The Rate and Extent of Chemical Change
Factors affecting rates of reaction
Collision Theory
How frequent the particles collide
The energy transferred during a collision.
Particles must collide with enough energy (activation energy)
Temperature
Concentration of solution
pressure of gas
surface area
catalysts
The particles will move faster
more collisions per second
The faster they move, the more energy they have (more successful collisions)
Same number of particles in the same volume
More collisions per second
More particles in the same volume
Provides an alternative pathway with lower activation energy
Powders have a greater surface area to volume ratio than lumps
More frequent collisions
Measuring Rates of Reaction
Precipitation and colour change
- Measure out 10cm3 of sodium thiosulfate with a measuring cyclinder
- Fill conical flask with ST
- Place the conical flask on a printed black cross
- Add 10cm3 of hydrochloric acid into the conical flask
- Swirl the solution and start the stopwatch
- Stop the clock when you can no longer see the cross (the solution will turn cloudy)
- Repeat the experiment again using lower concentrations
Repeat the whole experiment to take mean values
Reproducible: repeated by another person using a different technique and still get the same result
People have different eyesights so will stop the clock at different times
Change in mass
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Volume of gas produced
- Use measuring cylinder to place 50cm3 of hydrochloric acid into a conical flask
- Attach conical flask to a bung and delivery tube
- Place delivery tube into container filled with water
- Place an upturned measuring cylinder filled with water into the delivery tube
- Add a 3cm strip of magnesium to the hydrochloric acid and start the stopwatch
- Every 10 seconds, measure the volume of hydrogen gas trapped in the upturned measuring cylinder
- Repeat the experiment using different concentrations of hydrochloric acid
Reversible Reactions
Hydrated copper sulfate ⇌ anhydrous copper sulfate + water
Endothermic
Exothermic
The reaction gets hot as energy is released
Forward reaction takes place when you heat it (putting energy in)
Dynamic Equilibrium
Le Chatelier's Principle
Equilibrium is when the forward and reverse reactions take place at the same rate
If the conditions are changed in a reversible reaction at equilibrium then the system will try to counteract the change
Concentration
Temperature
Pressure
Concentration of products is increased
The reverse reaction if favoured so more reactants are produced
Concentration of products is decreased
The forward reaction is favoured so more products are produced
Temperature increases
Endothermic reaction if favoured
Temperature decreases
Exothermic reaction is favoured
Pressure increases
The reaction the fewer molecules is favoured
Pressure decreases
The reaction with the larger number of molecules
Number of molecules = coefficient
If the number of molecules on either side of the reaction are the same, pressure doesn't have an effect