What is continuous monitoring?
When the time graphs can be plotted from continuous measurements taken during the course of a reaction.
What are three methods of continuous monitoring?
- Monitoring by gas collection.
- Monitoring by mass loss.
- If a gas is not produced, colour change can also be measured via a colorimeter or by eye.
What happens in a colorimeter?
The wavelength of light passing through a coloured solution is controlled using a filter, the amount of light absorbed by a solution is measured.
How do you analyse/monitor rate by a colorimeter?
- Prepare a standard solution of known concentration of the coloured chemical in the reaction (iodine).
- Select a filter with the complementary colour of the coloured chemical (for indie it is a green/blue filter.
- Zero the colorimeter with water.
- Measure the absorbance readings of the standard solutions of iodine.
- Plot a calibration curve of absorbance against iodine concentration. You can now convert an absorbance reading into a concentration of iodine.
- Carry out the reaction between propanone and iodine. Take absorbance readings of the reacting mixture at measured time intervals.
- Use the calibration curve to measure the concentration of iodine at each absorbance reading.
- Plot a second graph of concentration of iodine against time. From the concentration-time graph, you can determine the order of reaction with respect to the coloured chemical (iodine).
How can the order with respect to a reactant be deduced off of a concentration-time graph?
The shape of a concentration-time graph for zero and first order reactions, only if all other reactant concentrations remain effectively constant.
How can zero order be deduced from a concentration-time graph?
Produces a straight line with a negative gradient as the rate of reaction does not change at all during the course of the reaction. The value of the gradient is equal to the rate constant, k.
How can you deduce a first order from a concentration-time graph?
Produces a downward curve with a decreasing gradient over time as the gradient decreases with time, the reaction gradually slows down.
What is the half-life of a concentration-time graph?
The time for the concentration of the reactant to halve is constant.
What can half-life be used to determine?
The rate constant of a first order reaction.
How can you deduce a second order from a concentration-time graph?
A downward curve, steeper at the start, but trailing off more slowly.
What is half-life (t 1/2)?
The time taken for the concentration of a reactant to decrease to half its original value.
What is the relationship with firs order reactions and half-life? Called?
First order reactions have a constant half-life with the concentration halving every half life. This pattern is called exponential decay.
How can a first order relationship be confirmed from a concentration-time graph?
By measuring the successive half-lives. If they are the same, the reaction is first order with respect to the reactant.
What are the two methods for determining k for a first order reaction?
- Calculating the rate constant from the rate.
- Calculating the rate constant from the half-life.
How do you calculate the rate constant from the rate?
- Draw a tangent to the curve on the concentration-time graph at a particular concentration.
- The gradient of the tangent is calculated giving the rate of reaction.
- The rate constant is calculated by rearranging the rate equation and substituting the value of rate (the gradient of the tangent) and the concentration at the position where the tangent has been drawn.
How do you calculate the rate constant from the half-life?
Make use of the exponential relationship for a constant half-life:
K = In2 (on calculator) / half life.
How can you plot a concentration-time graph from analyzation by a colorimetry?
- Prepare standard solutions of known concentration of the coloured chemical in the reaction.
- Select a filter with the complementary colour of the coloured chemical.
- Zero the colorimetry with water.
- Measures the absorbance readings of the standard solutions.
- Plot a calibration curve of absorbance against the coloured chemical concentration. You can now convert an absorbance reading into a concentration of the coloured chemical.
- Carry out the reaction and take absorbance readings of the reacting mixture at measured time intervals.
- Use the calibration curve to measure the concentration of the coloured chemical at each absorbance reading.
- Plot a second graph of concentration of the coloured chemical against time. From the concentration-time graph, you can determine the order of reaction with respect to the coloured chemical.
How is a concentration-time graph set up with constant half-lives?
- The concentration of in rate equation is on y-axis.
- The time in seconds is on the x-axis.
