2.2 - Rates of reaction Flashcards
What is the definition of rate of reaction?
The change in concentration of a reactant or product per unit of time.
What is the equation for rate of reaction + unit?
Rate = change in concentration/time
units = moldm3/s = moldm3s-1
Describe a standard reaction graph.
- rate is fastest at the start of a reaction since each reactant has its greatest concentration
- rate slows down as the reaction proceeds since the concentration of the reactants decreases
- rates becomes 0 when the reaction stops (e.g when one of the reactants has been used up)
How do you find the intitial rate of reaction?
To find the initial rate, you need to find the gradient.
MN/MP
Triangle
What are the factors effecting reaction rates?
- concentration of a solution
- surface area of a solid
- temperature of a reaction
- catalyst
- light
What is activation energy?
Ea
The minimum energy required to start a reaction by breaking of bonds.
What is collision theory?
- for a chemical reaction to take place, reacting molecules must collide effectively
- the greater the number of collisions, the higher the chance that one of them will be effective
- any factor that increases the chance of effective collisions will also increase the rate of reaction
What is an ‘effective collision’?
For a collision to be effective, the molecules must collide in the correct orientation and have sufficient energy
What is meant by an exothermic reaction?
The reacting chemicals lose energy and heat given out to the surroundings. Even though the products have a lower energy than the reactants, there still has to be an input of energy to break bonds and start the reaction.
What is meant by an endothermic reaction?
The enthalpy of the products is more than the enthalpy of the reactants and heat is taken in from the surroundings.
What is the equation for enthalpy change from an energy profile?
Enthalpy change = Eaf - Eab
(Eaf - foward reaction)
(Eab - backward reaction)
What effect does concentration (pressure) have on a reaction?
- if the concentration of a reactant increases, the reaction rate increases
- this is because there are more molecules in the same volume so the distances between the molecules are reduced and their is an increase in the number of collisions per unit time
- this means that there will be a greater chance that there will be more collisions with greater energy , hence the rate of reaction increases
Describe the feautures of a Maxwell Boltzmann energy distribution curve.
- the curves do not touch the energy axis
- the areas under the curves = number of molecules in the sample
- at higher temperature, the peak moves to the right with a lower height
- only the molecules with an energy equal to or greater than the activation energy are able to react
- at a higher temperature, many more molecules have sufficient energy to react so the rate increases significantly.
What happens when you increase the temperature a reaction?
- if the temperature increases, the reaction rate increases
- at higher temperatures the molecules have more kinetic energy and are moving faster
- more molecules have an energy that is greater than the acitivation energy and more collisions take place in a certain length of time
What do catalysts do and how do they work?
- increase the rate of a chemical reaction without being used up in the process
- does not take place in the reaction but can be recovered unchanged at the end of a reaction
- catalysts work by providing a different energy pathway for the reaction, the rate increases because the new pathway has a lower activation energy overall
