6.7- Surface area, concentration, temperature, and stirring affect reaction rate

Question 1

Why reaction rates are important

Answer 1

• Reaction rate is how fast a reaction proceeds
• Doesn’t mean more products are formed in a reaction
• E.g. a 100 m race
  o Runner can run fast or slow- only difference is how quickly the runner finishes the 100 m
  o A fast reaction has a high reaction rate
  o A slow reaction has a low reaction rate
• In the chemical industry, controlling the rate of reaction is vital
• Reactions too slow are not economical
  o Equipment used for too long
• Reactions that are too fast need to be controlled
• Cheap as possible

Question 2

Collision Theory

Answer 2

• For a chemical reaction to occur, the atoms, ions, or molecules must collide with enough energy (collision theory)
• E.g. hydrogen iodide
  o A gas and molecules move around quickly
  o Each hydrogen iodide molecule must collide with another hydrogen iodide molecule for a reaction
  o Some collisions do cause reaction
   hydrogen iodide molecule bounces apart with no reaction
• Only some collisions result in reactions
  o Molecules must collide in the correct orientation for a reaction to occur
  o A weak chemical bond forms between the iodide ions and hydrogen ions
  o Intermediate substance is unstable- only exists with a short time

Question 3

Increasing the rate of collisions

Answer 3

• To increase the rate of a reaction-> increased number of collisions occurring through increasing
  o Surface area of the particles reacting
   A metal such as magnesium reacts with dilute hydrochloric acid
   For a reaction to occur, hydrogen ions in the acid must collide with magnesium atoms
   There are more metals exposed to the hydrogen ions if the metal is in small pieces
   Because the reaction occurs on the surface of magnesium, breaking it into smaller pieces provides a larger surface area for the reaction to occur

Question 4

Concentration of the reactants

Answer 4

 In a dilute solution, the particles of the reactants are spread out in a solvent such as water
 There is a lot of space between reactant particles
 Mant more reactant particles in the same volume (closer together)
 Between magnesium and hydrogen ions, the reaction will occur faster if there are more hydrogen ions to give it volume
 Using a hydrochloric acid solution with higher concentration will speed up reaction

Question 5

Temperature of the reaction

Answer 5

 Particles in a hot substance have more kinetic energy than particles in a cold substance
* Vibrate faster in hot substance
 High temperature particles will collide more frequently and with more speed than low temperatures
 More frequent + increased likelihood of each collision = reactions happen faster at higher temperatures
 Slow moving gas molecules will be pushed apart by the repulsion of the electrons that orbit the atoms (never close enough to form new chemical bonds)
 Fast moving molecules can push through repulsion-> electrons move to different atoms
 Exception: enzyme reactions

Question 6

Stir and mix

Answer 6

 As chemical reactions proceed, the particles of the reactants get used up (fewer particles of reactants and fewer collision-> slows reaction
 To maintain the reaction rate-> Products of the reaction should be removed and replaced with more particles of reactants (mixing and stirring the reactants)