rate and extent of reactions

• What happens to the rate of chemical reactions over time?


  • Reactions are rapid initially
  • Reactions slow down over time
  • Reactions eventually stop
• What is the key idea in chemistry that explains the shape of the reaction rate curve?

  Collision Theory
• What does collision theory state about chemical reactions?


  Chemical reactions can only take place when reacting particles collide with sufficient energy
• How is the rate of a chemical reaction determined according to collision theory?


  By the frequency of successful collisions
• What does the term 'frequency' refer to in the context of chemical reactions?


  The number of successful collisions per second
• Why do reactions slow down over time according to the explanation given?


  Because the number of reactant molecules is running out
• What happens to the number of collisions per second as reactant molecules run out?


  The number of collisions per second decreases
• When does a chemical reaction stop?


  When all of the reactant molecules have run out
• How does the concentration of reactants affect the rate of reaction?


  • Higher concentration leads to more particles in the same volume
  • More particles result in more collisions per second
  • Rate of reaction is proportional to concentration
• What is the effect of doubling the concentration of reactants in a container?


  The number of collisions per second will also double
• How does the rate of reaction in a container with higher concentration compare to one with lower concentration?


  The rate of reaction in the higher concentration container will be faster
• What happens to the quantity of product when the concentration of reactants is increased?


  • The reaction produces more product at the end
  • The line on the graph for higher concentration is steeper
  • More reactant molecules lead to more product
• The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.
• For a chemical reaction to happen:

  • reactant particles must collide with each other
  • the particles must have enough energy for them to react

  A collision that produces a reaction is called a successful collision. The activation energy is the minimum amount of energy needed for a collision to be successful. It is different for different reactions.
• mean rate of reaction = amount of reactant used / time

  mean rate of reaction = amount of product formed / time
• The change in mass of a reactant or product can be followed during a reaction. This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass
• The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder.

  The units for rate are usually cm3 s-1 or cm3 min-1.
• The rate of a chemical reaction can also be measured in Mol s-1
• The gradient of the line is equal to the rate of reaction:

  • the steeper the line, the greater the rate of reaction
  • fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions
• The greater the frequency of successful collisions, the greater the rate of reaction. If the concentration of a reacting solution or the pressure of a reacting gas is increased:

  • the reactantparticles become more crowded
  • the frequency of collisions between reactant particles increases
  • the rate of reaction increases
• mean energy of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.
• The rates of two or more reactions can be compared using a graph of mass or volume of product formed against time
• For a given mass of a solid, large lumps have smaller surface area to volume ratios than smaller lumps or powders. If a large lump is divided or ground into a powder:

  • its total volume stays the same
  • the area of exposed surface increases
  • the surface area to volume ratio increases
• The greater the frequency of successful collisions, the greater the rate of reaction. If the surface area to volume ratio of a reacting solid is increased:

  • more reactantparticles are exposed at the surface
  • the frequency of collisions between reactant particles increases
  • the rate of reaction increases
• The greater the number of successful collisions the greater the rate of reaction 

  If the temperature of the reactant mixture increases: 

  • The energy (from the temperature kinetic energy given) increases 
  • Reactant particles gain kinetic energy and move more quickly 
  • frequency of particle collisions increases 
  • The proportion of successful collisions increases 
  • The reaction rate increases 
• The effect of temperature on the rate of reaction is due to two factors: frequency of collisions and energy of collisions. The increase in energy is usually the more important factor.
• A catalyst is a substance that:

  • increases the rate of a reaction
  • does not alter the products of the reaction
  • is not chemically changed or used up at the end of the reaction
• Only a very small mass of catalyst is needed to increase the rate of a reaction. Different substances catalyse different reactions.
• enzymes are biological catalysts
• catalysts do not appear in the overall chemical equation
• A catalyst provides an alternative reaction pathway that has a lower activation energy (than the uncatalysed reaction)
• catalysts

  using a catalyst does not change the frequency of collisions. However, it does increase the frequency of successful collisions because more particles have energy greater than the activation energy, therefore there are more successful collisions.
• method 1 (remember 2 because it needs to be repeatable)

  1. Use measuring cylinder to put 10cm^3 of sodium thiosulphate into a conical flask 
  2. Place conical flask onto a printed black cross
  3. Add 10cm^3 of hydrochloric acid into the conical flask 
  4. Swirl the solution and start a stopwatch 
  5. Look down at cross, and stop the stopwatch when you cannot see the cross 

  *Carry out the solution again using lower concentrations* 

  1. Repeat the entire experiment and calculate the mean values for the concentrations 

            > don’t include any anomalous results when calculating a mean    
• 1.  use a measuring cylinder to place 50cm^3 of hydrochloric acid in conical flask 
  2. Attach the conical flask --> bung --> delivery tube 
  3. Place delivery tube into a container filled with water 
  4. Place an upturned measuring cylinder, also filled with water, over delivery tube 
  5. Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch 

  (hydrogen gas captured in measuring cylinder)

  1. Every 10 secs measure the volume of hydrogen gas in the measuring cylinder 
  2. Continue until no more hydrogen is given off 

  Repeat with different concentrations of hydrochloric acid 
• In some chemical reactions, the products of the reaction can react together to produce the original reactants. These reactions are called reversible reactions. They can be represented in the following way:

  A + B ⇌ C + D
• directions of arrows in reversible reactions

  • the forward reaction is the one that goes to the right
  • the backward reaction is the one that goes to the le
• Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again.
• If a reaction is exothermic in one direction, it will be endothermic in the other direction. The same amount of energy is transferred in both the forwards and reverse reaction.
• Blue copper sulfate is described as hydrated. The copper ions in its crystal lattice structure are surrounded by water molecules. This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate. This reaction is reversible:

  hydrated copper sulfate ⇌ anhydrous copper sulfate + water
• When a reversible reaction happens in a closed container, it reaches a dynamic equilibrium. At equilibrium:

  • the forward and backward reactions are still happening
  • the forward and backward reactions have the same rate of reaction
  • the concentrations of all the reacting substances remain constant