2015 AP Chemistry free response 5a: Finding order of reaction | Chemistry | Khan Academy
TLDRThe video script discusses a chemistry experiment where blue food coloring is bleached using household bleach, causing a color change from blue to colorless. A spectrophotometer is used to study the absorbance over time, revealing that the reaction is first order due to a constant half-life and a linear graph of ln(concentration) versus time.
Takeaways
- 🔬 The script discusses a chemical reaction where blue food coloring is oxidized by household bleach, resulting in colorless products.
- 📚 The reaction is represented by a chemical equation, showing the transformation from blue to colorless.
- 🔍 A spectrophotometer at a wavelength of 625 nanometers is used to study the absorbance of the food coloring over time during the bleaching process.
- ⚗️ Bleach is present in large excess to keep the concentration of hypochlorite essentially constant throughout the reaction.
- 📈 The student generated graphs to analyze the reaction, including absorbance over time, ln(A) over time, and 1/[A] over time.
- 🧐 The task is to determine the order of the reaction with respect to the blue food coloring based on the provided graphs.
- 📊 The first graph, showing concentration over time, is not linear, indicating the reaction is not zero order.
- 🕒 The constant half-life observed in the first graph suggests that the reaction is first order, as the time to halve the concentration remains the same regardless of the initial concentration.
- 📚 The script references the first order integrated rate law and the concept that the half-life is constant for first order reactions.
- 📉 Graph II, showing ln(A) over time, is linear, which is consistent with the behavior of a first order reaction.
- 🔑 Two pieces of evidence from Graph I (constant half-life) and Graph II (linear ln(A) vs. time) confirm that the reaction is first order with respect to the blue food coloring.
Q & A
What causes the blue food coloring to change to a colorless product?
-The blue food coloring is oxidized by household bleach, which contains hypochlorite ion (OCI-), resulting in a color change from blue to colorless.
What is the significance of the bleach being in large excess during the reaction?
-The large excess of bleach ensures that the concentration of hypochlorite remains essentially constant throughout the reaction, which is important for maintaining a consistent rate of reaction.
What is the purpose of using a spectrophotometer in this study?
-A spectrophotometer is used to measure the absorbance of the food coloring over time during the bleaching process, providing data on how the concentration of the reactant changes.
Why is it important to determine the order of the reaction with respect to the blue food coloring?
-Determining the reaction order helps in understanding the kinetics of the reaction, which is essential for predicting how changes in the concentration of reactants will affect the reaction rate.
How can we tell if the reaction is not zero order by looking at the first graph?
-If the reaction were zero order, the concentration of the blue food coloring would decrease linearly over time. Since the graph is not linear, it indicates that the reaction is not zero order.
What does a constant half-life imply about the order of the reaction?
-A constant half-life implies that the reaction is first order because, in a first-order reaction, the half-life remains the same regardless of the initial concentration or the point in the reaction.
How can we confirm that the reaction is first order by looking at the second graph?
-In the second graph, if plotting the natural logarithm of the absorbance (ln A) against time results in a straight line, it confirms that the reaction is first order.
What is the relationship between the half-life of a reaction and the rate constant for a first-order reaction?
-The half-life of a first-order reaction is a constant value and is related to the rate constant (k) by the equation: half-life = 0.693 / k.
Why is it unnecessary to look at the third graph if we already determined the reaction is first order from the first two graphs?
-The third graph, which involves plotting 1/concentration over time, is typically used to determine if a reaction is second order. Since the first two graphs have already confirmed the reaction is first order, the third graph does not provide additional relevant information.
What does the non-linear decrease of absorbance over time in the first graph indicate about the reaction kinetics?
-The non-linear decrease of absorbance over time in the first graph indicates that the reaction is not following a simple zero-order kinetics, where the concentration would decrease linearly with time.
How can the rate law be determined from the given graphs and the reaction data?
-By analyzing the graphs, particularly the linearity in the ln(A) vs. time plot and the constant half-life observed, the rate law can be determined to be first order with respect to the blue food coloring.
Outlines
🧪 Chemical Reaction of Blue Food Coloring with Bleach
This paragraph discusses the chemical reaction between blue food coloring and household bleach, which contains hypochlorite ions (OCI-). The reaction leads to the oxidation of the blue coloring, resulting in colorless products. The process is studied using a spectrophotometer at a wavelength of 625 nanometers to measure the absorbance of the food coloring over time. The bleach is present in large excess to ensure the concentration of hypochlorite remains constant throughout the reaction. The student's data is used to generate graphs that will help determine the order of the reaction with respect to the blue food coloring.
📊 Analyzing Reaction Order Through Graphs and Half-Life
The second paragraph delves into the analysis of the reaction order by examining the absorbance of the blue food coloring over time through various graphs. The graphs include the concentration of A over time, the natural logarithm of A over time, and one over the concentration of A over time. The paragraph emphasizes that the reaction is not zero order due to the non-linear graph of concentration over time. Further analysis reveals that the reaction is first order, as evidenced by the constant half-life observed in the first graph. This is confirmed by the fact that it consistently takes 20 seconds for the concentration to halve, regardless of the starting point. Additionally, the paragraph mentions that a first-order reaction would result in a straight line when plotting the natural logarithm of the reaction versus time, which is supported by the second graph.
Mindmap
Keywords
💡Blue food coloring
💡Oxidized
💡Hypochlorite ion
💡Spectrophotometer
💡Absorbance
💡Reaction order
💡First order reaction
💡Half-life
💡Integrated rate law
💡Natural log
💡Concentration
Highlights
Blue food coloring can be oxidized by household bleach to form colorless products.
The chemical reaction involves bleach or hypochlorite with the blue food coloring.
A spectrophotometer at 625 nanometers was used to study the absorbance of food coloring over time.
Bleach is present in large excess to keep the concentration of hypochlorite constant throughout the reaction.
The order of the reaction with respect to the blue food coloring is to be determined from the graphs.
Absorbance is represented as A, and the study involves looking at concentration of A over time.
The natural logarithm of A over time and one over the concentration of A over time are also analyzed.
The reaction is not zero order as the concentration of A over time is not linear.
First order reactions have a constant half-life, which is a key characteristic used to identify the order.
The constant half-life of 20 seconds for the blue food coloring indicates a first order reaction.
Graph I shows that the time to halve the concentration remains constant, supporting the first order reaction.
Graph II confirms the first order reaction as ln of A versus time results in a straight line.
Two pieces of evidence from Graph I and II confirm the reaction is first order with respect to the blue food coloring.
The reaction's order was determined without needing to look at all the provided graphs.
Understanding the characteristics of first order reactions is crucial for analyzing the data.
The spectrophotometer data and the concept of half-life were effectively used to determine the reaction order.
The method demonstrates a practical application of kinetics in analyzing chemical reactions.
Transcripts
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