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Measuring rates of reaction experimentally

Change

in

mass

Change in mass

  • For reactions that produce gases, mass can be measured over time
  • Mass loss indicates amount of gas produced
  • Can't work if reaction vessel is sealed

Exploration

  • Adequate for reactions producing carbon dioxide or oxygen
  • Except for hydrogen since mass is too low to be measured accurately
  • This allows for continuous readings
  • This is because graph can be plotted directly of mass against time

Application

Release of carbon dioxide

from the reaction between a carbonate and dilute acid

Examples

Spectrophotometry

spectro

photometry

- Spectrophotometry is a powerful tool for measuring rates of chemical reactions. This technique involves measuring the absorption or transmission of light by a solution as a function of wavelength.

What happens

- In a typical spectrophotometric experiment, a solution containing a reactant and a chromophore (a molecule that absorbs light at a specific wavelength) is placed in a cuvette, and the cuvette is inserted into a spectrophotometer. The spectrophotometer passes a beam of light through the solution and measures the intensity of light that passes through the sample as a function of wavelength.

The rate of reaction

As the reaction proceeds, the concentration of the chromophore changes, resulting in a change in the amount of light absorbed or transmitted by the solution. By monitoring the change in absorption or transmission of light over time, the rate of the reaction can be determined.

The rate of the reaction is related to the change in concentration of the reactant or product over time, and this change can be calculated from the change in absorbance or transmittance measured by the spectrophotometer. The rate can be determined using Beer's law, which relates the concentration of a substance in a solution to its absorbance.

Rates of reaction

Video explanation

https://youtu.be/zuUvQN8KXOk

Video

What

Change in concentration measured using titration

Using titration

Titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution.

Titration takes time during which the reaction mixture in the sample will continue to react.

How

Small samples of the reaction mixture can be removed at regular intervals and The reaction in the sample stopped or slowed significantly by a procedure called quenching.

Quenching may be achieved by adding ice-cold water or stopping the reaction by adding another reagent. The concentration of a reactant or product can be determined by titration.

example

example

The reaction between hydrogen peroxide and acidified potassium iodide producing iodine, titrated against sodium thiosulfate, Na2S2O3 to determine its concentration.

H2O2(aq) + 2H+(aq) + 2I−(aq) → I2(aq) + 2H2O(l)

The reaction is titrated against sodium thiosulfate, Na2S2O3 to determine its concentration. Sodium carbonate, Na2CO3 is used to quench the reaction by neutralizing the added acid.

I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)

Using conductivity

Change in concentration measured using conductivity

  • The rate of reaction entailing ions can be examined by electrical conductivity method. The conductivity of such a solution depends upon the rate of changes of concentration of the reacting ions or the ions formed during the reaction. The conductivity will be proportional to the rate of changes in the concentration of such ions.
  • Conductivity can be measured directly using a conductivity meter which involves immersing inert electrodes in the solution

How

  • Conductivity is a measure of the ability of a substance to conduct an electric current.
  • In the context of a chemical reaction, changes in the concentration of reactants or products can affect the conductivity of the reaction mixture.
  • In general, as the concentration of ions in a solution increases, the conductivity of the solution also increases. This is because ions are the charge carriers in a solution, and a higher concentration of ions means more charge carriers available to conduct the current.

What

Change in volume of gas produced

Used when the product is a gas. Collecting the gas and measuring the change in volume at a regular time interval enebles a graph to be plotted.

It consists of a ground of a ground glass and a plunger which moves out wards as a glass collects and is caliberated to record the volume directly

A gas syringe cann be used

This can only be done if the gas collected has low solubility in water. The rate of reaction of a metal with a dilute acid to release hydrogen gas can be folowed in this way.

Mg(s) + 2HCl(aq) ! MgCl2(aq) + H2(g)

Displacement of water from an inverted burette

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