Rates of Reaction Flashcards | Knowt (2024)

• the steeper the slope the faster the reaction

• eg. initially the reaction very fast as we are making a lot of product in a short amount of time

  • as we have a large number of reactant molecule

• gradually the slope of the line becomes less steep which means the rate of the reaction is decreasing

  • as a lot of the reactant molecules have already reacted and turned into product

  • so there are fewer reactant molecules available to react

• at the end the slop of the line is flat which means the reaction has stopped

  • as all the reactants molecules have already reacted

quantity of product formed/time taken

OR

quantity of reactant used/time taken

• chemical reactions can only take place when the reacting particles collide with each other

  • the collisions must have sufficient energy

• the rate of chemical reaction is determined by the frequency of successful collisions

• reactions are rapid initially because we have a large number of reactant molecules

  • so there are a large number of collisions per second

• over time the reactions slow down because the number of reactant molecules is running out

  • so there are a smaller number of collisions per second

• the reaction then stops as all of the reactant molecules have ran out

  • so there are no collisions per second

• increase the concentration, more collisions per second and you increases the rate of the reaction

• increase the pressure, more collisions per second, increase the rate of the reaction

1. use a measuring cylinder to put 10cm of sodium thiosulfate solution into a conical flask

2. place the conical flask onto a printed black cross

3. add 10cm of hydrochloric acid into the conical flask

4. stir the solution and start a stopwatch

5. look down through the top of the flask

   • the solution will turn cloudy so stop the stopwatch when you can no longer see the cross

6. repeats the experiment using lower concentrations of sodium thiosulfate and calculate mean values for each concentration

   • do not include anomalous results

• people have different eyesight so some people can see the cross for longer than others so they may not get the same result

• fixed by using the same size printed cross

1. use a measuring cylinder to place 50cm of hydrochloric acid into a conical flask

2. attach the conical flask to a bung and delivery tube

3. place the delivery tube into a container filled with water

4. then place an upturned measuring cylinder also filled with water over the deliver tube

5. add a 3 cm strip of magnesium to the hydrochloric acid and start a stop watch

   • the produces hydrogen gas which is trapped in the measuring cylinder

6. every 10 secs measure the volume of hydrogen gas in the measuring cylinder until no more hydrogen is given off

7. repeat the experiment using different concentrations of hydrochloric acid

• both experiments show that the great the concentration, the faster the reaction takes place

• because it is shown by 2 experiments, this finding is reproducible

• particles in solution can only react with particles on the surface of the solid

• increase the surface area of a solid reactant, there are more collisions per second, the rate of the reaction increases

• smaller sized blocks of a solid reactant have a great surface area to volume ratio

  • so more particles on the surface so more collisions per second so increase in the rate of reaction

• if 2 particles with a small amount of energy collide a reaction will not occur as they cannot overcome the activation energy barrier

• if 2 particles with a large amount of energy collide a reaction will occur as they will overcome the activation energy barrier

• increasing temperature increases the rate of reaction

  • as there is increases the energy of the particles

  • so there are more collisions per second

  • each collision has more energy

  • so more particles can overcome the activation energy barrier and collide successfully

• catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy

  • so less energy is required to cross the activation energy barrier

  • so more particles can successfully collide per second

• catalysts are not included in the chemical equation as they are not used up

• different reactions need different catalysts

• enzymes act as catalysts in living organisms

when the forward and reverse reactions take place at exactly the same rate

• if you carry out a reversible reaction in a sealed container none of the reactants or products can escape, and eventually, equilibrium is achieved

• if a reversible reaction is exothermic in on direction (temperature increases), it is endothermic in the opposite direction (temperature decreases)

• the same amount of energy is transferred in each case

• if we change the concentration of the reactants or products, the reaction is no longer at equilibrium

• so the concentrations of all the substances will change until equilibrium is reached

• eg. if you increase the concentration of NO2, more N2O4 will be formed until equilibrium is reached again

eg. 2NO₂ ←→ N₂O₄

• 2NO₂ → = exothermic (temperature increases)

• ←N₂O₄ = endothermic (temperature decreases)

• if we increase the temperature of this reaction, then the equilibrium shifts to the left to reduce the temperature

  • as the reverse reaction is endothermic, so energy is taken in

  • so the amount of 2NO₂ would increase and the amount of N₂O₄ would decrease

• if we increase the temperature, then the equilibrium shifts to the right to increase the temperature

  • as the forward direction is exothermic, so energy is released

  • the amount of N₂O₄ would increase and the amount of 2NO₂ would decrease

• the pressure of a gas depends on the number of molecules

• if we increase the pressure on a reversible reaction at equilibrium, the position of the equilibrium shifts to the side with the smaller number of molecules

• if we reduce the pressure, the position of the equilibrium shifts to the side with the larger number of molecules

• if the number of molecules is the same on both sides, changing pressure has no effect on the position of the equilibrium

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