reaction rates and equilibrium

how do rates of reaction differ?

some happen very fast, while other reactions take ages

what does the rate of a reaction do?

measures how fast a reactant is being used up/ how fast a product is being formed

what is the rate of a reaction?

the change in concentration of a reactant or product in a given time

rate =?

change in concentration/ time

units of rate?

when and why is rate of a reaction fastest?

at the start of the reaction, as each reactant is at its highest concentration

when and why does the rate of a reaction slow down?

as the reaction proceeds, as the reactants are being used up and their concentrations decrease

when is the rate of a reaction zero?

once one of the reactants have been completely used up so the concentrations stop changing

concentration time graph example:

• shows the formation of a product over the course of a chemical reaction

what are the factors that 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

what is collision theory?

states that 2 reacting particles must collide for a reaction to occur. usually only a small number of collisions start a chemical reaction

what happens in most collisions?

the molecules collide but then bounce off each other and remain chemically unchanged

what are the 2 conditions that need to be met for a collision to be effective?

• the particles collide with the correct orientation

• the particles have sufficient energy to overcome the activation energy barrier of the reaction

how does increasing the concentration affect the rate of reaction?

• an increase in conc increases the number of particles in the same volume

• the particles are then closer together and collide more frequently

• there will therefore be more effective collisions in a given period of time and an increased rate

how does increasing the pressure of a gas affect the rate of reaction?

• the conc of the gas molecules increases as the same number of gas molecules occupy a smaller volume

• the gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time

how can the progress of a chemical reaction be followed?

• monitoring the removal (decrease in conc) of a reactant

• following the formation (increase in conc) of a product

what will the method chosen depend on?

the properties and physical states of the reactants and products in the reaction

what are other measurable properties that can be used to measure rate?

gas volume, mass of reactants/ products, colour

how can you determine the rate of a reaction if the reaction produces a gas?

• monitor the volume of gas produced at regular time intervals using gas collection

• monitor the loss of mass of reactants using a balance

what are volume of gas produced and mass loss both proportional to?

the change in concentration of a reactant/ a product. this means that the change in volume with time/ mass loss with time both give a measure of the rate of a reaction

monitoring the production of gas

eg. how can the rate of reaction between calcium carbonate and hydrochloric acid also be determined?

by monitoring the loss in mass of the reactants over time

equation for the reaction:

how does this work?

• add carbonate and acid to conical flask on a balance

• mass of flask and content recorded initially and at regular time intervals

• the reaction is complete when no more gas is produced so no more mass is then lost

• you can then plot a graph of mass lost against time

worked example:

what is a catalyst?

a substance that changes the rate of a chemical reaction without undergoing an permanent change itself

features of catalysts:

• the catalyst isnt used up in the reaction

• the catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place

• at the end of the reaction the catalyst is regenerated

how does a catalyst increase the rate of a chemical reaction?

it provides an alternative reaction pathway with a lower activation energy

enthalpy profile diagrams for catalysts in endothermic and exothermic reactions:

what is a homogeneous catalyst?

a catalyst with the same physical state as the reactants to form an intermediate. the intermediate then breaks down to give the product and regenerates the catalyst

one example of the many reactions of gases and liquids that use homogeneous catalysts:

another example of the many reactions of gases and liquids that use homogeneous catalysts:

what is a heterogeneous catalyst?

a catalyst that has a different physical state from the reactants. they are usually solids in contact with gaseous reactants/ reactants in solution

what happens to the reactant molecules?

they are absorbed (weakly bonded) onto the surface of the catalyst where the reaction takes place. after reaction, the product molecules leave the surface of the catalyst by desorption

some of the common industrial processes that use heterogeneous catalysts:

extra info: heterogeneous catalysts and atmospheric pollution:

what is estimated?

90% of all chemical materials are produced using a catalysts, which increase the rate of many industrial chemical reactions by lowering the activation energy. this then reduces the temp needed for the process and energy requirements

whats the case if a chemical process requires less energy?

then less electricity or fossil fuels are used

what does making the product faster and using less energy do do?

cut costs and increase profitability. therefore the economic advantages of using a catalyst outweigh any costs associated with developing a catalytic process

what does the modern focus on sustainability require?

industry to operate processes with high atom economies and fewer pollutants. using less fossil fuel will cut emissions of CO2, a gas linked to global warming

extra info: autocatalysis:

What did we learn about molecules in a gas and collisions at GCSE?

• molecules in a gas move at a high speed, colliding with each other and with the walls of the container they are held in

• The collisions are said to be elastic - the molecules dont slow down as a result of a collision and no energy is lost

what is the case with different molecules in the different states?

some molecules move slowly with low energy and some molecules move fast with a high energy

what do most molecules do?

move close to the average speed and have close to the average energy

What is the spread of molecular energies in gases known as?

the Boltzmann distribution

diagram to show Boltzmann distribution of molecular energies:

What does E_a stand for?

Activation energy - you can see from the diagram that only a small proportion of the molecules have more energy than Ea, that is enough energy to react

feature of the Boltzmann distribution:

• no molecules have zero energy - the curve starts at the origin

• The area under the curve is equal to the total number of molecules

• There is no maximum energy for a molecule - the curve doesn’t meet the x axis at high energy. The curve would need to reach infinite energy to meet the x axis

the effect of temperature on a Boltzmann distribution curve:

• as temp increases, the average energy of the molecules also increases

• A small proportion of the molecules will still have low energy, but more molecules have higher energy

• The graph is therefore stretched over a greater range of energy values

• The peak of the graph is lower on the y axis and further along the x axis - the peak is at a higher energy

• The number of molecules is the same, so the area under the curve remains the same

At a higher temperature:

• more molecules have an energy greater than or equal to the activation energy

• Therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction

• Collisions will also be more frequent as the molecules are moving faster, but the increased energy of the molecules is much more important than the increased frequency of collisions

the effect of a catalyst on activation energy:

• a catalyst provides an alternative reaction route with a lower activation energy

• Compared to activation energy, a greater proportion of molecules now have an energy equal to or greater than the lower activation energy

• In collision, more molecules will react to form products. The result is an increase in the rate of reaction