the rate & extent of chemical change (factors affecting rates of…
the rate & extent of chemical change
rates of reaction
rate -- how fast reactants --> products
slow - rusting of iron, chemical weathering (acid rain damage to limestone buildings)
moderate speed - magnesium reacting w an acid --> stream of bubbles
fast - burning, explosions
speed = amount of product formed or amount of reactant used up / time
steeper line - faster rate
over time line less steep - reactants used up
quickest - steepest + becomes flat in least time
rate depends on:
of reacting particles -- more collisions - faster - double frequency double rate
during a collision - have to collide with enough energy for collision to be successful.
- minimum amount of energy that particles need to react -- to break bonds in reactants + start reaction
a successful collision ends in the reactant particles forming products.
factors affecting rates of reaction
rate depends on:
concentration (solution) or pressure (gas)
presence of a catalyst
increase number of successful collisions between reacting particles.
- particles move faster so collide more frequently as they have more energy so more collisions will have enough energy to make the reaction happen.
- more particles per unit volume
- same number of particles occupy a smaller space
:arrow_right: collisions more frequent between reactant particles.
- solid - break into smaller pieces - increase surface area to volume ratio -- for same volume, particles have more area to work on - collisions more frequent.
- speeds up reaction without being used up (so not part of reaction equation) - decrease activation energy by providing an alternative reaction pathway with a lower activation energy.
enzymes = biological catalysts
measuring rates of reaction
rate of reaction = amount of reactant used or amount of product formed / time
^^ mean rate
rate cm^3/s or g/s or mol/s
rate at a particular time - draw tangent + find gradient
precipitation + colour change
record visual change if initial solution transparent + product is a precipitate which clouds solution (opaque)
observe mark through solution + measure how long it takes to disappear - fast disappears - quicker rate.
results subjective as different people may not agree on exact point when mark disappears or solution changes colour + graph can't be plotted.
if reactants coloured + products colourless - time how long it takes solution to lose colour
change in mass
use mass balance - as gas released (product), mass disappearing is measured on mass balance - quicker reading drops, faster reaction.
take measurements at regular intervals to plot rate of reaction graph
most accurate as mass balance is accurate but gas is released directly into the room.
the volume of gas given off
gas syringe measures volume of gas given off - more gas - faster.
gas syringes accurate to nearest cm^3 + can take measurements at regular intervals to plot rate of reaction graph but if reaction too vigorous - plunger can be blown out of end of syringe.
magnetism + HCl react to product H2 gas
adds volume of dilute HCl to a conical flask + place on mass balance.
add magnesium ribbon to acid + quickly plug flask with cotton wool.
start stopwatch + record mass on balance - take readings at regular intervals.
plot results in table + work out mass lost for each reading
repeat with more concentrated acid solutions.
control: amount of magnetism ribbon used + volume of acid used --> fair test
higher concentration --> faster rate of reaction
measure gas released with gas syringe.
sodium thiosulfate + HCl produce a cloudy precipitate
both clear solutions + form a yellow precipitate of sulfur together.
add set volume of dilute sodium thiosulfate to a conical flask.
place flask on a piece of paper w/ a black cross drawn on it.
add some dilute HCl acid to flask + start stopwatch.
watch cross disappear through cloudy sulfur + time how long it takes.
repeat with solutions of either reactant at a different concentration (only change one) -- depth of liquid must be kept the same each time.
higher conc. - quicker reaction so less time taken for cross to disappear
graph cannot be plotted.
as reactants react, their concentrations far so forward reaction slows down but as more products are made + their concentrations rise, the backward reaction speeds up.
after a while the forward reaction goes at same rate as backward one -- system is at equilibrium - both reactions still happen but there is no overall effect (dynamic equilibrium) - concentrations of reactants + products have reached a balance + won't change.
equilibrium only reached if the reversible reactions takes place in a closed system - none of the reactants or products can escape + nothing else can get in.
at equilibrium amounts of reactants & products aren't equal -- lies to right, conc. of products greater -- lies to left, conc, of reactants greater.
position of equilibrium depends on:
if reaction is endothermic in one direction, it will be exothermic in the other --- energy transferred from surroundings during endothermic r is equal to energy transferred to surroundings during exothermic r.
thermal decomposition of hydrated copper sulfate
hydrated copper sulfate ⇌ anhydrous copper sulfate + water
heat blue copper(II) sulfate crystals, drives water off to leave white anhydrous copper(II) sulfate powder - endothermic.
add drops of water to white powder, blue crystals form - exothermic.
le Chatelier's Principle
if the conditions of a reversible reaction at equilibrium are changed, the system will try to counteract the change.
change: system no longer at equilibrium so system responds to bring itself back to equilibrium.
decrease temp.: equilibrium moves in exothermic direction to produce more heat -- more products of exothermic reaction & fewer for endothermic.
increase temp.: equilibrium moves in endothermic direction to try to decrease it - more products for exothermic reaction & less for exothermic reaction.
increase pressure: equilibrium tries to reduce it by moving in direction where there are fewer molecules of gas.
decrease pressure: equilibrium tries to increase it + moves in direction where there are more molecules of gas.
increase conc. of reactants: system tries to decrease it by producing more products.
decrease conc. of products: system tries to increase it by reducing amount of reactants.