Chemistry Reaction Rates Quiz

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Questions and Answers

What is the correct description of an activated complex?

  • The heat required to bring reactants to a common activation energy.
  • The final arrangement of atoms after a reaction is complete.
  • A temporary arrangement of atoms as reactants transition to products. (correct)
  • A stable arrangement of atoms that exist prior to a reaction starting.

If the enthalpy of the reactants is $60kJ$ and the enthalpy of the products is $20kJ$, what is the change in enthalpy $(\Delta H)$ of the reaction and is the reaction exothermic or endothermic?

  • $\Delta H= +40kJ$; endothermic
  • $\Delta H= +40kJ$; exothermic
  • $\Delta H= -40kJ$; exothermic (correct)
  • $\Delta H= -40kJ$; endothermic

In an exothermic reaction, how do the energy levels of the reactants and products compare?

  • The products are at a lower energy level than the reactants. (correct)
  • The products' energy level is unpredictable relative to reactants.
  • The reactants and products are at the same energy level.
  • The products are at a higher energy level than the reactants.

In a reaction energy diagram, which of the following represents the activation energy?

<p>The difference in energy between the reactants and the transition state. (A)</p> Signup and view all the answers

If a catalyst is added to a reaction, what is the main effect on an energy diagram?

<p>It lowers the activation energy of the reaction. (D)</p> Signup and view all the answers

In the reaction Al + 3CuCl → AlCl3 + 3Cu, if the amount of Al decreases by 6 mol in two seconds, what is the corresponding change in the amount of Cu?

<p>An increase of 18 mol (B)</p> Signup and view all the answers

Given the reaction 2KBr + Mg → MgBr2 + 2K, if 12 mol of MgBr2 is produced in 2 seconds, how much K is produced in the same time?

<p>24 mol (A)</p> Signup and view all the answers

In a reaction where reactants X2 and Y2 form 2XY, what is the change in enthalpy if the energy of the reactants is 50kJ, and the energy of the products is 100kJ?

<p>+50kJ (B)</p> Signup and view all the answers

In the reaction 2KBr + Mg → MgBr2 + 2K, if 20 mol of KBr reacts, what amount of Mg will be needed?

<p>10 mol (C)</p> Signup and view all the answers

If the transition state energy for a reaction is 150kJ and the reactants have an energy of 50kJ, what is the activation energy?

<p>100kJ (D)</p> Signup and view all the answers

Consider the reaction: Al + 3CuCl → AlCl3 + 3Cu. If at 6 seconds there are 22 mol of Al, and at 8 seconds there are 18 mol of Al, what is the average rate of change of [Al] over that period?

<p>-4 mol/2s (D)</p> Signup and view all the answers

For a reaction with reactants at 100kJ and products at 25kJ, what is the enthalpy change?

<p>-75kJ (D)</p> Signup and view all the answers

A reaction has an enthalpy change of -20kJ. Is it endothermic or exothermic?

<p>Exothermic, because ΔH is negative (A)</p> Signup and view all the answers

What effect does increasing the concentration of reactants typically have on the rate of a chemical reaction?

<p>It increases the rate due to higher collision frequency. (B)</p> Signup and view all the answers

Which reaction will proceed faster, one with an activation energy of $10kJ$ or one with an activation energy of $150kJ$?

<p>The reaction with an activation energy of 10kJ (C)</p> Signup and view all the answers

How does decreasing the temperature affect the rate of a chemical reaction?

<p>It decreases the rate due to less collisions. (D)</p> Signup and view all the answers

What is the effect of increasing the volume on the rate of reaction?

<p>It will decrease the rate of reaction. (D)</p> Signup and view all the answers

If a reaction has reactants with a potential energy of 200kJ and products with a potential energy of 100kJ, what is the change in enthalpy?

<p>-100kJ (B)</p> Signup and view all the answers

In a potential energy diagram for a reaction, which of the following is not labeled on the Y-axis?

