Questions and Answers
Which chemical equation is consistent with the graph?
What is the collision theory based on?
The assumption that for a reaction to occur, the reacting compounds have to come in contact or collide with one another.
Is X a reactant or product of the reaction?
Product
How is the reaction rate changing as time progresses?
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What order would the reaction need to be to solve for the rate constant 𝑘 with only the information provided?
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What is the reaction order for A in the reaction A⟶products based on the concentration data?
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Which orders can be classified based on the effects of initial concentration on the reaction rate?
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What is the activation energy of the reverse reaction if the forward reaction has an activation energy of 𝐸a=56 kJ and an enthalpy of Δ𝐻=24 kJ?
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How many transition states are there in the reaction as described?
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Which mechanism or mechanisms are consistent with the observed rate law of the reaction H2(g)+I2(g)⟶2HI(g)?
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Determine the average rate of change of B from 𝑡=0 s to 𝑡=212 s.
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What is the instantaneous rate of the reaction at time 𝑡=40 s?
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What is the rise in concentration between the two reference points?
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What is the run between the two reference points?
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What is the slope calculated for the rise over run?
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What is the order with respect to A in the reaction 2A+3B⟶products?
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What is the order with respect to B in the reaction 2A+3B⟶products?
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What is the overall reaction order for 2A+3B⟶products?
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What will the initial rate be if [A] is halved and [B] is tripled for A+B⟶C+D?
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What will the initial rate be if [A] is tripled and [B] is halved for A+B⟶C+D?
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What is the value of x if the rate doubles when [A] is doubled?
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What is the value of x if the rate quadruples when [A] is doubled?
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What is the initial rate for the reaction C2H5Cl(g)⟶C2H4(g)+HCl(g) when [C2H5Cl]0 is 0.100 M?
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What is the rate constant k for the reaction C2H5Cl(g)⟶C2H4(g)+HCl(g) given initial concentration?
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What is the rate constant k for the reaction A+2B⟶C+D with given trial data?
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What is the rate constant k in units and values for the reaction A+B⟶C+D with trials?
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What is the value of the rate constant k for the reaction 2NO2(g)+O3(g)⟶N2O5(g)+O2(g)?
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How long would it take for the concentration of A to decrease from 0.790 M to 0.280 M for a first-order reaction with a given k?
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What is the mass of A remaining after 1.75 minutes if the initial mass is 17.93 g?
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What is the half-life of the reaction if 14.0% of a compound has decomposed?
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What is the order of the reaction that has a constant half-life of 105 s regardless of the initial concentration?
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What is the value of the rate constant for a reaction with half-life of 105 s?
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How long would it take for the concentration to decrease from 0.990 M to 0.300 M for a second-order reaction?
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What is the order of the reaction based on half-lives of 161 s and 205 s?
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What is the value of k for the reaction when given half-lives and concentrations?
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What will be the concentration after 215 s if the initial concentration is 0.00280 M?
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How long would it take for the concentration of A to decrease from 0.950 M to 0.280 M for a zero-order reaction?
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What is the reaction order if concentration data for A shows a plot of 1/[A] versus time is linear?
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What is the value of k for the reaction based on concentration and time data collected?
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Match the calculated rate constant with its appropriate units:
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Study Notes
Reaction Rates and Concentration Changes
- Reaction rates are analyzed by measuring the concentration of reactants and products over time.
- Plotting concentration vs. time reveals characteristic curves for reactants and products, indicating their behavior during the reaction.
- For the equation Y ⟶ 2X, the concentration of Y decreases while X increases, demonstrating that Y is a reactant and X is a product.
Collision Theory
- The collision theory states that chemical reactions occur when reactant particles collide effectively.
- Increasing reactant concentrations enhances the frequency of collisions, thereby speeding up the reaction rate.
- A higher number of reactants leads to faster reactions compared to fewer reactants.
Reaction Order and Integration
- A reaction order can be determined by plotting concentration data in various forms ([A] vs. t, ln[A] vs. t, or 1/[A] vs. t).
- A linear plot of 1/[A] vs. t indicates a second-order reaction, with a specific equation relating concentration changes over time.
- A first-order reaction shows a direct relationship between half-life and concentration changes.
Rate Constants and Enthalpy
- The rate constant (k) can vary based on the order of the reaction.
- Endothermic and exothermic reactions have different enthalpy values, affecting activation energies for forward and reverse reactions. For instance, with ΔH = 24 kJ and Ea = 56 kJ, the activation energy for the reverse reaction is 32 kJ.
Transition States and Mechanisms
- Reaction mechanisms can be assessed using energy diagrams, revealing transition states that correspond to maxima in energy levels.
- A mechanism with faster steps can be inferred from lower activation energies in the diagram.
