Concentration of the Reactants

Why more concentration means more rate. Collision theory (Section 7.7) already supplies the explanation: packing more reactant particles into the same volume raises the CHANCE that any two of them collide in a given second, so both the collision frequency and the reaction rate rise together with concentration.

A lab demonstration. Three conical flasks (A, B, C) are each made up to the same 50 ml total volume, but with increasing amounts of 0.1 M sodium thiosulphate (10, 20, 40 ml respectively, topped up with water). Adding 10 ml of 1 M HCl to each flask starts the reaction $Na_2S_2O_3+2HCl\rightarrow2NaCl+H_2O+SO_2+S\!\downarrow$, whose colloidal sulphur product gradually clouds the solution. Timing how long a cross mark, viewed from directly above the flask, takes to become invisible through the clouding solution gives a value of t for each flask -- and t is SHORTEST for flask C (highest thiosulphate concentration) and LONGEST for flask A (lowest concentration). Since $1/t$ is a direct measure of rate (shorter time = faster clouding = faster rate …