Collision theory.

Step 1 of 6

Molecules have to meet

Some reactions are over in a split second, like an explosion, and some take years, like rusting. The rate of reaction is how much reactant turns into product each second. To see what sets it, look at the molecules.

The flask holds molecules of nitrogen monoxide, $\mathrm{NO}$⁠, and of ozone, $\mathrm{O_3}$⁠, which react by $\mathrm{NO + O_3 \rightarrow NO_2 + O_2}$⁠. That is how nitrogen oxides destroy ozone in the stratosphere. Each flash is one reaction. Watch how many collisions there are, and how many of them end in a flash.

Step 2 of 6

A more crowded flask

Concentration sets how many reacting molecules the flask holds. The more molecules there are, the more often they meet. But by how much?

Here Temperature is fixed at 298 K, room temperature.

Step 3 of 6

Thinning it out

The rate depends on both reactants together, so it goes as the square of Concentration: $\text{rate} = k[\mathrm{NO}][\mathrm{O_3}]$$\text{rate} =$$k[\mathrm{NO}][\mathrm{O_3}]$⁠, and both concentrations move with the slider. Use that the other way round.

Step 4 of 6

Turning up the heat

Heating speeds up almost every reaction. A molecule’s mean speed grows as the square root of the temperature in kelvin, $\sqrt{T}$⁠, so from 298 K to 398 K the molecules are only $16\%$⁠ faster. Concentration is now fixed. Predict before you heat.

Step 5 of 6

The energetic few

Not every molecule has the same energy: at any temperature there are a few very slow ones, many middling ones and a few very fast ones. The chart shows that spread, and a line marks the activation energy, $E_a$⁠, the least energy a collision needs to react. The shaded area is the molecules beyond the line. Highlight energetic molecules is now On, so those molecules glow in the flask.

Step 6 of 6

Why heating works

When you heat, the whole curve shifts right and flattens. The $E_a$⁠ line stays where it is. Compare the chart at 398 K with the chart at 298 K.