<p>Reaction progress (D)</p> Signup and view all the answers

Which of the following does NOT typically increase the rate of reaction?

<p>Decreasing the surface area. (D)</p> Signup and view all the answers

A reaction has an initial potential energy of 35kJ. It reaches a transition state with 45kJ of energy and finishes with 15kJ of energy. How much energy would a catalyst use, with the assumption it lowers the activation energy by 50%?

<p>5kJ (B)</p> Signup and view all the answers

In a chemical reaction, if the concentration of a substance decreases over time, what is it classified as?

<p>A reactant (B)</p> Signup and view all the answers

Given a reaction where the rate of N$_2$O$_5$ consumption is 2.4 M/hour, and the reaction is 2N$_2$O$_5$ → 4NO$_2$ + O$_2$, at what rate is NO$_2$ formed?

<p>4.8 M/hour (C)</p> Signup and view all the answers

If the rate of change of [CH$_4$] is -1.25 mol/s, and the reaction is CH$_4$ + 2O$_2$ → CO$_2$ + 2H$_2$O, what is the rate of change of [O$_2$]?

<p>-2.5 mol/s (D)</p> Signup and view all the answers

Which of the following best describes the term 'instantaneous rate'?

<p>The rate of reaction at a particular moment. (D)</p> Signup and view all the answers

In the reaction 24NH$_3$ + 5O$_2$ → 4NO + 6H$_2$O, which of the following represents the correct rate expression relationship for NH$_3$ compared to others?

<ul> <li>Δ[NH$_3$] / Δt = 4 Δ[O$_2$] / Δt = 4 Δ[NO] / Δt = 6 Δ[H$_2$O] / Δt (B)</li> </ul> Signup and view all the answers

Consider a graph of concentration vs. time for a reaction. For a reactant, how will the slope of the tangent line change as the reaction progresses?

<p>The slope will become less negative. (D)</p> Signup and view all the answers

If a reaction has an instantaneous rate of 0.00469 g/s at the beginning, but decreases with time, what information does the average rate provide?

<p>The overall rate over a specified period. (C)</p> Signup and view all the answers

In the reaction CH$_4$ + 2O$_2$ → CO$_2$ + 2H$_2$O, if the rate of consumption of CH$_4$ is 1.25 mol/s, what is the rate of formation of H$_2$O?

<p>2.5 mol/s (B)</p> Signup and view all the answers

In the reaction $N_{2(g)} + 2O_{2(g)} \rightarrow 2NO_{2(g)}$, if the concentration of $NO_2$ increases by $0.45M$ over 3 minutes, what is the reaction rate of $NO_2$ in $M/s$?

<p>0.0025 M/s (B)</p> Signup and view all the answers

For the reaction $Li^+ + NO_3^- \rightarrow LiNO_3$, if the initial amount of $Li^+$ is 5g and after 30 seconds it is 0g, what is the reaction rate of $Li^+$?

<p>-0.167 g/s (B)</p> Signup and view all the answers

Which of these is NOT a valid way to measure the reaction rate of the reaction below? $Cu(s) + 4H^+(aq) + 4NO_3^-(aq) \rightarrow Cu^{+2}(aq) + 2NO_3^-(aq) + 2NO_2(g) + 2H_2O(l)$

<p>Change in color of the solution (A)</p> Signup and view all the answers

In the reaction $2NO_{2(g)} \rightarrow 2NO_{(g)} + O_{2(g)}$, if during a 400s period the concentration of $NO_2$ changes from 0.100 M to 0.030 M, what is the reaction rate of $NO_2$?

<p>-0.000175 M/s (C)</p> Signup and view all the answers

If a reaction shows a decrease in the concentration of a substance over time, is this species a reactant or a product?

<p>Reactant, because the reaction consumes it. (A)</p> Signup and view all the answers

If the rate of a reaction is reported as $-0.0033 M/s$, what does the negative sign indicate?

