Unit 6 Flashcards
(26 cards)
Rate of reaction
speed at which a chemical reaction takes place and can be expressed as the change in concentration of a particular reactant or product per unit time
Rate of reaction formula
Change in concentration of reactants / products (Mol dm^3) / Time (Secs)
How do rate of reaction graphs work?
- As a chemical reaction proceeds, the concentration of the reactants decreases and the concentration of the products increases
- The steeper the gradient, the quicker the rate of reaction
Average rate
- Rate of a reaction throughout the whole experiment.
- Calculated by measuring the slope of 2 points in the graph.
Instantaneous rate
- Rate of a reaction at a specific moment (time).
- Calculated by measuring the slope of a tangent line.
- Instantaneous rate decreases over time because the concentration is maximum in the beginning hence have the largest collision frequency. But as the reactant gets used, concentration decreases. So the collision frequency also decreases.
How to measure rate of reaction using colorimetry
A colorimeter or spectrophotometer measures the amount of light that passes through a solution. If a solution changes color during a reaction, this can be used to measure the rate
- intensity of light reaching the detector is measured every few seconds and the data is plotted to show how the concentration of the reactants or products changes with time
How to measure rate of reaction using changes in mass
Gas produced escapes from the reaction vessel, so the mass of the vessel decreases.
- Measuring changes in mass using a balance. The cotton wool in the neck of the flask allows the gas to escape whilst preventing the other reactants and products from leaving the container. But gas must be dense or change in mass value too small to calculate
How to measure rate of reaction using change in volume of gas
When gas produced, trapped and its volume measured over time
Syringe: Collecting a gas through water by displacement using an inverted measuring cylinder or burette. Only viable if the gas produced has a low water solubility
Inverted measuring cylinder: vVlume can be measured every few seconds and plotted to show how the volume of gas varies with time
How to measure rate of reaction using titrations
A titration can affect the rate of reaction and it cannot be done continuously. To overcome this, samples of the reaction mixture are taken at regular intervals during the course of the reaction
- quenching (reaction in each of the samples is deliberately stopped) to allow concentration to be determined by titration
- the rate of reaction can be calculated by determining the change in concentration with time
How to measure rate of reaction using conductivity
used to measure the rate of a reaction by monitoring changes in the electrical conductivity of the reaction mixture over time
- As reaction proceeds, concentration of ions in the solution may change, affecting its conductivity
- By measuring the conductivity at different time intervals, the rate of the reaction can be determined based on how quickly the conductivity changes
How to measure rate of reaction using clock reaction
more convenient to ‘stop the clock’ when a specific (visible) point in the reaction is reached instead of continuously monitoring the change in rate. Non-continuous methods in which the time taken to reach a fixed point is measured
Eg: piece of magnesium dissolves completely in hydrochloric acid, but only generates one piece of data for analysis
Collision theory
Explains how chemical reactions occur: When reactants come together the kinetic energy they possess means their particles will collide and some of these collisions will result in chemical bonds being broken and some new bonds being formed
Collision frequency and factors that alter it
The number of collisions between particles per unit time in a system. Can be altered by:
- Changing the concentration of the reactants
- Changing the total pressure
- Changing the temperature
- Changing the surface area of the reacting particles
Collision energy
Most collisions result in the colliding particles bouncing off each other, which become unsuccessful collisions and occurs when colliding species don’t have enough energy to break the necessary bonds. However, if sufficient energy, reaction occurs and collision successful
Activation energy
minimum energy the colliding particles need in order to react
- Collision Energy of colliding particles < Activation energy = Unsuccessful collision
- Collision Energy ≥ Activation energy = Successful collision
How does concentration affect rate of reaction?
The more concentrated a solution is, the greater the number of particles
- increase in concentration causes an increased collision frequency and therefore the frequency of successful collisions increases
How does pressure affect rate of reaction
When the pressure is increased, the particles have less space in which they can move
This means that the number of successful collisions increases due to an increased collision frequency
An increase in pressure, therefore, increases the rate of reaction
How does temperature affect rate of reaction
- At higher temperatures, the particles are moving faster, so collide more frequently leading to more successful collisions
- At higher temperatures, a higher proportion of the particles have the activation energy, thus higher proportion of collisions successful
How does surface area affect rate of reaction
Increase in surface area leads to more particles on surface able to collide with particles or other reactants, thus more total collisions and more successful collisions
How does catalysts affect rate of reaction
- catalyst provides the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction leading to more successful collisions
- catalyst itself doesn’t undergo permanent chemical change, thus not chemically changed
Enviornmental impact of using catalysts
- Reducing the energy requirements of processes as they enable reactions to occur at lower temperatures and pressures
- Reducing waste products as they can be reused and are only used in small quantities, increasing atom economy
- increasing the selectivity of processes, promoting specific reactions and suppressing undesired side reactions
Homogenous and heterogenous catalysts
- Homogeneous means that the catalyst is in the same state as the reactants
- Heterogeneous means that the catalyst is in a different state to the reactants
Biological catalysts
- Enzymes act as catalysts in biological systems, controlling many biochemical reactions within cells. They allow industrial reactions to happen at lower temperatures and pressures than usually needed, saving money and energy.
- Transition metals are often used as catalysts due to their ability to form more than one stable oxidation state
Maxwell Boltzmann distribution curve
graph that shows the distribution of energies at a certain temperature. In a sample of a substance:
- A few particles will have very low energy
- A few particles will have very high energy
- Many particles will have energy in between
