Describe a practical to investigate the rate of reaction using the mass loss method.
- measure mass of marble chips on their own and record
- place marble chips in conical flask and place on top of mass balance
- start timer, pour in hydrochloric acid and quickly put a fluffed up cotton ball in the neck of the flask
- measure and record the loss in mass from the carbon dioxide given off into the surroundings
- calculate rate of reaction using (mass of reactants lost/time)
Describe a practical to investigate the rate of reaction using the volume gas collection method.
- in conical flask with gas syringe attached (syringe pushed all the way in at this point) put marble chips with hydrochloric acid
- measure and note volume of gas collected at 5 second intervals until end point of reaction (no more fizzing)
- draw a graph of results
Describe a practical to investigate the rate of reaction using the disappearing cross method.
- draw an ‘x’ clearly on a white sheet of paper
- place a conical flask on top of this paper so that the ‘x’ may be seen through the bottom of the flask when looking from the top
- pour 50 cm^3 of sodium thiosulphate into the flask
- pour 10 cm^3 of dilute hydrochloric acid into the flask at the same time as starting a stopwatch
- stop the stopwatch when the yellow precipitate formed completely obscures the cross
- repeat four more times, increasing the volume of dilute hydrochloric acid added every time
- make sure to have the same person observe the cross to determine when it has ‘disappeared’ so that the same perception will be used + results are fairer
How do you calculate the rate of reaction at a specific time on a graph ?
Draw a tangent to the curve at the specified time, construct a right angled triangle and find the gradient.
Explain collision theory.
- in any reaction, particles collide
- to react, particles must collide with sufficient energy and force (activation energy)
What are the 5 factors which affect rate of reaction ?
- surface area to volume ratio
- temperature
- pressure (gases)
- concentration of solution
- presence of a catalyst
Formula for mean rate of reaction.
mean rate = mass of reactant used up/time
Why does SA:V ratio affect rate of reaction ?
larger surface area, more particles can collide at once and can react faster
Why does a higher temperature increase rate of reaction ?
- particles collide more frequently due to having more energy
- particles collide with more force due to having more energy
- a higher proportion of particles achieve the sufficient activation energy
What effect does increasing the pressure of a gas have on the rate of reaction and why, and does increasing the concentration of a solution have the same effect ?
- rate of reaction is faster
- gas is compressed
- particles are closer together so FREQUENCY OF COLLISION IS INCREASED
- yes, the same effect is reached
Explain how a catalyst works to increase the rate of reaction.
- provides alternative energy pathway with lower activation energy
- a higher proportion of particles can now achieve sufficient energy to react
- FREQUENCY OF EFFECTIVE COLLISIONS INCREASES
Why are catalysts useful in industry ?
- lower energy cost of reaction
- catalysts are not used up so can be used for a long time without needing replacement (only need to be replaced when ‘poisoned’ by reactants)
- better for environment as allows reactions to take place at lower temp + pressure, so less fossil fuels burnt, so less CO2 in atmosphere
If the forward reaction of a reversible reaction is endothermic, what will the reverse reaction be ?
exothermic
Describe a test for water (both forward and reverse reaction) using copper sulfate.
FORWARDS REACTION:
- start w/anhydrous copper sulfate powder
- add water
- observe colour change from white to blue
- this is an exothermic reaction - if water is added drop by drop, steam may be observed
REVERSE REACTION:
- start with hydrated copper sulfate
- heat
- observe colour change from blue to white
- endothermic reaction
Explain dynamic equilibrium.
When (in a closed system reversible reaction) the forward and reverse reactions are occurring at the same rate, so appear stable.
What is Le Chatelier’s Principle ?
If a system at equilibrium is disturbed, the position of equilibrium will shift to ensure that things equalise again.
What is the effect of temperature on equilibrium ?
- when forwards reaction is exothermic, if temp cool, equilibrium on right, meaning forwards reaction occurs more to heat up system
- causes higher yield of products
- if temp high, forwards reaction will happen less to cool system down, giving higher yield of reactants than products
What is the effect of pressure on equilibrium ?
- AFFECTS ONLY GASEOUS REACTIONS WHERE THERE ARE UNEQUAL RATIOS OF GAS MOLECULES ON EACH SIDE
- higher pressure forces position of equilibrium to the side of the reaction with fewer molecules of gas
- lower pressure shifts the position of equilibrium to the side of the reaction with more molecules of gas