<p>The concentration of a reactant is decreasing. (C)</p> Signup and view all the answers

If Substance 'B' has an average reaction rate of +0.0033M/s, what does this suggest about the substance?

<p>It is being formed in the reaction. (A)</p> Signup and view all the answers

Given an average reaction rate of -0.0033 M/s for substance A of a reaction, and a reaction time of 500s, what is the concentration change?

<p>-1.65M (A)</p> Signup and view all the answers

Flashcards

Reaction Rate

A measure of how quickly the concentration of reactants decreases or the concentration of products increases over a specific time period.

Average Rate of Reaction

A measure of a substance’s change in concentration over time.

Rate of Reaction of a Specific Species

The change in concentration of a substance over a specific time interval.

Reactant

A substance that is consumed during a chemical reaction.

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Product

A substance that is produced during a chemical reaction.

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Rate of product formation = Rate of reactant consumption

The rate at which a product is formed is the same as the rate at which a reactant is being consumed.

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Instantaneous Rate

The rate of a reaction at a specific instant in time.

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Average Rate

The rate of a reaction calculated over a specific time interval.

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Rate of reaction

The rate of change in concentration of a substance divided by the change in time.

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Rate Expression

The rate of a reaction is proportional to the change in concentration of each substance divided by its stoichiometric coefficient.

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Stoichiometry and Rate

The rate of formation of a product is related to the rate of consumption of a reactant by the stoichiometric coefficients in the balanced chemical equation.

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Reactant & Product Rate

The rate of consumption of a reactant is proportional to the rate of formation of a product multiplied by the ratio of their stoichiometric coefficients.

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Calculating Rate

The rate of a reaction can be calculated by dividing the change in concentration of a substance by the change in time.

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Rate of change of a species

The change in concentration of a substance over a specific time interval.

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Catalyst

A substance that speeds up the rate of a chemical reaction without being consumed itself.

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Inhibitor

A substance that slows down the rate of a chemical reaction.

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Activation Energy

The minimum amount of energy that reactant molecules must have for a successful reaction.

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Activated complex

The unstable arrangement of atoms at the peak of the energy profile during a reaction, representing the highest energy point where reactants are about to transform into products. It's like a mountain pass between two valleys.

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Change in enthalpy (ΔH)

The difference in energy between reactants and products, a positive value indicates energy is absorbed (endothermic) and a negative value indicates energy is released (exothermic).

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Exothermic Reaction

A reaction in which energy is released to the surroundings resulting in a decrease in enthalpy of the system. The products are at a lower energy level than the reactants.

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Endothermic Reaction

A reaction in which energy is absorbed from the surroundings resulting in an increase in enthalpy of the system. The products are at a higher energy level than the reactants.

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Enthalpy Change (ΔH)

Indicates whether a reaction releases or absorbs heat. A negative value means energy is released (exothermic), while a positive value means heat is absorbed (endothermic).

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Potential Energy Diagram

A visual representation of the energy changes that occur during a chemical reaction, showing the energy levels of reactants, products, and the activated complex.

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Study Notes

Reaction Rates and Data Analysis

  • Reaction rate calculation (Nitrous Oxide): A 0.45M increase in nitrous oxide concentration over 3 minutes equates to a reaction rate of 0.0025M/s.

Reaction of Lithium and Nitrate Ions

  • Reaction: Li⁺ + NO₃⁻ → LiNO₃
  • Initial conditions: 5g lithium ions, 10g nitrate ions, 0g lithium nitrate
  • 30 seconds later: 0g lithium ions, 5g nitrate ions, 5g lithium nitrate
  • Reaction rates:
    • Li⁺: -0.17g/s
    • NO₃⁻: +0.17g/s
    • LiNO₃: +0.17g/s

Measuring Reaction Rates

  • Methods: Reaction rates can be measured by monitoring mass, concentration, or pressure changes.

