Rate and extent of Chemical Change
Rates of Reaction
Rate of reaction is how fast the reactants are changed into products
The rusting of iron, and chemical weathering have very slow rates of reaction
Magnesium reacting with acid to produce bubbles has a moderate rate of reaction
Burning has a fast rate of reaction, but explosions are faster and release lots of gas
Collision theory
Particles must collide with enough energy in order to react
Rate of reaction depends on the collision frequency of reacting particles - the more collisions the faster the rate of the reaction
Activation energy is the minimum amount of energy needed for the particles to react
The higher the energy of the particles the faster the rate of reaction
Factors affecting rate of reaction
Temperature
Increasing temperature increases rate of reaction
Particles have more energy when increased temperature so increased movement so move faster
Faster = more collisions so more collisions provide the minimum energy (activation energy) needed to start the reaction
Therefore faster rate of reaction with higher temperature
Concentration of solution or pressure of a gas
If solution is more concentrated there are more particles in the same volume, and if pressure is increased in a gas then more particles in same space
More frequent collisions so faster rate of reaction
Surface area
If one reactant is a solid, then breaking it up increases surface area
Same volume of the solid, but particles around it have higher area to collide with, so more collisions so higher rate of reaction
Catalyst
Substance that speeds up a reaction without being used up itself
They all decrease the activation energy needed, so less energy is needed to start the reaction
Do this by providing an alternative reaction pathway
Reversible reactions
Equilibrium
As the reactants react, their concentrations will fall, so the rate of the forward reaction will slow down, but as more products made their concentrations rise, so the rate of the backwards reaction will increase
After a while the rate of forward and backwards reactions will reach equilibrium - they will go at the same rate
At equilibrium both reactions are still happening but there is no overall affect - the concentrations of reactants and products have reached a balance and won't change
Equilibrium can only be reached if the reversible reaction takes place in a closed system - none of the reactants or products can escape, and nothing can get in
Position of equilibrium
When in equilibrium amounts of reactants and products don't have to be equal
If equilibrium lies to the left, then concentration of reactants is greater than that of products
If equilibrium lies to the right concentration of reactants is greater than that of products
Position of equilibrium depends on
Temperature
Pressure (only in gases)
Concentration of the reactants and products
Endothermic or exothermic
In reversible reactions, if the reaction is endothermic in one direction it will be exothermic in another
The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
Eg. hydrates copper sulfate --> anhydrous copper sulfate + water (thermal decomposition)
Forward reaction is endothermic
If blue hydrates copper (II) sulfate crystals are headed it drives the water off and leaves copper (II) sulfate - endothermic
If you then add a few drops of water to the white powder blue crystals are formed - exothermic
Le Chatelier's Principal
If you change the conditions of a reversible reaction at equilibrium the system will try to counteract that change
Changes to temperature
If you decrease temperature the equilibrium will move in the exothermic direction to produce more heat - more products for the exothermic reaction and fewer for the endothermic reaction
If you raise the temperature the equilibrium will move in the endothermic direction to try and decrease it - more products for endothermic reaction and fewer for exothermic
Changes to pressure
Only affects gases
If pressure is increased the equilibrium tries to reduce it - move sin the direction with fewer gas molecules
If pressure is decreased the equilibrium tries to increase it - moves in direction where more molecules of gas
Need a balanced equation
Changes to concentration
If concentration of products or reactants is changed, the system will no longer be in equilibrium
If increase concentration of reactants system tries to decrease it by making more products
Decrease concentration of products the system tries to increase it by reducing amount of reactants