Chemical Kinetics Introduction

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Questions and Answers

What is the area of chemistry concerned with reaction rates called?

  • Thermodynamics
  • Physical chemistry
  • Chemical kinetics (correct)
  • Analytical chemistry

The rate of a chemical reaction is determined solely by the increase in concentration of the products.

False (B)

What formula expresses the rate of a chemical reaction?

Rate = ∆c/∆t

In the reaction A → B, the concentration of A decreases during the time interval, therefore ∆[A] is a ______ quantity.

<p>negative</p> Signup and view all the answers

Match the following examples with their respective rates of reaction:

<p>Rusting of iron = Slow Combustion of gasoline = Fast Ripening of fruits = Moderate Weathering of stone = Very slow</p> Signup and view all the answers

Which reaction would likely have the fastest rate?

<p>Combustion of gasoline (A)</p> Signup and view all the answers

Chemical kinetics only studies reactions that occur within a few seconds.

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

What does ∆c represent in the rate of reaction formula?

<p>Change in concentration of a substance</p> Signup and view all the answers

What is the average rate of formation of NO2 during the time period 300 – 400 s?

<p>3.7 × 10-5 mol L−1 s−1 (C)</p> Signup and view all the answers

The concentration of NO2 increases with time as the reaction proceeds.

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

What is the molarity of O2 produced when [N2O5] = 1.20 × 10–2 mol L−1 at 300 s?

<p>0.4 × 10–2 mol L−1</p> Signup and view all the answers

The molarity of NO2 produced per liter is calculated using the factor of _______ moles of NO2 per mole of N2O5.

<p>2</p> Signup and view all the answers

Match the following terms with their definitions:

<p>Molarity = Concentration of a solute per liter of solution Stoichiometry = Relationship between reactants and products in a chemical reaction Reaction rate = Change in concentration of a reactant or product over time Chemical equation = Represents the substances involved in a reaction</p> Signup and view all the answers

Which statement best explains why the rate of reaction decreases over time?

<p>The concentration of reactants decreases. (D)</p> Signup and view all the answers

How is the average rate of O2 formation determined during a specific time period?

<p>By calculating the slope of the hypotenuse of the triangle using ∆[O2] and ∆t.</p> Signup and view all the answers

At time t = 350 s, the rate of formation of NO2 can be calculated using the initial concentration of N2O5.

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

What is the rate expression for the reaction 3O2(g) → 2O3(g)?

<p>Rate = −1/3 ∆ [O2]/ ∆ t (A)</p> Signup and view all the answers

The rate of a reaction is always expressed as a positive quantity without needing adjustments for reactant disappearance.

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

If the rate of disappearance of I− is 2.0 × 10–5 mol L–1 s–1, what would be the rate of appearance of OI− in the reaction I−(aq) + OCl−(aq) → Cl−(aq) + OI−(aq)?

<p>2.0 × 10–5 mol L–1 s–1</p> Signup and view all the answers

The rate of formation of N2 from the reaction 2NO(g) + 2H2(g) → N2(g) + 2H2O(g) can be expressed as ___.

<p>2.5 × 10–5 mol L–1 s–1</p> Signup and view all the answers

Match the following reactions with their correct rate expressions:

<p>I−(aq) + OCl−(aq) → Cl−(aq) + OI−(aq) = Rate = −∆[I−]/∆t 3O2(g) → 2O3(g) = Rate = −1/3 ∆[O2]/∆t 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) = Rate = −1/4 ∆[NH3]/∆t 2NO(g) + 2H2(g) → N2(g) + 2H2O(g) = Rate = −1/2 ∆[NO]/∆t</p> Signup and view all the answers

For the reaction aA + bB → cC + dD, which expression correctly represents the rate?

<p>All of the above (D)</p> Signup and view all the answers

An observable property that can indicate the rate of reaction must be something that can be directly calculated as a concentration.

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

What common observable properties can be used to determine the change in concentration over time?

<p>Color change, temperature change, pressure change, mass change, appearance of a new substance.</p> Signup and view all the answers

What is the term for the minimum amount of energy needed for a reaction to occur?

<p>Activation energy (A)</p> Signup and view all the answers

Ineffective collisions can result in the formation of desired products.

