3.2.2 Reaction rates
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what does the rate of a chemical reaction measure
• measures how fast a reactants is used up or how fast a product is formed
what can the rate of a reaction be defined as + give formula + units
• the change in concentration of a reaction or product in a given time • rate = change in conc / time • moldm-3 / s = moldm-3s-1
when is the rate of a reaction the fastest
• at the start of a reactions as each reactant is in its highest conc
why does the rate of reaction slow down as reaction progresses
• as reactants are being used up their conc decreases + rate of reaction slows
when is the rate of reaction zero
• when the reactant has been completely used up, the reaction stops + rate is zero
what are the factors that can change the rate of a chemical reaction
• concentration (or pressure is gases) • temperature • use of catalyst • surface area of solid reactants
what does the collision theory state
• two reacting particles must collide for a reaction to occur
what does an effective collision lead to
• leads to a chemical reaction
a collision will be effective if it meets 2 conditions:
• particles collide with correct orientation • particles have sufficient energy to overcome the activation energy barrier of the reaction
how does increasing concentration affect rate of reaction
• increase in conc causes increases in number of particles per unit volume > particles are closer together > leads to higher frequency of collisions > more successful collisions per unit of time • causes an increased rate of reactions
how does increases surface area of solids affect rate of reaction
• more reactant particles are exposed/available for collision > higher frequency of collisions > more successful collision per unit time • causes increased rate of reaction
how does increases temp affect rate of reaction
• higher temp means particles have higher kinetic energy > more particles have energy above the activation energy or > particles move faster > higher frequency of collisions > more successful collisions per unit time • increased rate of reaction
how does a catalyst affect rate of reaction
• provides an alternative route with lower activation energy • more particles have energy above the activation energy • more successful collisions per unit time > increased rate of reaction
how can a rate of reaction be measured
• using gas syringe • inverted measuring cylinder • change in mass
what is a catalyst
• a substance that speeds up the rate of a reaction by providing an alternative reaction pathways of lower activation energy > its not used up in the chemical reaction
catalysts aren’t used up in reactions but they may do what
• react with a reactant to form an intermediate in a multistep reaction but will be regenerated by the end of the reaction
what are homogenous catalysts + how do they work
• catalysts that has the same physical state as the reactants • catalyst reacts with reactants to form an intermediate which breaks down to give the product > catalyst is regenerated
what are heterogenous catalysts + how do they work
• catalysts that has different physical state from the reactants • they tend to be solids in contact with gaseous or solutions of reactants • reactant molecules are adsorbed onto surface of catalyst (weakly bonded), where reaction takes place > after reaction, product molecules leave surface of the catalyst by desorption
why does catalysis serve great economic importance
• products made faster (higher rate) + at lower cost (less energy) > this increases profitability
why is catalysis good for sustainability
• catalysts lower temp + energy needed for reactions by lowering activation energy • lower energy requirements mean less fossil fuels are used > this reduces CO2 emissions
what is the Boltzmann distribution
• the spread of molecular energies in gases
what does the area under the Boltzmann distribution represent
• the total number of molecules
why does the line of a Boltzmann distribution never meet the x-axis
• there is no maximum energy for a molecule
what does the shaded region of a Boltzmann distribution represent
• where the successful collisions + reactions take place > shows the molecules that have more energy than Ea, that is enough energy to react