3.4 RATES AND EQUILIBRIUM Flashcards
How do you work out the rate of reaction?
Rate (mol dm-3 s-1) = change in concentration (mol dm-3) / time (s)
What does the rate of reaction measure?
The rate of reaction measures how fast a reactant is being used up or how fast a product is being formed.
What happens to rate throughout a reaction?
At the start of a reaction, the rate is the fastest as each reactant is at its highest concentration. As the reaction proceeds, the rate slows as the reactants are being used up and the concentration decreases. Once all the reactants have been used up, the concentrations stop decreasing and the rate of reaction is zero.
What is collision theory?
Collision theory states that two reacting particles must collide for a reaction to occur.
What factors affect the rate of reaction?
The factors that affect the rate of reaction are:
- concentration = higher conc = faster rate
- temperature = higher temp = faster rate
- catalyst = usually increases rate
- surface area of solid reactants = higher SA = faster rate
For collision theory, must collisions be effective or ineffective?
Collisions must be accurate and effective for a reaction to occur.
Collisions will be effective if:
- particles collide with correct orientation
- particles have sufficient energy to overcome the activation energy barrier of a reaction
What happens to rate if the pressure is increased?
When a gas is compressed, the pressure is increased and therefore the rate of reaction will also increase. Concentrated gas molecules increase as the same number of gas molecules occupy a smaller volume. Gas molecules move closer together and collide more frequently and effectively.
What is a catalyst?
A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself by providing an alternative reaction pathway of an altered activation energy.
- AE lowered for faster rate
- AE raised for slower rate
What are the features of catalysts?
Features of catalyst:
- not used up in a chemical reaction
- regenerated at end of chemical reaction
- may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
What are the negatives of catalysts?
Negatives:
- require specific conditions to work
- specific to particular reactants
What are homogeneous catatlysts?
Homogeneous catalysts have the same physical state of the reactants
- forms an intermediate that breaks down to give the product then regenerate the catalyst
- e.g making esters
What is a heterogeneous catalyst?
Heterogeneous catalysts have a different physical state than the reactants, usually solids
- after product molecules leave surface through desorption
- catalyst remains unchanged
- reaction takes place when reactant molecules are absorbed onto surface
- e.g haber process to make ammonia
What is the sustainability and economic importance of catalysts?
Sustainability and economic importance:
- catalyst lowers AE which increases ROR
= reduces temp needed and energy required
- less energy = less electricity/fossil fuels used
= makes product faster
= cuts cost, increases profit
- economic advantages outweigh cost in a catalytic process
- sustainability requires industry to operate processes with high atom economy and fewer pollutants
= less fossil fuels = less co2 emissions
What is the Boltzmann distribution curve?
Boltzmann curve:
- proves catalysts work to increase rate
- no molecules have zero energy (never touches x axis)
- area under curve = total number of molecules
- no maximum energy for a molecule
What is the effect of temperature on the Boltzmann curve?
Effect of temperature:
- shifts to right
- more molecules have energy increased or equal to AE
- increased proportion of collisions, increased ROR
- collisions more frequent as molecules move faster, increased energy