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

What factor increases the reaction rate by causing particles to collide more frequently?

<p>Temperature</p> Signup and view all the answers

The reaction between an acid and a base, such as HCl and NaOH, occurs rapidly compared to the decomposition of ________.

<p>hydrogen peroxide</p> Signup and view all the answers

Match the following reactions with their characteristics:

<p>HCl + NaOH = Rapid acid-base reaction H2O2 = Slow decomposition reaction Sodium with water = Explosive reaction Calcium with water = Moderate reaction rate</p> Signup and view all the answers

Which factor does NOT affect the rate of a chemical reaction?

<p>Color of reactants (D)</p> Signup and view all the answers

Homogeneous reactions occur in different phases.

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

What type of collisions lead to reactions forming new products?

<p>Effective collisions</p> Signup and view all the answers

What type of reaction occurs only at the interface between two phases?

<p>Heterogeneous reaction (D)</p> Signup and view all the answers

An increase in concentration of reactants generally decreases the rate of reaction.

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

What is the effect of temperature on the rate of reaction?

<p>Increasing the temperature generally increases the rate of reaction.</p> Signup and view all the answers

A __________ is a substance that increases the rate of a reaction by providing a different reaction mechanism with lower activation energy.

<p>catalyst</p> Signup and view all the answers

Match the following factors with their effects on reaction rates:

<p>Concentration = Increases contacts between reacting particles Temperature = Increases kinetic energy of molecules Surface area = Increases the rate of solid reactions Catalyst = Lowers activation energy</p> Signup and view all the answers

Which of the following statements is true regarding the effect of pressure on reaction rate?

<p>Pressure only affects gaseous reactants (A)</p> Signup and view all the answers

Finely divided solids react more slowly than larger bodies of the same substance.

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

What is activation energy in the context of chemical reactions?

<p>The minimum energy required to start a chemical reaction.</p> Signup and view all the answers

What is the initial rate of reaction calculated from?

<p>The slope of a tangent at the initial point (D)</p> Signup and view all the answers

According to collision theory, all collisions between molecules result in a reaction.

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

What must happen for a chemical reaction to take place according to collision theory?

<p>Molecules must collide with proper orientation and sufficient energy.</p> Signup and view all the answers

The rate of reaction is directly proportional to the number of ______ per second.

<p>collisions</p> Signup and view all the answers

At what time is the instantaneous rate of reaction calculated in the given example?

<p>2 seconds (C)</p> Signup and view all the answers

The slope of the tangent drawn to the curve represents the instantaneous rate of reaction.

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

In the provided gas-phase reaction, what compounds are being produced?

<p>I2 and HCl.</p> Signup and view all the answers

Flashcards

Chemical Kinetics

The study of how fast chemical reactions happen and the factors that influence their speed.

Rate of reaction

The change in the concentration of a reactant or product over a period of time.

Half-life

The time it takes for the concentration of a reactant to decrease to half its initial value.

Factors affecting reaction rate

They are the conditions that affect the rate of a reaction, such as temperature, concentration, surface area and catalysts.

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Temperature effect

An increase in temperature increases the rate of reaction.

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Concentration effect

An increase in concentration of reactants increases the rate of reaction.

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Surface area effect

An increase in surface area increases the rate of reaction.

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Catalyst

A substance that speeds up a reaction without being consumed in the process.

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Why does the rate of reaction decrease over time?

The rate of reaction decreases as the reaction progresses.

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What is the rate of reaction?

The change in concentration of a reactant or product over time.

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What is the average rate of reaction?

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

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What is the instantaneous rate of reaction?

The rate of reaction at a specific point in time.

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How do you find the instantaneous rate of reaction?

The slope of the concentration vs. time graph at a given time point.

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How do concentrations change during a reaction?

The concentration of reactants decreases and the concentration of products increases over time.

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What is the relationship between the chemical equation and the rate of reaction?

A chemical equation represents the stoichiometry of a reaction.

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How do you calculate the rate of reaction from the concentration of reactants and products?

The rate of a reaction can be calculated using the change in concentration of any reactant or product over time.

