Rate and Extent of Chemical Change

• Collision theory: for a reaction to take place the particles much collide with enough energy

  Activation energy: minimum energy required to start the reaction (for a successful collision)

  Catalyst: substance that changes the speed of a chemical reaction without chemically altering the product or itself

  Turbidity: cloudiness or haziness of a fluid caused by large numbers of individual particles

  Reversible reaction: reactions that proceed in both forward & reverse direction
• • Collision theory explains how various factors affect rates of reactions

  o According to this theory, chemical reactions can occur only when

  ▪ reacting particles collide with each other

  ▪ with sufficient energy

  o The minimum amount of energy that particles must have to react is called the activation energy

  • Ways to increase number of successful collisions

  o Increase collision frequency

  o Increase energy of particles
• Factors which affect the rates of chemical reactions

  • Concentration & Pressure (only in gases)

  o More particles in a smaller space

  • Surface area

  o Increased surface area to volume ratio e.g. many small marbles more likely to collide than one large marble

  • Temperature

  o Increases kinetics energy of particles meaning they move faster and collide more

  o The temperature increases the energy within the particles

  • Catalysts
• • Catalysts change the rate of chemical reactions but

  are not used up during the reaction & don’t change

  the products of the reaction

  • Different reactions need different catalysts

  • Enzymes act as catalysts in biological systems

  • Catalysts increase the rate of reaction by providing a

  different pathway for the reaction that has a lower

  activation energy
• RPA 5 - investigate how changes in concentration affect the rates of reactions

  1. Support a gas syringe with a stand, boss & clamp

  2. Using a measuring cylinder add 50cm3 of dilute hydrochloric acid to a conical flask

  3. Add 0.4g of calcium carbonate to the flask, immediately connect the gas syringe & start the timer

  4. Every 10 seconds record the volume of gas produced

  5. When the reaction is complete (gas volume plateaus) stop

  6. Repeat steps 1-5 with different concentration of hydrochloric acid
• By a method involving a change in colour or turbidity REACTION

  
  

  • Sodium thiosulfate solution reacts with dilute hydrochloric acid:

  o sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur

  o Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

  • Sulfur forms a cloudy yellow precipitate during this reaction – overtime this increases the turbidity of the

  solution – time taken for the solution to become almost opaque is a way to measure the reaction time
• METHOD

  1. Using a measuring cylinder add 50cm3 of dilute sodium thiosulfate solution to a conical flask

  2. Place the conical flask on a piece of paper with a black cross drawn on it

  3. Using a different measuring cylinder add 50cm3 of dilute HCl (0.5mol/dm3), immediately start the timer

  4. Time how long it takes for the cross to disappear through the cloudy sulfur

  5. Repeat this reaction with different concentrations on HCl (e.g. 1, 1.5, 2 mol/dm3
• • If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.

  o The same amount of energy is transferred in each case.
• • Dynamic equilibrium has 3 conditions:

  o Rate of forward reaction is equal to the rate of backwards reaction

  o Concentration of product & reaction remains constant

  o Must be in a closed system, otherwise reactants and products would escape
• o Equilibrium lies to left: concentration of products greater

  o Equilibrium lies to right: concentration of reactants greater
• • Le Chatelier’s Principle

  o When a change is applied to a system in dynamic equilibrium, the system reacts in such a way to oppose

  the effect of the change
• The effect of changing concentration

  • If concentration of reactants or products changes, equilibrium is lost

  o Concentration of reactants increases: more products will be formed

  o Concentration of products decreases: reduction in the amount of reactants
• The effect of temperature changes on equilibrium

  • Temperature of a system at equilibrium is decreased:

  o Equilibrium will move towards the exothermic reaction – to increase temp.

  o More products for exothermic reactions & fewer products for endothermic reactions

  • Temperature of a system at equilibrium is increased:

  o Equilibrium will move towards the endothermic reaction – to decrease temp.

  o More products for endothermic reactions & fewer products for exothermic reactions
• The effect of pressure changes on equilibrium (gases only)

  For gaseous reactions at equilibrium:

  • Increase in pressure

  o equilibrium shifts towards side with the smaller number of molecules

  • Decrease in pressure

  o equilibrium shifts towards side with the larger number of molecules

  • Number of molecules shown by the symbol equation for that reaction