- Nature and concentration of the catalyst.
- Temperature: as the kinetic energy increases, the number of collisions increases, thus increasing the reaction rate
Heterogeneous reactions occur between reactants that are in different states. Factors that affect the reaction rate are the same as homogeneous with the addition of:
- Surface area to volume ratio: breaking up a solid increases the surface area which increases the amount of collision, therefore reaction rate increases.
- Stirring: stirring keeps the solid suspended in the solution so that the solid's maximum surface area is exposed. Stirring increases the rate of collision between reactant particles and so increases the rate of reaction.
3. There is a risk of explosion when several elements are present:
- an oxidant: the oxygen in air, for example
- a fuel: either gas or vapors (hydrocarbons, solvents, varnishes, dilutants, gas, alcohol, dyes, perfumes, chemical products or reagents for manufacture of plastics) or powders and dust (magnesium, aluminum, sulfur, cellulose, cereals, carbon, wood, milk, resins, sugars, starch, polystyrenes or fertilizer).
- a source of combustion: hot spot, flame or spark
For example, when filling a grain silo, the concentration of dust is very high. The environment then is dangerous: a rise in temperature, or even a spark, can trigger an explosion. The more surface area on which collisions can occur, the faster the reaction. You can hold a burning match to a large chunk of coal and nothing will happen. But if you take that same piece of coal, grind it up very, very fine, throw it up into the air, and strike a match, you’ll get an explosion because of the increased surface area of the coal.
4. Safety measures to prevent explosions include controlling the dust, eliminating ignition sources, and limiting the effects of explosions to minimize injuries. Initial preventative steps would be to contain combustible dust to areas that are properly designed and located, with ignition sources either eliminated or controlled. Equipment or spaces such as ducts, dust collectors, vessels, and processing equipment that contain combustible dust should be designed in a manner to prevent leaks to minimize the escape of dust into work areas. Any dust that settles on workplace surfaces should be removed through a routinely implemented housekeeping program. Areas or equipment potentially subject to explosions, including the dust collection system, should also be designed to relieve pressure in a safe manner, or be provided with proper suppression, explosion prevention systems, or an oxygen-deficient atmosphere.