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Topic 6 The Rate And Extent of Chemical Change By Bethan Poole (6.2…
Topic 6 The Rate And Extent of Chemical Change
By Bethan Poole
6.1 Rate of Reaction
6.1.1 Calculating rates of reaction
Quantity of reactant/product measured in g or cm3
Rate of reaction measured in g/s or cm3/s or mol/s
Rate of reactions using tangents
Difference in y/ difference in x =gradient
Measuring the amount of products formed
if one product is a gas measure the total volume of gas produced in cubic cm with a gas syringe and the time it takes for the reaction to happen
Measuring the time it takes for a reaction mixture to become opaque or change colour
Time how long it takes for the mixture to change colour
Rate of reaction= 1/ time taken for the solution to change colour
6.1.2 Factors which affect the rates of chemical reactions
Pressure/Concentration
If increased
The particles become more crowded
There is a greater chance of the particles colliding
The rate of reaction increases
Surface area
The bigger the surface area the more the particles are exposed to the other reactant, therefore more successful collisions
The rate of reaction increases
Temperature
If increased
Reactant particles move quicker
They have more enrgy
The particles collide more often
The rate of reaction increases
Presence of Catalysts
6.1.3 Collision theory and activation energy
Chemical reactions can only happen if the reactant particles collide with enough energy.
The more frequently the particles collide, the greater the proportion of collisions with enough energy so the higher the rate of reaction
The minimum amount of energy needed for the reaction to happen is called the activation energy
6.1.4 Catalysts
Work by:
Make collisions more successful
Speed up the reaction
Provide a larger surface for the molecules to attach to, which increases the chances of bumping into each other
Reduce energy needed for a successful collision
They are not included in the chemical equation for the reaction
They help to reduce costs by speeding up reactions
Examples
Catalyst:
VANADIUM OXIDE
Reaction catalysed: Making sulphuric acid
Catalyst:
PLATINUM
Reaction catalysed: Making nitric acid from ammonia
Catalyst:
IRON
Reaction catalysed: Making ammonia from nitrogen and hydrogen
Increases the rate of reaction by providing a different pathway that has a lower activation energy
6.2 Reversible reactions and dynamic equilibrium
6.2.4 The effect of changing conditions on equilibrium
Le Chatelier's Principle
states that if a system in equilibrium is subject to a change in conditions, then the system shifts to resist the change
They help (with rate of reactions) to determine optimum conditions industrial processes (Haber Process etc.)
6.2.5 The effect of changing concentration
The system will no longer be in equilibrium
So will adjust so it can reach equilibrium once more
If the concentration of one of the reactants is increased then:
More products will be formed to reach equilibrium
If the concentration of one of the reactants is decreased then:
The position of equilibrium shifts so more reactants are made to reach equilibrium
6.2.3 Equilibrium
Happens when a reversible reaction occurs in apparatus which prevents escape of reactants or products
Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate
6.2.6 The effect of temperature changes on equilibrium
Endothermic reaction
Increase Temp.
Make more products (the yield increases)
Decrease Temp.
Makes less products (the yield decreases)
Exothermic reaction
Decrease Temp.
Makes more products (the yield decreases)
Increase Temp.
Makes less products (the yield decreases)
Increase
Increase in temp.
favours endothermic reaction
Decrease in temp.
favours exothermic reaction
6.2.2 Energy changes and reversible reactions
If a reaction is exothermic in one direction it is endothermic in the other.
the same amount of energy is transferred in each case
e.g
Hydrated copper sulfate (blue)
anhydrous copper sulfate (white) + water
The forward reaction is endothermic and the backwards reaction is exothermic in this case
6.2.7 The effect of pressure change on equilibrium
For gaseous reactions at equilibrium:
Increase of pressure
Favours the side of the reaction which produces the least number molecules
Decrease in pressure
Favours the side of the reaction that produces a greater number of molecules
Nitrogen + Hydrogen
Ammonia
N2 + 3H2
2NH3
6.2.1 Reversible Reactions
Some reactions are reversible
Means the reactants will make the products and the products will make the reactants
The direction of reversible reactions can be changed by changing the conditions
e.g.
Ammonium chloride
Ammonia + Hydrogen Chloride
NH4Cl
NH3 + HCl
Heating will favour the will the forwards reaction as it is endothermic as it takes in heat
Cooling will favour the backwards reaction because it is exothermic as it gives out heat
Symbol for reversible reactions
