Rate of reaction
Rate of reaction = change in concentration of reactant/product per unit time
Amount of reactant used (or product made) divided by time
How to measure rate of reaction?
Continuous method
measurements are taken over the duration of the reaction, concentration-time graph is drawn and
the half-life is measured to work out orders of reaction.
Continuous rate data
concentration of one substance is followed throughout the experiment. The concentrations of the
reactants not being followed must be in large excess in the experiment so their concentrations stay
virtually constant and does not affect the rate.
Electrical conductivity
during a reaction there may be a change in the number of ions which will affect the level of
electrical conductivity e.g. increasing the number of ions the electrical conductivity would increase
Amount of mass lost
for reactions that produce a gas, place the reaction vessel on a balance and measure the mass loss
as gas is lost (fume cupboard used as some gases produced could be toxic
Volume of gas produced
gas syringe used to measure volume of the gas over a speci c time period or could measure the
volume of water displaced by the gas using an inverted cylinder
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, Change in pH of a reaction
pH of a reaction could change over time eg if H+ ions are produced or used up
Use a pH meter to measure the pH of the reaction at regular time internals and use this to calculate
the H+ ion concentration
Titration
monitoring change in concentration of a reactant/product by taking small samples (aliquot) at
regular time intervals and titrating them
When the aliquot is taken, the reaction must be slowed down straight away otherwise the
concentration would continue to change whilst you were conducting the titration.
Methods of slowing down the reaction
diluting with deionised (distilled) water
cool it down
add a chemical to stop the reaction (quenching agent)
Colour Change
Colorimeter is used
A colorimeter measures the absorbance of light in a coloured sample. The more concentrated the
sample is, the darker its colour so more light is absorbed
The colour of the substance can be measured at regular time intervals (monitoring colour change)
I₂(aq) + CH3COCH₂I(aq) → I-(aq) + H+(aq) (Reaction goes from brown to colourless as all products are
colourless)
a calibration curve needs to be plotted by using xed concentrations and measuring the
absorbance for each one and plotting the results
The calibration curve can then be used to nd out the concentration
Measurement of optical activity
If there is a change in the optical activity through the reaction, this could be followed in a
polarimeter
CH3CHBrCH3(l) + OH-(aq) → CH3CH(OH)CH3(l) + Br-(aq)
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, Calculating rate from a graph
found from gradient
The bigger the section of graph used to calculate the gradient, the better
we calculate rate by working out gradient of a tangent if given a curved line of best t (gives rate at
a speci c time)
fi fi
Rate of reaction = change in concentration of reactant/product per unit time
Amount of reactant used (or product made) divided by time
How to measure rate of reaction?
Continuous method
measurements are taken over the duration of the reaction, concentration-time graph is drawn and
the half-life is measured to work out orders of reaction.
Continuous rate data
concentration of one substance is followed throughout the experiment. The concentrations of the
reactants not being followed must be in large excess in the experiment so their concentrations stay
virtually constant and does not affect the rate.
Electrical conductivity
during a reaction there may be a change in the number of ions which will affect the level of
electrical conductivity e.g. increasing the number of ions the electrical conductivity would increase
Amount of mass lost
for reactions that produce a gas, place the reaction vessel on a balance and measure the mass loss
as gas is lost (fume cupboard used as some gases produced could be toxic
Volume of gas produced
gas syringe used to measure volume of the gas over a speci c time period or could measure the
volume of water displaced by the gas using an inverted cylinder
fi
, Change in pH of a reaction
pH of a reaction could change over time eg if H+ ions are produced or used up
Use a pH meter to measure the pH of the reaction at regular time internals and use this to calculate
the H+ ion concentration
Titration
monitoring change in concentration of a reactant/product by taking small samples (aliquot) at
regular time intervals and titrating them
When the aliquot is taken, the reaction must be slowed down straight away otherwise the
concentration would continue to change whilst you were conducting the titration.
Methods of slowing down the reaction
diluting with deionised (distilled) water
cool it down
add a chemical to stop the reaction (quenching agent)
Colour Change
Colorimeter is used
A colorimeter measures the absorbance of light in a coloured sample. The more concentrated the
sample is, the darker its colour so more light is absorbed
The colour of the substance can be measured at regular time intervals (monitoring colour change)
I₂(aq) + CH3COCH₂I(aq) → I-(aq) + H+(aq) (Reaction goes from brown to colourless as all products are
colourless)
a calibration curve needs to be plotted by using xed concentrations and measuring the
absorbance for each one and plotting the results
The calibration curve can then be used to nd out the concentration
Measurement of optical activity
If there is a change in the optical activity through the reaction, this could be followed in a
polarimeter
CH3CHBrCH3(l) + OH-(aq) → CH3CH(OH)CH3(l) + Br-(aq)
fi fi
, Calculating rate from a graph
found from gradient
The bigger the section of graph used to calculate the gradient, the better
we calculate rate by working out gradient of a tangent if given a curved line of best t (gives rate at
a speci c time)
fi fi
