1.3 Rates of Reaction Flashcards
What factors affect rate?
Concentration
Surface area of reactants
Temperature
Catalysis
How does concentration affect rate?
- Increase in the concentration of a solution or the pressure of a gas, will result in an increase in the rate of reaction
- If the concentration of a solution or the pressure of a gas is increases, there are more particles in a given volume. Therefore there will be more collisions in a set amount of time and the probability of successful collisions becomes higher.
How does the surface area of reactants affect rate?
If the surface area of a solid is increases, there are more particles exposed to react. Therefore there will be more successful collisions in a set amount of time and the rate of reaction increases
How does temperature affect rate?
- The rates of most chemical reactions increase dramatically for only small increases in temperature
- For many gaseous reactions, the rate is approximately doubled by a rise in temp of about 10ºC
- An increase in temperature causes:
1. An increase in the average velocity of the particles, resulting in a greater number of collisions per second
2. An increased number of particles which possess the activation energy Ea.
Define activation energy (Ea)
The minimum amount of energy required for a reaction to occur.
Hoe do catalysts affect the rate?
Catalysts provide an alternative pathway with a lower activation energy, Therefore the proportion of particles with Ea is increased and the rate of reaction increases.
Define catalyst
a substance that increases the rate of a chemical reaction, without getting used up in the process
What are the two categories of catalyst?
homogenous and heterogenous
What is a homogenous catalyst?
catalyst is in the same physical state as the reactants and products
What is a heterogenous catalyst?
The catalyst is not in the same physical state as the reactants and products
Define rate of reaction
the change in concentration of a reactant or product with respect to time
What is the equation used to calculate rate?
Rate = change in concentration of reactant or product/time taken for change to take place
Describe the relative rate over the course of a normal reaction
Reactions are fastest at the start (greater likelihood of collisions), and get slower as the reactant concentration drops
What is the progressive rate method?
- Reaction is allowed to progress and take readings (absorbance/pH), or take samples at various times (sample should be quenched to stop reaction, and titrated.)
- Plot a graph of concentration against time for the reactant, shape of graph gives order with respect to the reactant monitored.
How do you quench a reaction?
Quenching can be carried out by rapid cooling, or adding large quantities of cold water/quenching chemicals
What is the initial rate method?
Method 1:
- Initial rate calculated by drawing a tangent to the curve at t=0 on the concentration/time graph and calculating the gradient
- As the reactants are consumed during the reaction, the rate and therefore gradient decreases
Method 2:
- Measure the time taken for a certain amount of product to be formed, eg. Clock reactions (measure the time it takes for eg. Appearance of a precipitate, disappearance in solid or change in colour)
- Initial rate taken as 1/t
- Measure the initial rate of several sections with varying initial concentrations. Plot graph and determine the order of reaction.
How do you calculate gradient?
Gradient = (y2 - y1)/(x2 - x1)
How can you measure the release of gas in a reaction?
A gas syringe can be used to measure gas volume (same as gas concentration)
The change in mass can be recorded using an electronic balance
How can you measure colour change in a reaction?
Colorimeter can be used to measure its concentration, as well as a calibration curve that relates absorbance values to known concentrations of solutions of the coloured substance