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Instantaneous Rate

The rate of a reaction at a specific moment in time, determined by the slope of a tangent line on a concentration-time graph.

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Initial Rate

The rate of a reaction at the very beginning, when all reactants are present in their initial concentrations.

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Collision Theory

The theory that chemical reactions happen due to energetic collisions between molecules, where the molecules must collide with sufficient energy and in the correct orientation.

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Effective Collisions

Collisions between reactants that result in the formation of products.

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Ineffective Collisions

Collisions between reactants that do not lead to product formation.

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Rate Expression for Reactants

A negative sign is used in the rate expression for reactants because their concentration decreases over time.

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Rate Expression for Products

A positive sign is used in the rate expression for products because their concentration increases over time.

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Stoichiometry and Rate

The rate of product formation is directly proportional to the rate of reactant disappearance. The ratio is determined by the stoichiometric coefficients.

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General Rate Expression

Rate = -1/a ∆[A]/∆t = -1/b ∆[B]/∆t = 1/c ∆[C]/∆t = 1/d ∆[D]/∆t. Where a, b, c, and d are the stoichiometric coefficients for the reaction.

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Determining Reaction Rate

The rate of reaction can be determined by observing a measurable change in a property over time, such as color change, temperature change, pressure change, mass change, or the appearance of a new substance.

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Rate of Reaction Measurement

The rate of reaction is the change in concentration over time. This can be measured through techniques like spectrophotometry or titrations.

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Rate of Reaction Summary

The rate of a reaction is defined in terms of the disappearance of reactants or the appearance of products. The rate expression is written in terms of the change in concentration per unit time and is affected by stoichiometry.

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Molecular Orientation

The specific arrangement of molecules needed for them to interact and form new bonds.

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Activation Energy

Minimum energy required for reactants to overcome the energy barrier and start a reaction.

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Nature of Reactants

The inherent tendency of a substance to react, affecting how quickly a reaction occurs.

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Surface Area

The amount of exposed surface area of a reactant, which affects the rate of reaction.

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Homogeneous Reactions

Reactions involving reactants in the same physical state (e.g., gas and gas).

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Heterogeneous Reactions

Reactions involving reactants in different physical states (e.g., solid and liquid).

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Study Notes

Chemical Kinetics Introduction

  • Chemical reactions occur at varying speeds
  • Some reactions take days, others seconds
  • Chemical kinetics studies reaction rates
  • "Kinetic" implies movement or change
  • Chemical kinetics measures how fast reactant/product concentrations change

Reaction Rate

  • Measures the change in reactant or product concentration per unit time
  • Determined by monitoring reactant decrease or product increase
  • Expressed as ∆c/∆t
  • ∆c = Change in concentration, ∆t = Change in time
  • Rate is positive, ∆[A]/∆t or -∆ [A]/∆t

Rate Expressions for Complex Reactions

  • For more complex reactions, rate expressions consider stoichiometric coefficients
  • Rate = -1/a∆[A]/∆t = 1/b ∆[B]/∆t = 1/c ∆[C]/∆t= 1/d ∆[D]/∆t

Determining Reaction Rate

  • Observable property changes over time (e.g., color, pressure, mass)
  • Measurements track concentration changes over time
  • Example: Thermal decomposition of Nâ‚‚Oâ‚… to NOâ‚‚ + Oâ‚‚
  • Concentration changes can be monitored using pressure, color, or other measurable properties
  • Rate is calculated using concentration data, and time
  • Average rate is calculated using two concentration values in time interval ∆t

Instantaneous Rate of Reaction

  • Rate at a specific time, t
  • Calculated from the slope of a tangent line to a concentration vs. time graph at point t.
  • Initial rate is the slope of a tangent at t = 0

Factors Affecting Reaction Rates

  • Nature of Reactants: Some reactions are faster than others due to chemical nature
  • Surface Area: Larger surface area increases reaction rate for heterogeneous reactions
  • Concentration: Higher concentration of reactants leads to more frequent collisions, hence faster reaction
  • Temperature: Higher temperatures increase kinetic energy, leading to more frequent and energetic collisions, and thus a faster reaction
  • Presence of a Catalyst: Catalysts reduce activation energy; speeding up the reaction but are not consumed in the process.

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