The rate and extent of chemical change
Rates of reaction
Rate of a chemical reaction- how fast reactants are changed into products.
Slowest reaction- rusting of iron. Other slow- chemical weathering (acid rain)
Moderate speed- metal magnesium reacting with an acid to produce gentle stream of bubbles.
Fast reaction- burning; explosions are even faster- release a lot of gas in a fraction of a second.
The steeper the line is n a graph, the faster the rate of reaction. The line becomes less steep when reactants are used up. The quickest reactions have steepest lines and become flat in the least time.
The more collisions there are, the faster the reaction is; the higher the frequency, the higher the rate of reaction. Also, the particles have to collide with enough energy for collision to be successful.
Activation energy- minimum amount of energy that particles need to react. Particles need this energy to break the bonds in the reactants and start the reaction.
Factors that increase the number of collisions or the amount of energy particles collide with each other, will increase the rate of reaction.
Factors affecting rates of reaction
Rate of reaction depends on:
Temperature.
Concentration of a solution or pressure of a gas.
Surface area.
Pressure of a catalyst.
More collisions increase the rate of reaction.
When temperature is increased, particles move faster. If move faster, 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.
If a solution is more concentrated, more particles move about in the same volume of water (or other solvent). When the pressure of a gas is increased, the same number of particles occupies the smaller space. This make collisions more frequent.
If one reactant is a solid, breaking into smaller pieces increases surface area to volume ratio. Therefore, the particles will have more area to work on for the same volume of the solid- collisions will be more frequent.
Catalyst- substance that speeds up a reaction without being used up; it is not a part of the overall reaction equation. Different catalysts needed for different reactions, but they all decrease the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy. Enzymes- biological catalysts that catalyse reactions in living things.
Measuring rates of reaction
When a product or reactant is a gas, usually measure amount in cm3.
If solid- grams.
Time- seconds.
So units for rate of reaction is cm3/s or g/s.
You can also measure amount of product or reactant in moles- unit of rate is mol/s.
There are 3 ways to measure rate of reaction:
1) Precipitation and colour change.
2) Change in mass (usually gas given off).
3) Volume of gas given off.
Finding reaction rates from a graph
Rate of a reaction graph shows amount of product formed or amount of reactant used up on the y-axis and time on x-axis.
Mean rate of whole reaction ➡ overall change in y-value / time taken for reaction
On the graph, mean rate ➡ change in y / change in x
You can also draw a tangent and find gradient.
Reversible reactions
As reactants react, their concentrations fall- forward reaction slows down. But as more products made, their concentrations rise, backwards reaction will speed up. After a while, the forward reaction will be going exactly the same rate as the backward one- system is at equilibrium.
At equilibrium, both reaction are still happening, but no overall effect (dynamic equilibrium). This means concentrations of reactants and products have reached a balance and won't change.
Equilibrium only reached if reversible reaction takes place in a 'closed system'. This means no reactants or products are able to escape and nothing can get in.
When a reaction is at equilibrium, it doesn't mean amounts of reactants and products are equal.
If equilibrium lies to the right, concentration of products is greater than reactants.
If equilibrium lies to the left, concentration of reactant is greater than products.
Position of equilibrium depends on conditions as well as reaction itself:
-Temperature.
-Pressure (only if equilibria involves gases).
-Concentration of reactants and products.
In reversible reactions, if reaction is endothermic in one direction, exothermic in the other. The energy transferred from surroundings by endothermic reaction is equal to energy transferred to surroundings during exothermic reaction.
Le Chatelier's principle
Le Chatelier's principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract change- can be used to predict the effect of any changes you make to a reaction system.
Temperature- all reactions are exothermic in one direction and endothermic in the other.
If decrease temperature, equilibrium will move in exothermic direction to produce more heat- get more products for the exothermic reaction and fewer products for the endothermic reaction.
If increase temperature, equilibrium will move in endothermic direction to decrease temperature- get more products for endothermic reaction and fewer for exothermic.
Pressure (only affects equilibrium if gas involved).
If increase pressure, equilibrium tries to reduce it- moves to direction with fewer molecules of gas.
If decrease pressure, equilibrium tries to increase it- moves in direction with more molecules of gas. You can use balanced symbol equation for a reaction to find which side has more molecules of gas.
Concentration- If you change concentration of either the reactants or products, system will no longer be at equilibrium. So, the system responds to bring itself back to equilibrium again.
If increase concentrations of reactants, system tries to decrease it be making more products.
If decrease concentration of products, system tries to increase it again by reducing amount of reactants.