C7. Chemical Reactions Flashcards
Use collision theory to explain the effect of the size of reactant/surface area on the rate of reaction
- When the size of a reactant is smaller, the total surface area exposed to collision with the particles of other reactants increases
- The frequency of collision between the reacting particles increases
- Greater chance of successful collisions / Higher number of successful collisions in a given unit of time between reacting particles
- Resulting in higher rate of reaction
Use collision theory to explain the effect of concentration on the rate of reaction
- When the concentration of the solution of reactant increases, the number of particles per unit volume of the solution of this reactant increases
- The frequency of collision between the reacting particles increases
- Greater chance of successful collisions / Higher number of successful collisions in a given unit of time between reacting particles
- Resulting in higher rate of reaction
Use collision theory to explain the effect of temperature on the rate of reaction
- An increase in temperature will cause the kinetic energy of the reacting particles to increase, and the reacting particles move faster
- More colliding particles will have sufficient energy to react / More colliding particles are able to overcome the activation energy
- Greater chance of successful collisions / Higher number of successful collisions in a given unit of time between reacting particles
- Resulting in higher rate of reaction
Use collision theory to explain the effect of catalyst on the rate of reaction
- A positive catalyst provides an alternative path for the reaction between the reacting particles to take place, which requires lower activation energy
- More colliding particles are able to overcome the lower activation energy
- Greater chance of successful collisions / Higher number of successful collisions in a given unit of time between reacting particles
- Higher rate of reaction
Collision theory
In order for a reaction to occur:
- reacting particles must collide with each other
- the collision must have sufficient energy to be successful
Photochemical reactions
Chemical reactions that only happen in light; the rate of reaction is influenced by light
Film photography
- Film is coated with a gel containing tiny grains of silver bromide
- When light hits the film, silver ions are reduced to form silver (atoms)
- The silver atoms cluster together and block the transmission of light, causing the film to darken
* where brighter light strikes, decomposition is faster, giving more silver - The film is developed to produce negatives - unreacted AgBr is washed away, leaving clear and darker areas on film
- The film is printed - light shone through film onto photographic paper, also coated with silver bromide
- Light passes through clear areas easily, causing AgBr on photographic paper to decompose
- Unreacted AgBr is washed away, leaving a B&W image of the original scene, made of silver particles
Explosive combustion with fine powders (in flour mills)
Flour particles are tiny and have a very large surface area. If a lot of flour particles are suspended in the air, and they are close enough together, a spark from a machine could cause an explosion - when 1 flour particle ignites, it starts a rapid chain reaction with other particles.
Explosive combustion with gases (methane in coal mines)
In coal mines, methane and other flammable gases collect in the air. At certain concentrations they form an explosive mixture with the air. A spark could set off an explosion.
*coal dust also reacts explosively in air
Acidified potassium manganate (VII)
An oxidising agent
Changes from purple to colourless
Potassium iodide
A reducing agent
Changes from colourless to red-brown
Potassium dichromate (VI)
An oxidising agent
Changes from orange to green
