rate of a chemical reaction
how fast the reactants are changed into products
3 ways to measure rate of reaction
precipitation/ colour change
measuring amount of reactant used
measuring amount of product formed
rate of reaction formula(s)
amount of reactant used or amount of product formed (cm³) / time(s)
understanding graphs for rate of a reaction
steeper line- the faster the rate of reaction
over time line becomes less steep- reactants are used up
quickest reactions- steepest lines- flat in the least time
flat line shows reaction has finished
collision theory
reactions can only take place when particles collide with enough energy. reaction rates explained by collision theory:
collision frequency- more collisions, faster reaction. doubling frequency doubles rate
energy transferred during a collision- particles need to collide with enough energy for the collision to be successful (activation energy)
factors affecting rates of reaction
temperature
surface area
concentration
catalysts
affect of temp on rate
• increasing temperature increases rate of reaction
• more energy so particles move faster
• collide more frequently
• more energy so more of the collisions will have enough energy to make the reaction happen
affect of conc on rate
if a solution is more concentrated there are more particles in the same volume of water or solvent.. particles are more crowded
(similarly when pressure of gas is increased, same number of particles occupy smaller space)
collisions between reactant particle more frequent
double conc= double rate
affect of surface area on rate
catalyst is a substance that speeds up a reaction without being used in the reaction itself.
catalyst
catalyst is a substance that speeds up a reaction without being used in the reaction itself.
affect of catalyst on rate
using a catalyst will increase the rate of reaction
it decreases the activation energy needed for the reaction
it provides an alternative reaction pathway with a lower activation energy
measuring colour change
you can record visual change in a reaction if initial solution is transparent and product is a precipitate which clouds the solution
observe a mark through the solution and how long it takes to disappear. faster it disappears quicker the reaction
time how long it takes to change colour
results are subjective though and you can't plot a graph
observing colour change (disappearing cross) practical method
use measuring cylinder to put 40cm cubed of 8 g/ dm cubed sodium thiosulfate into conical flask and place it on the cross
measure 10 cm cubed HCl
pour acid into flask. gently swirl and start stop clock
look down through top of flask, stop when you can no longer see cross
collect results and repeat 3 times
repeat with different concentrations of sodium thiosulfate
plot graph- mean time taken for cross to disappear in s and sodium thiosulfate conc g/dm cubed
disappearing cross method variables
independent- sodium thiosulfate concentration
dependent- time taken for cross to disappear
controls- volume of solution (HCl and sodium thiosulfate), size and thickness of cross, person judging
measuring the volume of gas produced practical method
measure 50 cm cubed of 1M HCl using a measuring cylinder and put into the conical flask
set up equipment fixing a gas syringe to a clamp stand and push it to 0
add 3cm Mg strip to conical flask and put the bung on and start the stop clock
record the volume of H2 gas given off every 10 seconds for 120 seconds or until no more is given off
repeat this using 1.5 and 2.0 mol HCl
measuring volume of gas practical variables
independent- concentration acid (HCl)
dependent- volume of gas produced
controls- volume of acid, length of Mg wire
calculating rates using tangents
draw a dot/ cross at specific time in the question
draw a line that sits on the curve and goes through the dot/ cross
using the squares, draw an appropriate triangle (length doesn't matter)
calculate values for x and y
gradient= change in y/ change in x
reversible reactions
• a physical change is reversible. no new substances are made (e.g. ice melting, puddles drying)
• occurs in forward and backwards directions.
hydrated copper sulfate+ heat
• blue before heating
• white after heating
• endothermic
it boiled then the water began to evaporate and the copper sulfate formed white crystals
anhydrous copper sulfate + water
• white before water
• blue after water
• exothermic
when water was added the white powder turned blue and the liquid was warm
equilibrium
equilibrium is the state in which a forward and backwards reaction happens at the same rate and the concentration of the reactants and the products are constant in a closed system
Le Chatelier's principle
equilibrium will shift if the conditions are changed in the system to counteract them
affect of changing concentration on equilibrium
increasing conc sends equilibrium to opposite side
decreasing conc sends equilibrium towards same side
affect of changing pressure on equilibrium
only affects if in gaseos state
increasing pressure sends to side with less molecules
decreasing pressure sends to side with more molecules
affect of changing temperature on equilibrium
increasing temp to endothermic direction
decreasing temp to exothermic direction
affect of catalyst on equilibrium
a catalyst doesn't affect equilibrium
has an equal effect on both forwards and backwards reaction