Rate of reaction: change in concentration of a reactant or product in a given time. Unit: mol dm⁻³ s⁻¹.
Average rate: total change in concentration / total time. Rate at one moment: gradient of the tangent to the concentration-time curve.
Collision theory: particles must collide with enough energy (at least the activation energy) and the correct orientation for a reaction.
Activation energy: the minimum energy colliding particles need to react. Activated complex (transition state): the temporary high-energy state formed in an effective collision.
Catalyst: speeds up a reaction without being used up, by giving an alternative pathway with lower activation energy. It does not change the overall energy change or the products, and cannot make an impossible reaction occur. In a reversible reaction it lowers the activation energy of both directions. Enzymes: biological catalysts (proteins), each works on one reaction.
Must-know points
As a reaction proceeds, reactants fall and products rise, and the rate falls (curve gets flatter). A steep slope means a fast reaction. A flat line means the reaction has stopped (reactants used up or equilibrium reached).
Factors with collision theory: higher concentration (more particles per unit volume, more collisions); higher pressure for gases (same reason); larger surface area of a solid (more exposed particles, powdered zinc reacts faster than a lump); higher temperature (faster particles, more frequent and more energetic collisions, more particles reach the activation energy, shown by the Maxwell-Boltzmann curve); catalyst (lower activation energy).
Physical changes used to follow a rate: change in mass, formation of gas (in an open flask mass falls; in a closed vessel pressure rises), temperature change. Colour of the reactants is not used here.
Fast reactions: acid-base neutralisation, explosions. Slow: fermentation, digestion, rusting.
Role in the food industry: finds the best time to harvest and transport so produce reaches the market at its best taste, texture and nutrition; identifies what causes degradation in transport; helps slow spoilage by controlling temperature and humidity, so shelf life is longer; helps design storage and transport methods that keep nutritional value.
Past-paper questions with model answers
Role of chemical kinetics in the food industry, any four aspects (D, Sec C 1.5 x 4): use the four bullets above, a sentence each.
Rate of reaction meaning and how it is measured (B, Sec C): rate is change in concentration of a reactant or product per unit time. Measure by following the concentration of a product at regular times, or the volume or mass of gas given off, or the mass lost.
Graph question, rate between 5 s and 20 s (B): rate = (concentration at 20 s − concentration at 5 s) / (20 − 5). Read both values from the graph and divide by 15 s. Unit mol dm⁻³ s⁻¹.
Horizontal line at the end of the graph (B, M): the reaction has stopped (all the limiting reactant is used up, or equilibrium has been reached), so there is no further change in concentration.
Two reactions A and B, which is faster (M): the one with the steeper line, shown by calculating rate = Δconcentration / Δtime for each from the graph; the larger value is the faster reaction.