Measuring Rates of Reaction — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions

Measuring rates follow a change over time – volume of gas (gas syringe or upturned measuring cylinder), mass of the flask as gas escapes, or time for a precipitate to hide a cross. The faster the change, the higher the rate.

On a graph of product against time, the curve is steepest at the start (fastest rate), then levels off when a reactant is used up. A faster reaction has a steeper initial gradient but, with the same amounts of reactants, reaches the same final amount.

Explaining rates reactions happen when particles collide with at least the activation energy. Anything that increases the frequency of successful collisions increases the rate.

Higher concentration or pressure puts more particles in the same volume.

Activation energy is the energy barrier on an energy level diagram, from the reactants up to the peak. Reactions with high E_a are slow at room temperature.

Catalysts provide an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful. They are not used up, are chemically unchanged at the end and are needed only in small amounts. Enzymes are biological catalysts.

marble chips of different sizes with acid; keep mass of carbonate, acid volume, concentration and temperature constant. Powder gives the fastest mass loss.

add different solids (e.g. MnO₂, CuO, PbO₂) of equal mass to hydrogen peroxide and measure oxygen volume in a gas syringe over time. The best catalyst gives the steepest initial gradient; filter, dry and reweigh to show the catalyst is unchanged.

time how long a cross under a flask takes to disappear when dilute HCl is added to sodium thiosulfate solution (a cloudy sulfur precipitate forms). Dilute the thiosulfate with water to change concentration, keeping total volume constant. A shorter time means a faster rate.

warm the acid to set temperatures in a water bath, add a fixed length of magnesium ribbon and time how long it takes to disappear. Keep acid volume, concentration and magnesium length the same. Higher temperatures give shorter times.

Comparing curves for the same amounts of reactants, a faster reaction (higher concentration, pressure, temperature or surface area) gives a steeper start, levels off sooner and finishes at the same final amount of product.

Rule of thumb for many reactions in solution or between gases, a rise of about 10 °C roughly doubles the rate, because a much larger fraction of collisions now has energy of at least E_a.

Particle size cutting a solid into smaller pieces keeps its volume but increases the exposed surface. A 3 cm cube has 54 cm² of surface; cut into 27 cubes of side 1 cm it has 162 cm², so collisions with the other reactant are more frequent.

Collision frequency doubling the concentration (or the surface area) roughly doubles the number of collisions per second. Raising gas pressure puts the same number of particles into a smaller volume.

Reaction profiles show energy as the reaction proceeds. The rise from reactants to the peak is E_a; the gap between reactants and products is ΔH. Products lower than reactants: exothermic, ΔH negative. Products higher: endothermic, ΔH positive.

Catalysts in industry a catalyst is written above the arrow, not as a reactant, e.g. iron in the Haber process. Catalysts raise production rate and cut energy costs because lower temperatures can be used. Transition metals and their compounds are common catalysts.