Rates of Reaction
What Is Rate of Reaction?
The rate of reaction tells us how quickly a reaction happens — how fast the reactants are used up or how fast the products are formed. A fast reaction (like an explosion) is over in a fraction of a second. A slow reaction (like rusting or the weathering of stone) may take months or years. Being able to control the rate of a reaction is vital in industry — too fast can be dangerous, too slow is uneconomical.
You can measure rate in several ways: the volume of gas produced per unit time, the mass of reactant lost per unit time (using a balance), or the time taken for a colour change or for a solution to become opaque (disappearing cross experiment). The choice of method depends on the particular reaction and what can be conveniently measured.
Collision Theory
Collision theory states that for a reaction to occur, the reacting particles must: (1) collide with each other, (2) collide with sufficient energy (at least the activation energy), and (3) collide with the correct orientation. The rate of reaction depends on the frequency of successful collisions — collisions that have enough energy and the right orientation to result in a reaction. Not every collision leads to a reaction; most collisions are unsuccessful because the particles bounce off each other without reacting.
For a reaction to happen, particles must collide with enough energy (the activation energy) and the correct orientation. Increasing the frequency of successful collisions increases the rate of reaction.
Factors Affecting Rate
Four main factors affect the rate of a chemical reaction:
Temperature — increasing the temperature gives particles more kinetic energy, so they move faster and collide more frequently. More importantly, a greater proportion of collisions have energy equal to or exceeding the activation energy, so the rate increases significantly.
Concentration (or pressure for gases) — increasing the concentration means there are more particles in a given volume, so they collide more frequently. For gases, increasing the pressure pushes particles closer together, having the same effect.
Surface area — breaking a solid into smaller pieces increases the surface area to volume ratio. More of the solid is exposed to the other reactant, so collisions happen more frequently and the rate increases. This is why powdered marble chips react faster with acid than large lumps, even though the total mass of marble is the same. In the extreme case, a fine powder of a combustible material in air can cause a dust explosion because the surface area is so enormous that the reaction rate is dangerously fast.
Interpreting Rate of Reaction Graphs
Rate graphs typically plot volume of gas produced (or mass lost) on the y-axis against time on the x-axis. The gradient (steepness) of the line shows the rate — a steeper gradient means a faster reaction. The line levels off when one of the reactants is used up (the reaction is complete). The final amount of product is the same regardless of rate (unless the amount of reactant changes). Two curves on the same graph — one steep and one gradual — that level off at the same point indicate the same amount of reactant was used but one reaction was faster than the other.
You can also use \(\frac{1}{t}\) as a measure of rate, where \(t\) is the time taken for a certain amount of reaction to occur. A shorter time means a faster rate, so \(\frac{1}{t}\) is proportional to rate.
Catalysts
A catalyst is a substance that increases the rate of a reaction without being used up itself. It works by providing an alternative reaction pathway with a lower activation energy, so more particles have enough energy to react. On a reaction profile, the catalyst lowers the energy barrier (the peak). The catalyst itself is unchanged at the end of the reaction and can be reused. Each catalyst is specific to a particular reaction or type of reaction — an iron catalyst works for the Haber process but not for decomposing hydrogen peroxide (which uses manganese dioxide).
Catalysts speed up reactions by lowering the activation energy. They are not used up and are not changed permanently. Different reactions need different catalysts. Enzymes are biological catalysts that control reactions in living organisms.
Gas syringe method: collect gas produced in a gas syringe and record the volume at regular time intervals. Works well for reactions producing \(CO_2\) or \(H_2\) (e.g. marble chips and acid, magnesium and acid). Mass loss method: place the reaction on a balance and record the mass at regular intervals as gas escapes. Disappearing cross: time how long it takes for a cross viewed through the solution to disappear as the solution becomes opaque (sodium thiosulfate and acid producing sulfur). In all methods, change only one variable at a time and keep everything else constant.