Kinetics

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Kinetics
Rate of reaction - The speed of the reaction. It can be measured as the 'rate of formation of
product' (e.g. collecting gaseous product in a syringe) or the 'rate of removal of reactant'. The
speeds of reactions are very varied, e.g. a faster reaction example is magnesium reacting with
hydrochloric acid to form magnesium chloride. Different ways to measure the rate of reaction can be
found by using methods of colorimetry, mass change and conductivity.
Rate equation ­ This shows the relationship between the rate and the powers to which the reactant
concentrations are raised. If two reactants, A and B, react together then measurements of reaction
rates with different concentrations of A and B could give the rate equation: rate = k[A]x[B]y
Order of reaction - It shows the number of molecules of a reactant that are involved in the rate-
determining step e.g. if reactions are second order with respect to x, there'll be two molecules of x in
the rate determining step. The orders of reaction are a consequence of the mechanism of the
reaction and can only be found from rate experiments; they cannot be predicted from the balanced
equation.
Rate constant - the constant of proportionality in the rate expression. k is the rate constant, which is
constant at a particular temperature. When the temperature increases, the rate of a reaction
increases too because the rate constant increases. The rate constant k is only a constant for a
particular temperature. Changing the temperature changes the value of k because the proportion of
molecules that have the required energy (greater than the activation energy) is increased and the
colliding particles have a greater average energy.
Half-life - the time needed for any reactant concentration to fall to half of its initial value. In
radioactive decay, the half-life is the length of time after which there is a 50% chance that an atom
will have undergone nuclear decay. It varies depending on the atom type and isotope, and is usually
determined experimentally.

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Rate-determining step - A step in a mechanism which solely determines the rate of a reaction e.g. it
might be an initiation step with a high activation energy. Catalysts can also be rate-determining
steps.
Activation energy ­ The minimum energy needed by reactant particles (molecules or ions) before
products can form. Catalysed reactions have lower activation energies.
Heterogeneous catalyst ­ a catalyst that is in a different physical state to a reactant.…read more

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During the rate of reaction experiment, read the absorbance from the meter at regular intervals, and
then use the calibration curve to convert those values into concentrations.
Gas production
When a gas is formed from a solid reacting with a solution, it can
be collected in a gas syringe.
For more accurate results you can measure the volume of gas
given off at regular time intervals throughout the reaction or
measure the time to make a fixed volume of gas.…read more