What Is the Difference Between Reaction Time & Reaction Rate?

Reaction Rate

• Chemicals with an affinity for each other will change when brought together. An ice cube and a pinch of salt turn into a saltwater solution. Hydrogen and oxygen, with help from a spark, explode into light, give off heat and turn into water. Chemists call these changes reactions; atoms and molecules rearrange themselves in new ways, forming new products. The speed at which that new product (which may look nothing like its ingredients) comes into the world is called the reaction rate.

Reaction Time

• Imagine a car traveling a steady 50 mph. In one hour it travels 50 miles, in two hours 100 miles, and so forth, propelled by the reaction of gasoline and air in the presence of a spark. The reaction rate doesn't change, a steady foot on the gas; the reaction product, miles traveled, increases steadily and constantly. Then the gas runs out and the car stops. The reaction time tells us how long it took the substances being combined to complete their changeover. Gasoline and air became heat, mechanical energy and air pollution. The reaction rate is the given, knowing how much air and fuel we started with. Chemists also use reaction time and rate to reason backwards, if the starting product amounts are unknown. If our car gets 20 miles to a gallon (reaction rate) and stopped moving after four hours at 50 miles an hour (200 miles, reaction time), it must have started with 10 gallons of gasoline in the tank.

Changes

• Scientists look for ways to speed up the reaction rate so they can reduce the reaction time. They increase the solvent, the substance doing the dissolving, while keeping constant the solute, the substance that the scientist is trying to dissolve. More molecules of solvent collide with the same molecules of solute, and a faster reaction rate and a shortened reaction time are achieved.

  Alternatively, the scientists grind down a solid block of solute into powder, maximizing the surface area for solvent molecules to attack. Or the solvent plus solute, the solution, is heated to move molecules faster so they'll collide more frequently. What scientists do at the workbench, manufacturers scale up on the factory floor. Time is money, and it's more efficient to smash and pound and heat a product than wait for nature to take its course, assuming the cost of the extra energy doesn't cancel out the gain in speed.

Math

• Scientists don't just measure reaction rates by how quickly the product is made. They also measure how fast the concentration of each ingredient is used up. As the ingredient is used up, its concentration, measured in moles (the quantity of the ingredient's molecules) in liters per second, goes down. The reaction rate declines, unless (as on the workbench or in the factory) the ingredient is constantly topped off.

  However, not all ingredients --- the reactants --- are created equal. Sometimes the concentration of one or the other has to be altered to make a noticeable difference in the reaction rate; sometimes, both. Sometimes scientists must double that concentration and sometimes square it. These are called the "orders" of reaction, and they depend on the role each ingredient plays in the reaction, assuming we keep the temperature and other factors constant.