Measuring Reaction Rates
Reaction rates are measured by observing the changes in the concentrations of
reactants or products within a particular time frame.
fig. 1
Rates of Disappearance and Appearance of Chemical Species
This expression relates the rates of disappearance and appearance of chemical species
in the reaction A + 2B --> 3C.
 Reaction rate is calculated using the formula rate = Δ[C]/Δt, where Δ[C] is
change in product concentration during time period Δt. The rate of reaction can
be observed by watching the disappearance of a reactant or the appearance of a
product over time.
 Various methods are used for detecting reaction products. Lit splints are used for
testing the presence of a hydrogen gas product. When carbon dioxide gas is
produced, it turns limewater milky. If a reaction produces oxygen or carbon
dioxide, the gas syringe method can measure volume.
 The amount of product precipitate formed in a period of time can be measured to
estimate the reaction rate. This is done by measuring the amount of time it takes
for the forming precipitate to obscure the visibility of a cross through a conical
flask in which the reaction takes place.
 If a gas is formed during a reaction, the reduction in the mass of the reaction
solution can be measured. This method will produce reliable results for oxygenand carbon dioxide-producing reactions, but it is not accurate for hydrogenproducing reactions because of hydrogen's low mass.
Product
A chemical substance formed as a result of a chemical reaction
Reaction Rate
How fast or slowly a reaction takes place
Gas syringe
An item of laboratory equipment used to withdraw a volume of gas from a closed
chemical system, for measurement and/or analysis
EXAMPLES
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Figure 1 demonstrates the gas syringe method that is used for measuring the volume of
gas produced in a particular reaction. This method can be used for measuring oxygen
or carbon dioxide production. As an example, decomposition of hydrogen peroxide
leads to production of water and oxygen. In this experimental setup, the produced
oxygen will push against the plunger, and the markings on the syringe will indicate the
volume of the gas produced.
fig. 2. Gas Syringe Method
In a reaction that produces a gas, the volume of the gas produced can be measured
using the gas syringe method.
fig. 3
Measuring Reaction Rates When Precipitates Are Formed
At the beginning of the reaction between sodium thiosulphate and hydrochloric acid,
when no precipitate has been formed, the cross at the bottom of the conical flask can be
clearly seen.
Source URL: https://www.boundless.com/chemistry/chemical-kinetics/reaction-rates/measuring-reaction-rates/
Saylor URL: http://www.saylor.org/courses/chem102#1.2.1
Attributed to: Boundless
www.saylor.org
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