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topic 6-the rate and extent of chemical change (factors affecting the…
topic 6-the rate and extent of chemical change
rates of reaction
reactions can go at different rates
the rate of chemical reaction= how fast the reactants are changed into products
slow reaction=chemical weathering (rusting iron)
fast reaction=burning
moderate speed reaction= magnesium reacting with acid
graphs
steeper the line, the faster the rate of reaction
overtime line levels off as reactants are used up
the quickest reactions have the steepest lines and become flat in the least time
collision energy
the more collisions, the faster the reaction
particles have to collide with enough energy for the reaction to be successful (activation energy)
activation energy is needed to break bonds in reactants to start reaction
factors affecting the rates of reaction
depends on:
temperature, the concentration
of solution/
pressure
of gas,
surface area,
presence of a
catalyst
more collisions increases RofR
increasing temperature increases RofR
particles move faster
faster movement causes more frequent collisions
faster=more energy to break bonds
increasing the concentration/ pressure increases RofR
more particles in solution/small space
more frequent collisions
increasing surface area, increases RofR
breaking a solid up into smaller pieces increases surface area to volume ratio
particles around it have more to collide with- increasing frequency
catalyst increases RofR
SPEEDS UP without being USED UP
decrease the activation energy by providing an
alternative pathway
with lower activation energy
enzymes= biological catalysts
measuring rates of reaction
rate of reaction= amount of reactant used or product formed / time
precipitation and colour change (cross test)
visual change when solution goes from transparent to cloudy
put cross underneath solution and measure the time or it to disappear-the faster it goes, quicker reaction
if products are coloured and reactants are transparent vice versa, measure time taken for colour change to occur
SUBJECTIVE!!!!!!!!
change in mass
1) measuring RofR that produces gas uses mass balance
2)as the gas is produced the mass decreases
3)the faster the mass drops, the faster RofR
4)plot graph
the volume of gas produced
1)gas syringe
2)the more gas produced in a time interval, the faster RofR
two rates experiments (practical)
Mg and HCl
1) add set volume of dilute HCl in conical flask and put on mass balance
2)add magnesium ribbon and plug opening with cotton wool
3)start stopwatch and record mass on balance in regular intervals
4)plot results in table and work out mass lost at each reading, plot graph
5)repeat with more concentrated acids but keep amount of Mg and volume of acid the same
sodium thiosulfate and HCl
1) add set volume of sodium thiosulfate to conical flask
2)place conical flask on black cross and add HCl and start stopwatch
3)time how long it takes for cross to disappear
4)repeat with different concentrations
5)the higher the concentration the quicker the reaction/ less time for cross to disappear
finding reaction rates from graphs
mean rate= change in y/ change in x
tangent
gradient=change in y/ change in x
to find that rate of reaction at a particular point
amount of product formed/ reactant used up
reversible reactions
reversible reactions will reach
equilibrium
as reactants react, their concentrations fall- forward reaction slows down
But as more products are made, their concentration rises- backward reaction will speed up
after a while for the forward reaction will go at the exact same rate as the backward reaction- system is at equilibrium
at equilibrium both reactions occur but
no overall effect
-concentrations of reactants and products are balanced and wont change
only happens in
closed system
-no products/reactants can escape
position of equilibrium
if equilibrium is on the right, concentration of products is greater
if equilibrium is on left, the concentration of reactants is greater
position depends on: temp, pressure, concentration
endo or exo
exo in one direction, endo in other
energy transferred from surroundings by endo = energy transferred to surroundings by exo
hydrated copper sulfate <==>anhydrous copper sulfate+ water
forward= endo, backward = exo
when heated hcs produces white acs-endo
add water to white powder yo get blue crystals again-exo
le chatelier's principle
=idea that if you
change conditions
of a reversible reaction at equilibrium, the system will try to
counteract
the change
temp changes
if temp is decreased the equilibrium will move in exo direction- more products for exo direction and fewer for endo
If temp is increased the equilibrium will move to endo- more products for endo and less products for exo reaction
pressure change
only involves gases
if you increase pressure, equilibrium will try to reduce it- shifts to where there are fewer molecules of gas
if you decrease pressure the equilibrium will try to increase it- shifts to where there are more molecules of gas
concentration change
if you change the conc of either products or reactants system is
no longer
at equilibrium
increase conc of reactants system will try to decrease it- makes more products
if you decrease conc of products then system tries to increase it- reducing amount of reactants