Rates and surface area to volume ratio
Dividing lumps
For a given The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a solid, large lumps have smaller The total area of an object, for example the area of a skydiver's body facing the air as they fall, or the area of a car tyre that touches the road. to 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. ratios than smaller lumps or powders. If a large lump is divided or ground into a powder:
- its total volume stays the same
- the area of exposed surface increases
- the surface area to volume ratio increases
Lumps vs powders
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 surface area to volume ratio of a reacting solid is increased:
- more 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. are exposed at the surface
- 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 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 faster reaction with the powder:
- gives a steeper line
- finishes sooner
Question
Icing sugar is made from a fine powder, whereas granulated sugar is made from much larger crystals. State and explain which will burn more quickly in air.
Icing sugar will burn in air more quickly than granulated sugar because it has a much larger surface area to volume ratio than granulated sugar. This means that successful collisions with the oxygen molecules in the air will be more frequent.
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