C5.2 - Controlling reactions Flashcards
(26 cards)
Rate of reaction
how quickly a reaction happens
Rate of reaction equation
rate of reaction = amount of reactants used or amount of products formed ÷ time
3 examples of measuring the rate of reaction
Precipitation
Change in mass (usually gas given off)
The volume of gas given off
How do measure rate of reaction in a precipitation reaction?
method works for any reaction where mixing two colourless solutions produces a precipitate which makes the solution cloudy
pour one solution into a flask with the other solution
place flask on a piece of paper that has a cross on it
observe the cross through the mixture
measure how long it takes for the mark to be obscured
faster it disappears - faster the reaction
result is subjective - different people may not agree on exactly when the mark disappears
How do you measure rate of reaction using change in mass?
can measure the rate of a reaction that produces a gas using mass balance
as gas is released the lost mass is easily measured on the balance
as the reading on the balance drops - mass is lost
use the results to plot a graph of change in mass against time
method released gas produced into the room - if gas is harmful need to take safety precautions (do experiment in fume cupboard)
How do you measure rate of reaction using volume of gas given off?
involves the use of gas syringe to measure the volume of gas given off
more gas given off during a set time interval - faster the rate of reaction
can use your results to plot a graph of gas volume against time elapsed
ensure you use right size gas syringe - if reaction is too vigorous, plunger can blow out of the end of syringe
Suggest an experiment which can be used to show how surface area affects reaction rate?
reaction of hydrochloric acid and marble chips
Method for reaction of hydrochloric acid and marble chips
in a flask place dilute HCl and marble chips
attach a gas syringe to the flask
measure volume of CO2 gas produced using gas syringe
take readings at regular time intervals and record results in a table
plot a graph of results - time on x-axis, volume on y-axis
repeat experiment with same volume and concentration of acid and same mass of marble chips but they are smaller in size
repeat experiment with powdered chalk
What are marble chips and chalk made out of?
calcium carbonate
CaCO3
Finer particles mean marbles have…
larger surface area
Rate of reaction of finer particles
finer particles - means greater the surface area of the solid reactants
reaction goes faster
Rate of reaction if a greater mass of marble chips is added
extra surface area gives a faster reaction
also more gas made overall
Suggest an experiment which can be used to measure the effects of changing the concentration on the rate of reaction
reaction of Magnesium metal and dilute HCl
reaction gives off hydrogen gas - measure mass loss of gas using mass balance
higher concentration gives…
a faster reaction
reaction finishes sooner
Calculating rate of reaction from a graph
If graph is amount of products formed (or reactants used up) against time - gradient of graph is rate of reaction
gradient = change in y ÷ change in x
Relation between rate and time
rate is inversely proportional to time
1/time = rate of reaction
What does rate of reaction depend on?
temperature
concentration (pressure for gasses)
size of particles
Collision theory
rate of chemical reaction depends on -
collision frequency of reacting particles - more successful collisions, the faster the reaction is
energy transferred during collisions - particles have to collide with enough energy for collision to be successful
Increasing the temperature…
particles move faster - have more collisions
increases energy of collisions since particles are moving faster
so more successful collisions - more particles collide with enough energy to react
so reaction rate increases
Increasing concentration (or pressure) …
if solution is made more concentrated - more particles of reactants in same volume
collisions are more likely - reaction rate increases
increasing pressure of a gas - particles are more crowded
frequency of collisions between particles will increase - reaction rate increases
Smaller solid particles (or more surface area)…
if one reactant is a solid - breaking it into smaller pieces will increase its surface area to vol ratio
particles around it will have more area to work on - frequency of collisions increases
reaction rate increases for solids with a larger surface area to volume ratio
Catalyst
substance which increases the rate of reaction without being chemically changed or used up in the reaction
How do catalysts work?
decrease the activation energy needed for a reaction occur
they do this by providing an alternative reaction pathway that has a lower activation energy
How do you find out if a substance is a catalyst for a reaction?
measure the reaction rate between two solutions without the solid you think may be a catalyst present
and measure again with a know mass of the solid
if rate increases but solid appears to be unchanged it could be a catalyst
to check that none of the solid has been used up - filter it out at the end of the experiment, dry it and measure the mass to check if its still all there
