Chapter 10 Reaction Rates and Equilibrium Flashcards
What is the rate of a chemical reaction?
How fast a reactant is used up or how fast a product is formed
Can be defined as the change in concentration of a reactant or product in a given time (mol dm^-3 s^-1)
Concentration time graph
Rate of reaction is fastest at the start of the reaction as each reactant is at its highest concentration (steepest)
Rate of reaction slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease (less steep) - but concentration of products continues to increase at a slower rate
Once one of the reactants is completely used up (limiting reactant)m concentration stops changing and rate of reaction is zero (horizontal line)
What factors can change the rate of a chemical reaction?
Concentration (or pressure - when reactants are gases)
Temperature
Use of a catalyst
Surface area of solid reactants
Collision theory
Two reacting particles must collide for a reaction to occur - only a small number of collisions result in a chemical reaction, most of the time they just bounce off each other
When is a collision effective?
When the particles collide with the correct orientation and when particles have sufficient energy to overcome the activation energy barrier of the reaction
What does increasing concentration do?
Rate of reaction increases as there is an increase in the number of particles in the same volume - thus the particles are closer together and collide more frequently ; therefore there will be more effective collisions leading to an increased rate of reaction
How does increasing pressure affect rate of reaction?
Concentration of gas molecules increases as they are more crowded - thus more collisions leading to more effective collisions
How to follow the progress of a reaction?
Monitor decrease in concentration of reactant
Follow formation of a product (increase in concentration)
Reactions that produce gases?
Monitor volume of gas produced at regular time intervals
Monitor loss of mass of reactants using a balance
THESE ARE BOTH PROPORTIONAL TO THE CHANGE IN CONCENTRATION of a reactant or product ; both give a measure of rate of reaction
Practical to measure gas production?
Reactant put in a conical flask and bung is taken off
Initial volume of gas in measuring cylinder is recorded (MAKE SURE THERE ARE NO AIR BUBBLES BEFORE YOU START - water level should be close to zero)
Add catalyst or second reactant and then put on bung - which should have a delivery tube attached coming to the cylinder
Gas produced, water displaced, measure volume of gas produced at regular intervals until no more gas produced
OR YOU COULD USE A GAS SYRINGE (more precise scale)
What is done after collecting the gas?
Graph plotted to show volume of gas produced against time - to find initial rate of reaction a tangent is drawn at t=0 ; gradient gives reaction rate (cm^3/s)
Monitoring the loss of mass of reactants using a balance?
Rate of reaction can also be determined by monitoring the loss in mass of reactants over a period of time ; carbonate and the acid are added to a flask on a balance and then the mass is recorded initially and at regular time intervals - complete when no more mass is lost ; graph of mass lost against time is plotted (opposite to volume of gas produced and time)
What is a catalyst?
A substance that changes the rate of a chemical reaction without undergoing any permanent change itself - it is not used up in the reaction
Offers an alternative reaction pathway with a lower activation energy
DRAW EXO AND ENDO DISGRAMS WITH Ea and Ec
Lower activation energy
No change to energy change
Homogenous catalysts?
Catalysts that have the same physical state as the reactants - the catalyst reacts with the reactants to form an intermediate ; this then breaks down to give the product and regenerates the catalyst
Example of homogenous catalyst?
Ozone depletion - Cl. Radicals act as catalyst with the depletion of ozone (O3) into O2
Both gases
What happens if the reactants are different states?
BOTH reactants have to be the same state as the catalyst
Heterogenous catalyst?
Different physical state from the reactants - they are usually solids in contact with gaseous reactants or reactants in solution ; reactants are firstly adsorbed (weakly bonded) to the surface of the catalyst and after the reaction takes place, they leave the surface of the catalyst through desorption
2 reactions that involve heterogenous catalysts?
Haber process - make ammonia iron (s) is catalyst and the reactants are N2 and 3H2
Hydrogenation of Alkenes - reactants are gases with a solid nickel catalyst
What are catalytic converters?
Contain a catalyst like platinum/palladium where harmful gases are turned into less harmful ones (carbon monoxide oxidised to carbon dioxide and nitrogen monoxide to nitrogen) - they are non-toxic
EXPENSIVE HOWEVER
HONEYCOMB STRUCTURE TO INCREASE SURFACE AREA FOR HIGH RATE OF REACTION
Catalyst effects on industry?
Increases rate of reaction by lowering activation energy - reduces temperature needed for the process and energy requirements
Less electricity/fossil fuels used and this cuts costs - increasing profitability ; outweigh the costs in developing a catalytic process
Sustainability now means high atom economy and few pollutants - less fossil fuels = less emissions of CO2 (global warming)
Boltzmann distribution?
Exemplifies the spread of molecular energies in gases - some molecules move slowly with low energy and some molecules move fast with high energy ; most molecules move close to the average speed and have the average energy
LINE IS MARKED WITH Ea SHOWING THE ACTIVATION ENERGY AND THAT ONLY A SMALL PROPORTION OF THE MOLECULES HAVE MORE ENERGY THAN Ea TO REACT
Features of Boltzmann Distribution?
No molecules have zero energy (starts at origin)
X axis = energy
Y axis = number of particles with given energy
Area under graph = number of total particles
No maximum energy for a molecule - does not meet the x-axis at high energy and the curve would need to reach infinite energy to meet the x-axis
Effect of temperature on Boltzmann?
Average energy of molecules also increases - more molecules have higher energy ; graph is now stretched over a greater range of energy values and the peak of the graph is lower on y-axis but further along on x-axis
Number of molecules is the same so area is the same
Peak is at higher energy
