Rates, concentration and pressure
The greater the The total number of times an event occurs. of A collision between reactant particles that has enough energy for a reaction to happen., the greater the rate of reaction. If the The concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. of a reacting Mixture formed by a solute and a solvent. or the Force exerted over an area. The greater the pressure, the greater the force exerted over the same area. of a reacting gas is increased:
- the A substance that reacts together with another substance to form products during a chemical reaction.A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. become more crowded
- the frequency of collisions between reactant particles increases
- the rate of reaction increases
Note that the mean The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.
Graphs
The rates of two or more reactions can be compared using a graph of The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). or The volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml. of A substance formed in a chemical reaction. formed against time. The graph shows this for two reactions.
The curved red and green lines are the reaction data. The straight dashed lines at the start of the two reactions are called A straight line that just touches a point on a curve. A tangent to a circle is perpendicular to the radius which meets the tangent.. They follow the curve at the beginning of the reaction. The steeper the tangent, the faster the reaction.
Tangents can also be drawn at other points on the graph. As the reaction carries on, the steepness of the tangent to the curve decreases (eg the blue dashed line) so the reaction is slowing down.
Calculating the rate of reaction from the gradient of a tangent
To calculate the rate at the start of a reaction, you need to find the mathematical gradient (steepness) of the gradient. Use this equation:
\(rate~of~reaction = \frac{change~in~mass~or~volume~of~product}{change~in~time}\)
For example, using the green tangent below:
\(Rate\ of\ reaction\ =\ \frac{60\ cm^{3}}{5\ s}\)
Rate of reaction = 12 cm3/s - 1