Collision Theory - 4th Flashcards
when can a chemical reaction only happen?
- if the reactant particles collide with enough energy
what is activation energy?
- the amount of energy the particles need to react is called activation energy
what will happen to particles which collide with less energy than the activation energy?
- will just rebound and remain unchanged
what does the overall rate of reaction involving millions of particles depend on?
- collision frequency
- percentage success
what is collision frequency?
- how often do the particles collide with each other
what is percentage success?
- what percentage of the collisions have enough energy?
what are four common factors which affect rate of reaction?
- surface area
- concentration
- temperature
- catalysts
why does a higher solid surface area increase the rate of reaction?
- more of the solid is exposed to the other particles
- so the collision frequency is higher
- so there are more successful collision per unit time
why does a higher temperature a increase the rate of reaction?
- 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 does a higher solution concentration/gas pressure increase the rate of reaction?
- there are more particles per unit volume
- so the collision frequency is higher
- so there are more successful collisions per unit time
why does a catalyst increase the rate of reaction?
- 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
how can you increase the concentration of a solution/gases?
- by dissolving more solute particles
how can you increase the pressure of a solution/gases?
- by making the reaction vessel smaller
how can the effect of concentration on rate be demonstrated?
- by reacting sodium thiosulphate solution with hydrochloric acid to make sodium chloride, sulphur solid , sulphur dioxide and water
what is the chemical equation for the disappearing cross experiment?
- Na2S2O3 (aq) + 2HCl (aq) -> 2NaCl (aq) + S (s) + SO2 (g) + H20
describe the observations of the disappearing cross experiment?
- in this reaction, a solid (sulphur) is slowly produced
- after a while, you cannot see through the mixture any longer
- this is measured by observing a black cross under the mixture
what are the variables for the disappearing cross experiment?
- input : the concentration of the Na2S2O3 (aq)
- output : the time taken for the cross to disappear
- control : total volume of solution, concentration of HCl (aq), temperature
how is rate calculated?
- rate (s^-1) = 1/time taken (s)
what is the relationship between rate and concentration?
- they are proportional
how is surface area increased?
- by cutting or grinding up the solid
how is the effect of surface area on rate demonstrated?
- by reacting calcium carbonate marble chips with hydrochloric acid to form calcium chloride, water and carbon dioxide gas
what is the chemical equation for the marble chip experiment?
- CaCO3 (s) + 2HCl (aq) -> CaCl2 (aq) + H2O (l) + CO2 (g)
what are the variables in the marble chip experiment?
- input : the sizes of the CaCO3 (s) chips
- output: the volume of CO2 (g)
- control: the mass of CaCO3(s), the volume and concentration of HCl (aq), temperature
how is the volume of carbon dioxide measured in the marble chip experiment?
- using a gas syringe or inverted measuring cylinder in water
what are the results of the marble chip experiment on a graph of volume of CO2 against time?
- a higher SA has a steeper start than the one with a lower SA because the rate has increased, so the volume of CO2 is increasing faster
- it has the same final volume of CO2- this is because the total amount of reactants hasn’t changed, so eventually the reaction produces the same amount of CO2
how can temperature be increased?
- by heating
how can the effect of temperature on rate be demonstrated?
- by reacting magnesium with hydrochloric acid to produce magnesium chloride and hydrogen gas
what is the chemical equation for the magnesium and acid experiment?
- Mg (s) + 2HCl (aq) -> MgCl2(aq) + H2
describe what happens in the magnesium and acid reaction?
- the magnesium disappears
what are the variables in the magnesium and acid experiment?
- input: the temperature of the HCl (aq)
- output: the time taken for Mg (s) to disappear
- control: mass and SA of Mg (s) and the volume and concentration of HCl (aq)
what are the results of the magnesium and acid experiment on a graph of rate against temperature?
- rate isn’t proportional to temperature so it isn’t a straight line
- it doesn’t go through the origin because the rate isn’t zero at 0 degrees
what are catalysts?
- are substances (usually solids) which increase rate and remain chemically unchanged
how can the effect of a catalyst on rate be demonstrated?
- using the decomposition of hydrogen peroxide
- 2H2O2 (aq) -> 2H2O (l) + O2 (g)
describe the results of the hydrogen peroxide experiment?
- very very slow reaction, usually taking days to finish
- it is catalysed by manganese (IV) oxide (MnO2), which is a black solid
- this causes it to place much faster, taking only a few seconds to finish
what are the variables of the hydrogen peroxide experiment?
- input: which catalyst is being used
- output: volume of O2 (g)
- control: volume and concentration of H2O2 (aq), temperature, mass and surface area of catalyst
what are the results of the hydrogen peroxide experiment on a graph of volume of O2 gas against time?
- increases
- most effective catalysts increase rate more effectively
- results in a steeper line on the graph
- the initial volume of the reactants is the same, so all lines would eventually flatten out at the same total volume of O2 (g) if you waited long enough
- the MnO2 (s) - catalysed decomposition of hydrogen peroxide is used to produce oxygen in the lab