Decomposition of NO₂

  • Reaction: 2NO₂(g) → 2NO(g) + O₂(g)
  • Data analysis (400s time period):
    • NO₂: -0.000175M/s
    • NO: +0.000175M/s
    • O₂: +0.0000875M/s

Reaction of Reactant A to Product E

  • Data:
    Time (min) Mass of A (g)
    0 25.0
    1.0 20.0
    2.0 17.0
    3.0 15.0
    4.0 13.0
    5.0 12.0
  • Rate over 5 minutes: -2.6g/min
  • Rate between 2 and 4 minutes: -2.0g/min

Average Reaction Rates for Substances A and B

  • Data: Plotted concentration vs. time
  • Species A: Reactant (concentration decreases)
  • Species B: Product (concentration increases)
  • Average rates:
    • A: -0.0033 M/s
    • B: +0.0033 M/s

Average Rates of Each Species (NO₂ and N₂O₄)

  • Data: Concentration vs. time
  • NO₂: +0.06M/s
  • N₂O₄: -0.45M/s

Reaction Rate at 150s for Substances A and B

  • Data from graph. Rates calculated by visual estimation.

Average Reaction Rates from 0-1200s

  • Species:
    • N₂O₅: -0.00021M/s
    • NO₂: +0.00042M/s
    • O₂: +0.00010M/s

Determining Reactants and Products

  • NO₂: Product
  • O₂: Product
  • N₂O₅: Reactant

Rates of N₂O₅ Consumption and Product Formation

  • NO₂ Formation rate: 4.8M/hr
  • O₂ Formation rate: +1.2M/hr

Coefficients in the Rate Expression for NH₃

  • Based on the reaction coefficients given
    • Δ[NH₃] / Δt = -4 / 5 Δ[O₂] / Δt = -4/5 Δ[NO] / Δt = -4/5 Δ[H₂O]/Δt

Methane Combustion and Reaction Rates

  • Reaction: CH₄ + 2O₂ → CO₂ + 2H₂O
  • CH₄ Rate: -1.25 mol/s
  • O₂ Rate: -2.5 mol/s
  • H₂O Rate: +2.5 mol/s
  • CO₂ Rate: +1.25 mol/s

Decomposition of Iron (III) Hydroxide

  • Balanced equation: 2Fe(OH)₃ → Fe₂O₃ + 3H₂O
  • Mass of Fe(OH)₃ required (1L, 0.096mol/L solution): 10.3g
  • Rate of loss of Fe(OH)₃: -0.00172 M/s

Rates of Product Formation in Fe(OH)₃ Decomposition

  • H₂O: +0.0026M/s
  • Fe₂O₃: +0.00086M/s

Acetaldehyde Decomposition

  • Balanced equation: CH₃CHO → CH₄ + CO
  • Mass of CH₃CHO (1L, 0.00667M): 0.29g
  • Rate of Acetaldehyde decomposition (42-105s): -0.000013M/s

Reaction Rates and Species Amounts in Chemical Reactions

  • Analysis of changing amounts of species in various reactions

Factors Affecting Reaction Rates

  • Concentration: Higher concentration leads to more collisions, increasing rate.
  • Temperature: Higher temperature increases particle energy and collision frequency, increasing the rate.
  • Volume: Decreasing volume leads to less space between particles, increasing collisions, but decreasing the rate.
  • Catalysts: Catalysts lower activation energy, increasing the reaction rate.
  • Pressure: Increasing pressure leads to more collisions, increasing rate.
  • Surface Area: Larger surface area increases reaction sites, increasing rate.

Energy Diagrams and Reaction Rates

  • Activation energy: Minimum energy needed for reaction to occur.
  • Activated complex: Intermediate unstable state during a reaction
  • Enthalpy change: Difference in energy levels between reactants and products
  • Endothermic: Energy is absorbed by the reaction
  • Exothermic: Energy is released from the reaction

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