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The Rate and Extent of Chemical Change (Factors Affecting Rates of…
The Rate and Extent of Chemical Change
the rate of chemical reaction is how fast the reactants are changed into products
one of the
slowest
is the rusting of iron
other slow reactions include chemical weathering
an example of a
moderate speed
reaction is the metal magnesium reacting with an acid to produce a gentle steam of bubbles
burning is a fast reaction, but explosions are even faster and release a lot of gas, explosive reactions are over in a fraction of a second
RATE OF REACTION = AMOUNT OF REACTANT USED OR AMOUNT OF PRODUCT FORMED / TIME
the steeper the line on the graph, the faster the rate of reaction
over time the line becomes less steep as the reactants are used up
the quickest reactions have the steepest lines and become flat in the least time
The rate of a chemical reaction depends on:
the
collision frequency
of reacting particles. The more collisions there are the faster the reaction is
the
energy transferred
during the collision. Particles have to collide with enough energy for a successful collision
the minimum amount of energy that particles need to react is called the activation energy. Particles need this much energy to break the bonds in the reactants to start the reaction
factors that increase the number of collisions or the amount of energy particles collide with will increase the rate of reaction
Factors Affecting Rates of Reaction
Temperature
when the temperature is increase, the particles all move faster
if they're moving faster, they collide more frequently
the faster they move the more energy they have, so more of the collisions will have enough energy to make the reaction happen
The
concentration
of a solution or
pressure
of gas
if solution is made more concentrated, it means more particles knocking about in the same volume a solvent = more collisions
similarly, when the pressure of a gas is increased, it means that the same number of particles occupies a smaller space= more collisions
Surface area
if one of the reactants is a solid, then breaking it up into smaller pieces will increase its surface area to volume ratio
this means for the same volume of solid, the particles around it will have more area to work on, so there will be collisions more frequently
Catalyst
a substance that speeds up a reaction, without being used up in the reaction itself
different catalysts are needed for different reactions, but they all work by decreasing the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with with a lower activation energy
enzymes are biological catalysts= they catalyse in living things
Calculating the mean reaction rate from a graph
1) a rate of reaction graph shows the amount of product formed or amount of reactant used up on the y-axis and time on the x-axis
2) to find the mean rate for the whole reaction, work out the overall change in the y-value and divide this by the total time taken for the reaction
if asked to find mean rate for whole reaction remember the reaction finishes as soon as the line on the graph goes flat
Draw a tangent to find the reaction rate at a particular point
