Kinetics definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

25 terms

Collision theory, activation energy and the rate of reaction

Rate of reaction: The change in the concentration of a reactant or a product per unit time, measured in mol dm⁻³ s⁻¹.
Initial rate: The rate of reaction at the start, when t = 0, found from the gradient of the tangent to a concentration–time graph at t = 0.
Activation energy: The minimum energy that colliding particles must have for a reaction to take place.
Successful collision: A collision between particles that have at least the activation energy and are in the correct orientation, so that it leads to a reaction.
Collision frequency: The number of collisions between reactant particles per unit time in a given volume.
Transition state: The highest-energy arrangement of atoms on the route from reactants to products, in which bonds are partly broken and partly formed.
Enthalpy profile: A diagram of enthalpy against the progress of a reaction, showing the reactants, the transition state at the peak and the products, from which Ea and ΔH are read.
Mean rate: The total change in the amount or concentration of a substance divided by the time interval over which it changed.
Tangent: A straight line that touches a curve at one point without crossing it there; its gradient is the rate at that moment.

Maxwell–Boltzmann distribution: The distribution of the energies of the molecules in a gas at a given temperature, shown as the number of molecules with each energy.
Most probable energy: The energy at the peak of the Maxwell–Boltzmann distribution, the energy possessed by the greatest number of molecules.
Mean energy: The average energy of the molecules, which lies to the right of the most probable energy because the distribution is not symmetrical.
Catalyst: A substance that increases the rate of a reaction without being changed in chemical composition or amount, by providing an alternative reaction route with a lower activation energy.
Heterogeneous catalyst: A catalyst in a different phase from the reactants, such as solid iron in the Haber process.
Homogeneous catalyst: A catalyst in the same phase as the reactants, such as iodide ions dissolved with hydrogen peroxide.

Continuous monitoring: Following a single reaction mixture by reading a property that changes, at intervals, through the whole reaction.
End point: The moment at which a fixed, observable change has taken place, such as a cross disappearing, when the timing is stopped.
Independent variable: The variable that is deliberately changed from one run to the next.
Dependent variable: The variable that is measured in each run to see the effect of the change.
Control variable: A variable kept the same in every run so that it cannot affect the result.
Calibration curve: A graph of a measured property, such as absorbance, against known concentrations of a substance, used to convert a reading into a concentration.
Resolution: The smallest change in a quantity that an instrument can show, such as 0.01 g on a balance reading to two decimal places.
Random error: An error that makes repeated readings scatter unpredictably on both sides of the true value; its effect is reduced by repeating the measurement and taking a mean.
Systematic error: An error that shifts every reading from the true value in the same direction by a similar amount; repeating the measurement neither reveals it nor reduces it.
Percentage uncertainty: The uncertainty in a measurement divided by the measured value and multiplied by 100.