Average and Instantaneous Rates
- Average rates can be calculated from concentration changes over time intervals and are affected by stoichiometric relationships.
- Instantaneous rates are determined by the slope of the tangent line on a concentration vs. time graph at any given moment.
Reaction Mechanisms Consistency
- Only mechanisms that align with the observed rate laws and additional catalytic effects (such as light in specific reactions) are deemed valid.
- Mechanisms must eliminate intermediates from rate laws to maintain consistency.
Factors Affecting Rates
- Increasing temperature or adding a catalyst boosts reaction rates by enhancing molecular speeds and collision frequencies.
- Decreasing reactant concentration or surface area reduces reaction rates, as fewer particles are available for collision.
Classification of Rate Laws
- The order of reaction toward a reactant is indicated by its exponent in the rate law (e.g., for rate = k[B]^2, order in B = 2, order in A = 0).
- The overall order of the reaction is determined by summing the individual orders, guiding predictions of rate changes with concentration adjustments.
Reaction Rate Calculations
- Reaction rates respond predictably to concentration changes, and adjustments can be calculated based on exponents in the rate laws.
- Realistic initial rates can be calculated from modifications in the concentrations of reactants.
Summary of Chemical Process
- Understanding reaction dynamics, mechanisms, and the factors influencing rates helps predict product formation and optimize chemical reactions.### Reaction Rates and Orders
- Doubling the concentration of a second-order reactant increases the reaction rate by a factor of four (2²).
- Reducing concentration of a second-order reactant by half decreases the reaction rate by a factor of four (1/4).
- Calculation example: If the initial rate is 0.0890 M/s, halving concentration gives a rate of 0.0668 M/s.
Determining Reaction Order
- In rate laws, the exponent ( x ) indicates the order of the reaction with respect to a reactant ( A ).
- If the rate doubles when ( [A] ) is doubled, ( x = 1 ) (first-order relationship).
- If the rate quadruples when ( [A] ) is doubled, ( x = 2 ) (second-order relationship).
- Zero-order means the rate is independent of concentration.
Rate Constant Calculations
- For the reaction ( C_2H_5Cl \rightarrow C_2H_4 + HCl ):
- Initial rates and concentrations lead to a rate constant ( k = 6.67 \times 10^{-30} ) s⁻¹.
- For the reaction ( A + 2B \rightarrow C + D ):
- Understanding the initial rates shows the reaction is second-order with respect to ( A ) and zero-order with respect to ( B ), leading to ( k = 0.141 ) M⁻¹⋅s⁻¹.
- For the reaction involving ( 2NO_2 + O_3 ):
- Calculation results in a rate constant ( k = 45000 ) M⁻¹⋅s⁻¹.
First-Order Reactions
- A first-order reaction has a rate constant.
- For ( A \rightarrow \text{products} ) with ( k = 0.550 ) s⁻¹, the time to decrease concentration from 0.790 M to 0.280 M is calculated to be 1.89 s.
- The mass of ( A ) remaining after 1.75 min with ( k = 0.0203 ) s⁻¹ is found to be 2.1 g.
Half-Life in Reactions
- The half-life of a first-order reaction is constant and can be used to determine ( k ).
- For a half-life of 202 min calculated from 14% decomposition over 44 min, ( k ) is deduced from the average lifetime.
- For a second-order reaction, decreased half-life indicates dependency on the initial concentration.
Concentration Change Over Time
- For a second-order reaction with ( k = 0.160 ) M⁻¹⋅s⁻¹, time for concentration to change from 0.990 M to 0.300 M is 14.5 s.
- For a zero-order reaction, with ( k = 0.0270 ) M⋅s⁻¹, time to decrease from 0.950 M to 0.280 M is calculated to be 24.8 s.
Plotting Concentration Data
- Different types of concentration-time plots can indicate the reaction order.
- For reaction data showing a linear plot of ( 1/[A] ) versus ( t ), the reaction order is determined as second-order with ( k = 0.0333 ) M⁻¹⋅s⁻¹.
Rate Constant Calculation Using Arrhenius Equation
- The Arrhenius equation ( k = Ae^{-Ea/(RT)} ) is essential for calculations.
- Activation energy must be converted to J/mol and temperature to Kelvin for accurate calculations.
- Live example results in ( k = 0.0332 ) s⁻¹ for a reaction at 55.0 °C with ( Ea = 84.0 ) kJ/mol and ( A = 7.83 \times 10^{11} ) s⁻¹.
Studying That Suits You
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Test your understanding of reaction rates with these flashcards from CHEM 123, Chapter 14. Each card explores key concepts, including the measurement of reactant and product concentrations over time. Perfect for reviewing essential chemical principles and enhancing your grasp of kinetics.
