to collide, increasing the frequency of successful collisions.
  6. [SL] Using collision theory, explain why increasing temperature increases rate.
    Higher temperature means: 1. Particles move faster so collisions are more frequent. 2. A much greater proportion of particles have energy greater than or equal to Ea.
  7. [SL] How does a catalyst increase the rate of a reaction?
    A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of colliding particles have sufficient energy to react.
  8. [SL] What does the peak of a Maxwell-Boltzmann distribution curve represent?
    The most probable kinetic energy possessed by the largest number of particles in the sample at a given temperature.
  9. [SL] How does the M-B distribution curve change when temperature increases?
    The curve flattens, becomes broader, and the peak shifts to the right, showing a larger proportion of particles have higher kinetic energies.
  10. [SL] How is a catalyst shown on a Maxwell-Boltzmann distribution?
    The curve shape stays the same, but the vertical Ea line shifts to the left, increasing the shaded area of particles with enough energy to react.
  11. [SL] On an energy profile diagram, what does the highest peak represent?
    The transition state (activated complex). Activation energy is the energy difference between the reactants and this highest peak.
R2.2 Rate Expressions & Arrhenius (HL): read all 9 questions and answers
  1. [HL] What is the order of a reaction with respect to a specific reactant?
    The exponent to which the concentration of that reactant is raised in the experimentally determined rate equation.
  2. [HL] How is the overall reaction order determined?
    The overall order is the sum of the individual orders (exponents) with respect to each reactant in the rate equation.
  3. [HL] What two variables affect the rate constant k?
    The rate constant depends on: 1. Absolute temperature. 2. Activation energy of the specific reaction.
  4. [HL] State the Arrhenius equation and identify factor A.
    k = Ae^(-Ea/RT). A is the Arrhenius factor (frequency factor), which accounts for the frequency of collisions with proper spatial orientations.
  5. [HL] What does the exponential term e^(-Ea/RT) represent?
    The fraction of collisions or molecules that possess adequate kinetic energy to overcome the activation energy barrier at temperature T.
  6. [HL] How is Ea determined from an Arrhenius plot (ln k vs 1/T)?
    The slope of the straight line = -Ea/R. Therefore Ea = -slope x R.
  7. [HL] Define the rate-determining step.
    The slowest elementary step in a reaction mechanism. It acts as a bottleneck that determines the overall rate of the reaction.
  8. [HL] What does molecularity mean in the context of an elementary step?
    The number of reacting particles (atoms, molecules, or ions) taking part in a single elementary step: unimolecular (1), bimolecular (2), or termolecular (3).
  9. [HL] How does the rate-determining step relate to the order of a reaction?
    For a single-step mechanism, the order equals the molecularity (particles reacting in that step). In multi-step mechanisms, any species appearing in the rate equation is involved in the rate-determining step or a fast equilibrium preceding it; reaction order does not always equal particle count in the RDS alone.
R2.3 Equilibrium & Le Chatelier (SL): read all 10 questions and answers
  1. [SL] What is dynamic equilibrium?
    A state where the rate of the forward reaction equals the rate of the backward reaction. Concentrations of reactants and products remain constant.
  2. [SL] Why must equilibrium involving gases occur in a closed system?
    A closed system ensures that none of the reactants or gaseous products can escape, which is required for the system to reach and maintain equilibrium.
  3. [SL] State Le Chatelier's Principle.
    If a change is made to a system at dynamic equilibrium, the position of equilibrium will shift to minimise or oppose the effect of that change.
  4. [SL] How does increasing the concentration of a reactant affect equilibrium?
    The equilibrium shifts to the right (towards products) to reduce the increased concentration by consuming some of the added reactant.
  5. [SL] How does increasing pressure affect a gaseous equilibrium?
    Equilibrium shifts in the direction that produces fewer gas molecules, to decrease the pressure.
  6. [SL] What if there are equal gas moles on both sides and pressure changes?
    Changes in pressure will have no effect on the position of equilibrium.
  7. [SL] How does increasing temperature affect an exothermic equilibrium?
    Equilibrium shifts in the endothermic direction (to the left) to absorb the excess heat.
  8. [SL] What effect does a catalyst have on chemical equilibrium?
    No effect on the position of equilibrium. A catalyst increases the rate of both forward and reverse reactions equally. It only helps equilibrium be reached faster.
  9. [SL] Why is a catalyst still used in industry if it does not change equilibrium position?
    Because it causes the reaction to reach equilibrium much faster, improving the efficiency and economics of the process.
  10. [SL] At dynamic equilibrium, are reactions still occurring?
    Yes. Both forward and reverse reactions continue at equal rates, but there is no net change in concentrations of reactants or products.
R2.3 Kc, Q & ICE Tables (HL): read all 10 questions and answers
  1. [SL] Write the equilibrium expression for Kc for aA + bB = cC + dD.
    Kc = ([C]^c [D]^d) / ([A]^a [B]^b), where square brackets represent equilibrium concentrations.
  2. [HL] How are pure solids treated in a Kc expression?
    Pure solids are NOT included in the equilibrium constant expression for heterogeneous equilibria.
  3. [SL] What does K >> 1 indicate?
    Products are strongly favoured. The equilibrium lies far to the right and the reaction almost goes to completion.
  4. [SL] What does K << 1 indicate?
    Reactants are strongly favoured. The equilibrium lies far to the left and the reaction hardly proceeds.
  5. [HL] How does increasing temperature affect K for an endothermic reaction?
    K increases because the equilibrium shifts towards products, increasing product concentrations and decreasing reactant concentrations.
  6. [SL] Is K affected by changes in pressure or concentration?
    No. K is constant at a specified temperature and is NOT affected by changes in concentration or pressure.
  7. [HL] What is the reaction quotient (Q) and how is it calculated?
    Q is calculated using the same expression as K, but with non-equilibrium concentrations at a specific moment in time.
  8. [HL] If Q < K, which direction does the reaction proceed?
    The reaction proceeds to the right (towards products) until equilibrium is reached.
  9. [HL] If Q > K, which direction does the reaction proceed?
    The reaction proceeds to the left (towards reactants) until equilibrium is established.
  10. [HL] What does ICE stand for in an ICE table?
    Initial, Change, and Equilibrium. These represent the stages used to determine equilibrium concentrations from starting amounts using stoichiometric ratios.
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