What is a rate of reaction?
The rate of a reaction is a measure of how quickly a A substance that reacts together with another substance to form products during a chemical reaction. is used up, or a A substance formed in a chemical reaction. is formed.
Collision theory
For a chemical reaction to happen:
- reactant A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. must collide with each other
- the particles must have enough energy for them to react
A collision that produces a reaction is called a A collision between reactant particles that has enough energy for a reaction to happen.. The The minimum amount of energy that colliding particles must have for them to react. is the minimum amount of 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. needed for a collision to be successful. This is different for different reasons.
Image caption, 1. Two pairs of particles move towards each other
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Measuring rates of reaction
There are different ways to determine the rate of a reaction. The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.
In addition, how long a reaction is observed for depends on the rate of reaction. Reactions can vary from being almost instantaneous to taking years to complete. In the lab, reactions are usually followed over a few seconds or minutes.
Question
Rusting is a slow reaction. Give four examples of a very fast reaction.
Combustion, explosions, neutralisation reactions and precipitation reactions are very fast reactions.
Measuring mass
The change in The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a reactant or product can be followed during a reaction. This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small The sum of the relative atomic masses of the atoms in a chemical formula., Mr. The units for rate are usually g/s or g/m.
Measuring volume
The change in 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 reactant or product can be followed during a reaction. This method is used when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down Long glass tube with a tap and marked with volume measurements, used in titrations. or measuring cylinder. The units for rate are usually cm3/s or cm3/min.
Graphs
The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.
The Another word for steepness. On a graph, the gradient is defined as being the change in the 'y' value divided by the change in the 'x' value. It defines how steep a line is. of the line is equal to the rate of reaction:
- the steeper the line, the greater the rate of reaction
- fast reactions finish sooner (when the line becomes horizontal) than slow reactions