4.3 - Rates of reaction Flashcards
when can a chemical reaction only happen
if the reactant particles collide with enough energy
what is activation energy (Ea)
the amount of energy the particles need to react
what is the equation for rate of reaction
rate = collision frequency x percentage success
what does the overall rate of a reaction depend on
collision frequency - how often the particles collide with each other
percentage success - what percentage of the collisions have enough energy
what are the factors that affect the rate of a reaction in terms of collision frequency and percentage success
solid surface area - collision frequency
solution concentration - collision frequency
temperature - percentage success
catalysts - percentage success
why does higher solid surface area increase rate
-more of the solid is exposed to other particles
- so the collision frequency is higher
- so there are more successful collisions per unit time
why does higher solution concentration/ gas pressure increase rate
- there are more particles per unit volume
- so the collision frequency is higher
- so there are more successful collisions per unit time
why does higher temperature increase rate
- the particles have more kinetic energy
- so the percentage of collisions with enough energy is higher
- so there are more successful collisions per unit time
why do catalysts increase rate
- they provide an alternative route with a lower activation energy
- so the percentage of collisions with enough energy is higher
- so there are more successful collisions per unit time
for solutions and gases why does increasing the concentration or pressure increase the rate
- there are more particles per unit volume
- so collision frequency is higher
- so there as more successful collisions per unit time
how can you increase the concentration
dissolve more solute particles so there are more particles per unit volume
how can you increase the pressure
by making the reaction vessel smaller
what experiment explored the affect of concentration on rate
disappearing cross experiment
what can the effect of concentration on rate be demonstrated by
reacting sodium thiosulphate solution with hydrochloric acid
what is the chemical equation for reacting sodium thiosulphate with hydrochloric acid
Na2S2O3(aq) + 2HCl(aq) –> 2NaCl(aq) + S(s) + SO2(g) + H2O (l)
what is the input variable in the disappearing cross experiment
the concentration of the NaS2O3(aq)
what is the output variable in the disappearing cross experiment
the time taken for the cross to disappear
what are the control variables in the disappearing cross experiment
total volume of solution
concentration of HCl (aq)
temperature
what is rate calculated from in the disappearing cross experiment
time taken
rate (S-1) = 1/ time taken
what would the graph for the disappearing cross experiment/ concentration effect on rate look like and why
would be proportional and show a straight line through the origin
because the rate is proportional to concentration
for solids why does the surface area increase the rate
- more of the solid is exposed to the other particles
- so the collision frequency is higher
- so there are more successful collisions per unit time
how is increased surface area achieved
by cutting or grinding up the solid
what experiment explores the affect of surface area on rate
the marble chip experiment
what can the effect of surface area on rate be demonstrated by
reacting calcium carbonate chips with hydrochloric acid
what is the chemical equation of reacting calcium carbonate chips with hydrochloric acid
CaCO3(s) + 2HCl(aq) –> CaCl2(aq) + H2O(l) +CO2(g)
what is the input variable in the marble chip experiment
the sizes of the CaCO3(s) chips
what is the output variable in the marble chip experiment
the volume of CO2(g)
what are the control variables in the marble chip experiment
-mass of CaCO3(s)
- volume and concentration of the HCl(aq)
- temperature
what is the volume of CO2 measured in, in the marble chip experiment
using a gas syringe or inverted measuring cylinder in water
what does the graph of the marble chip experiment/ effect of surface area on rate look like and why
the line from the smaller chips has a steeper start
– because the rate has increased so volume of CO2 increases faster
both lines have the same final volume
– because the total amounts of reactants hasn’t changed so eventually produces the same amount of CO2
why does increasing temperature increase the rate
- the particles have more kinetic energy
- so the percentage of collisions with enough energy is higher
- so there are more successful collisions per unit time
although particles also collide more often the effect is not significant
how can increasing temperature be accomplished
by heating
what experiment explores the effect of temperature on rate
magnesium and acid experiment
what can the effect of temperature on rate be demonstrated by
reacting magnesium with hydrochloric acid
what is the chemical equation of reacting magnesium with hydrochloric acid
Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)
what happens in the magnesium and acid experiment
the magnesium disappears
what is the input variable of the magnesium and acid experiment
the temperature of the HCl (aq)
what is the output variable of the magnesium and acid experiment
the time taken for Mg(s) to disappear
what are the control variables in the magnesium and acid experiment
mass and surface area of Mg(s)
volume and concentration of HCl(aq)
how can rate be calculated in the magnesium and acid experiment
time taken
rate (S-1) = 1/ time taken
what would the graph of the magnesium and acid experiment/ effect of temperature on rate look like and why
curve upwards
not a straight line
– because rate isn’t proportional to temperature
doesn’t go through the origin
–because the rate isn’t 0 at 0 degrees
what are catalysts
usually solids which
- increase rate
- remain chemically unchanged
how do catalysts work
they increase rate because
- they provide an alternative route with a lower activation energy
- so the percentage of collisions with enough energy is higher
- so there are more successful collisions per unit time
what experiment explores the effect of catalysts on rate of reaction
hydrogen peroxide experiment
what can the effect of a catalyst on rate be demonstrated by
using the decomposition of hydrogen peroxide
what is the chemical equation for the hydrogen peroxide experiment
2H2O2(aq) –> 2H2O(l) +O2(g)
why are catalysts not in the reaction equation for the hydrogen peroxide experiment
they are uchanged during the reaction
what is the speed of the reaction in the hydrogen peroxide experiment without a catalyst
very slow, takes days to finish
what is the catalyst in the hydrogen peroxide experiment
manganese(IV) oxide (MnO2) - which is a black solid
what is the speed of the reaction in the hydrogen peroxide experiment with a catalyst
takes place much faster, takes only a few seconds to finish
how can you measure the effectiveness of catalysts
comparing how they affect rate
what is the input variable in the hydrogen peroxide experiment
which catalyst is being used
what is the output variable in the hydrogen peroxide experiment
volume of O2(g)
what are the control variables in the hydrogen peroxide experiment
volume and concentration of H2O2(aq)
temperature
mass and surface area of catalyst
what would the graph of the hydrogen peroxide experiment/ how catalysts affect rate look like and why
more effective catalysts increase rate more effectively which results in a steeper line on the graph
the initial amounts of reactants is the same so all lines would flatten out eventually at the same volume of oxygen if you waited long enough
