C5.2 - controlling reactions Flashcards
define ‘rate of reactions’
a measure of how quickly reactants are used
or
a measure of how quickly products are made
what is the equation to calculate rate of reactions? (reactants)
amount of reactant used
———————————
time taken
what is the equation to calculate rate of reactions (product)?
amount of product formed
————————————
time taken
what happens as a gas syringe fills?
- plunger moves outwards
- record the end place of plunger (with graduations marked on side)
how do you measure the volume of gas produced?
1) place dilute hydrochloric acid in a conical flask connected to a gas syringe
2) add piece of magnesium to acid, stopper the flask and start stop watch
3) record amount of hydrogen produced (in gas syringe) at regular intervals
state two things you must remember when measuring how volume of gas changes
- push the gas plunger all the way in (make sure reading starts at 0)
- make sure the flask is stoppered as soon as experiment begins (so all hydrogen is collected)
what is the instantaneous rate of reaction?
the rate at a particular time (by drawing a tangent to the curve)
how do you calculate the mean rate of reactions?
change in volume
———————
change in time
why do reactions go faster at higher temperatures?
- particles gain energy
- collide more often
- greater PROPORTION of particles have enough activation energy
what 2 conditions must be met for reactions to occur?
- reactant particles must collide
- particles must have enough energy (activation energy)
define a successful collision
a collision that leads to a reaction
how do you investigate the effect of temperature?
using the ‘disappearing cross experiment’
- mix sodium thiosulfate and hydrochloric acid
(time it takes for cross to disappear at different temperatures)
how does the rate of reaction relate to reaction time?
it is inversely proportional
1/reaction time
why do reactions go faster at higher concentrations?
- particles become more crowded
- collide more often
- higher rate of successful collisions
when the concentration increases, does the energy stored in the particles change?
no
does the limiting reactant affect the amount of product made?
yes, they will be proportional
how does a smaller particle size affect rate of reactions?
- more reactant particles available for collisions (higher surface area to volume ratio)
- collisions more likely, so particles collide more often
- increased successful rate of collisions
why do powders result in fast reactions?
- very large surface area to volume ratio
define activation energy
minimum energy required for particles to react
how do you investigate particle size on rate of reactions?
- use gas syringe to measure volume of co2 produced
- how it changes with powder vs lump
describe characteristics of a catalyst (4)
- increases rate of reaction
- remains unchanged (mass)
- specific to particular reactions (like enzymes)
- high surface to volume ratio
define enzymes
proteins that act as catalysts in biological systems
how do catalysts work? - 4 mark
- provide alternate pathway
- lowers activation energy
therefore. ..
- greater PROPORTION of particles have the activation energy - or more
- rate of successful collisions increase
do catalysts affect how much energy the particles have (increase/decrease)?
no, they lower the energy required for the reaction to occur
Why do reactions go faster with powders, detailed.
The particles in a substance in the solid state can only vibrate about fixed positions. They cannot move from place to place. This means that only the particles at the surface can take part in collisions. The rate of reaction increases as the surface area increases because:
- more reactant particles are available for collisions
- collisions are more likely, so particles collide more often
The energy stored in the particles does not change, but because the rate of collisions increases, the rate of successful collisions increases.
Why smaller lumps react faster than bigger lumps?
The total surface area available for collisions is larger
Why is the fact that a small amount of catalyst will catalyse the reaction between a large amount of reactants useful?
Because catalytic converters use platinum, rhodium, and palladium.
These metals are very expensive, so they are coated onto an inert ceramic ‘honeycomb’ .
This uses only a few grams of catalyst, provides a large surface area for the reactions, and allows exhaust gasses through.
Pb - define catalyst
A substance that speeds up a reaction, WITHOUT BEING USED UP ITSELF
