2.2 Rates of reaction Flashcards
(25 cards)
Define rate of reaction.
The change in concentration of a reactant or product per unit time.
What is the unit for rate of reaction?
moldm^-3s^-1
Describe the natural progression of reactions.
The rate is the fastest at the start of a reaction since each reactant had its greatest concentration.
The rate slows down as the reaction proceeds as the concentration of the reactants decreases.
Rate becomes zero when the reaction stops, i.e. when one of the reactants has been used up.
How do you measure the rate of reaction on a graph?
Find the gradient
What does the collision theory state?
That for a reaction between two molecules to occur, an effective collision must take place.
The rate of reaction is a measure of how frequently effective collisions occur.
How does concentration affect the rate of reaction?
Increasing concentration, increases rate of reaction.
For reactions involving gases, increasing pressure will increase the rate of reaction, as pressure is proportional to concentration.
If there is an increase in concentration of reactants there are more molecules in a given volume. Distances between the molecules are reduced so there is an increase in the number of collisions per unit time. This means that there is a greater chance that the number of effective collisions increases, hence the rate of reaction increases.
For gaseous reaction, increasing the pressure is the same as increasing the concentration.
How does temperature affect the rate of reaction?
Increasing temperature, increases rate of reaction.
If there is an increase in temperature, there will be an increase in kinetic energy of the molecules and so they will move faster. This means that more molecules will have enough energy to react on collision, meaning that the reaction rate will increase.
How does particle size affect rate of reaction?
In a reaction involving a solid, breaking down the solid into smaller pieces increases the rate of reaction.
Reducing the particle size of a solid increases the surface area so again the molecules are closer together and there is an increase in the number of collisions per unit time leading to an increase in reaction rate.
How does an addition of a catalyst affect reaction rate?
A catalyst increases the rate of a chemical reaction without itself undergoing a permanent change
How does light affect rate of reaction?
Some reactions are more vigorous when carried out in bright light.
Define activation energy.
The minimum energy required to start a reaction by breaking of bonds
What does the Boltzmann energy distribution curve show?
The distribution of molecular energies in a gas.
The energies of a few molecules are almost zero.
Most molecules have an energy around an average value.
Only a minority have values that equal or exceed the activation energy.
Tell me about the Boltzmann distribution curve of molecules at higher temperatures.
At higher temperatures, the average molecular energy will increase.
Some molecules will still be almost motionless but at any one time, many more molecules will have a higher energy.
At higher temperatures, the distribution flattens and the peak moves to the right, with a lower height.
More molecules have sufficient energy to react and so the rate increases significantly
What is a catalyst?
A substance that increases the rate of reaction of a chemical reaction without being used up in the process.
The catalyst can be recovered at the end of the reaction unchanged.
How do catalysts work?
They work by providing an alternate pathway for the reaction.
Reaction rate increases because catalysts lower the activation energy.
Tell me about the Boltzmann distribution for catalysts.
At the same temperature, a greater proportion of colliding molecules will achieve minimum energy required to react
Activation energy is lower.
What are the 2 classes of catalysts.
Homogenous
Heterogenous
Define homogenous catalysts.
In the same physical state as the reactants
takes an active part in the reaction than being a spectator
Define heterogenous catalysts.
In a different physical state from the reactants
What are enzymes?
Biological catalysts.
List the benefits of using enzymes.
- Low temperatures and pressures used, saving energy and costs
- Operate in mild conditions and do not harm fabrics or food
- Biodegradable, disposal no problem
- Allow reactions to take place which form pure products, no side reactions. Removes need for complex separation techniques.
List the ways you can study rates of reactions.
- Changes in gas volume:
recorded using gas syringe at various times - Changes in gas pressure (changes in number of moles of gas):
using a manometer - Change in mass:
using weighing scales - Colorimetry (change of colour as reaction proceeds):
using colorimeter
Describe how you would show how the rate changes in a chemical reaction using gas collection method (experiment).
- Set up apparatus, ensuring that one reactant is in excess
- Start the reactant by shaking the magnesium into acid and start a stopwatch
- Measure amount of hydrogen given off at constant intervals
- Stop stopwatch when hydrogen is no longer being produced.
- Repeat experiment with different conc. of acid, temp of acid, particle size of Mg ensuring all other factors are kept constant.
- Draw graph of results
Describe how to perform an iodine clock reaction.
- For a trial, add 10 cm^3 H2SO4, 10 cm^3 Na2S2O3, 15 cm^3 KI and 9 cm^3 deionised water from burettes into a conical flask.
- Add 1 cm^3 of starch solution.
- Measure 5 cm^3 H2O2 from a burette into a test tube.
- Rapidly pour the H2O2 into the flask, simultaneously start a stopwatch and mix thoroughly
- When the blue colour appears, stop the stopwatch
- Repeat using 5 different concentrations of peroxide, ensuring that the total volume of the mixture is 50 cm^3
- The concentration of peroxide should vary by at least threefold to ensure that a good spread of results.
- Plot a graph of 1/time against volume of peroxide will give the relationship between hydrogen peroxide and rate
