rates of chemical reactions AOS2 Flashcards
what information does a chemical eq give?
the chemical equation for a reaction indicates the nature of the reactants + products but it provides no info about the way in which the reaction proceeds (speed + intensity + how products form)
how is the rate of reaction calculated?
found by change in concentration/time (in seconds), the usual unit for rate of reaction is moles per litre per second
how to experimental determine the rate of a reaction?
measure how much of a reactant is being used up or how much product is being formed in a given time period this could possibly involve measuring changes in gas volume/mass over time (for gases) + changes in pH or of colour which could be used through colorimetry
what does a steeper initial gradient mean on a graph?
it indicates that the initial rate of production of a gas (or other substance) is greater than the rate of reaction later in the experiment. the greater the gradient of the tangent, the faster the reaction rate and the point which acts as the maximum gradient also shows the maximum rate
how can changes in gradient be found on graphs?
changes in the rate of reaction can be found by drawing gradients of graphs of the change in mass/volume of a reacting mixture over time
what’s the collision theory ->
background info on why collisions occur in reactions?
chemical reactions occur as a result of collisions between the reacting particles, this idea is part of the collision theory of reaction rates. during chemical reactions, particles (atoms, molecules, ions) collide + are rearranged to produce new particles and in a reaction in order to form the products the (covalent?) bonds within molecules must be broken which also requires energy
the principles of the collision theory?
according to collision theory for a reaction to occur the reactant + product must collide with each other, collide with sufficient energy to break the bonds within the reactants and collide with the correct orientation to break the bonds within the reactants which allows for the formation of new products. IF A COLLISION DOESN’T MEET ALL OF THE REQUIREMENTS, THEN NO REACTION OCCURS
why does a reaction require activation energy?
for a reaction to occur between reactant molecules, the molecules must collide with a certain minimum amount of energy, unless the min amount of energy is met/exceeded the colliding molecules will rebound and simply move away from each other without reacting
what is activation energy?
it is the minimum energy that collision must possess for a reaction to occur, when the energy of a collision is greater than/equal to the activation energy can occur. activation energy (Ea) can be represented on an energy profile diagram
what’s an energy profile diagram?
activation energy can be represented on an energy profile diagram which represents the potential energies of the reactants + the products over the course of the reaction. the energy change during a reaction is represented on an energy profile diagram as delta H which is the difference in energy between the reactants + the products. the peak on an energy profile diagram represents the activation energy which is the min energy that must be absorbs to break the bonds of reactions so that a chemical reaction can progress. the activation energy is measured form the energy of the reactants to the top of the peak.
what’s an exothermic reaction?
this reaction releases more heat energy during the reaction than it absorbs so they consequently result in an increase in the temp of the surroundings (up then down)
what’s an endothermic reaction?
it absorbs more heat energy during the reaction that it releases so the temp of the surroundings decreases (down then up)
what’s the transition state?
this occurs once the activation energy is absorbs which prompts a new arrangement of the atoms to occur which occurs at the stage of max potential energy in the reaction: the activation energy. bond-breaking + bond-forming are both occurring at the stage and the arrangement of atoms is unstable hence the atoms in the transition state rearrange into the products as the reaction progresses
what does the magnitude of the activation energy determine?
it determines how easy it is or a reaction to occur and therefore what proportion of collisions result in a successful reaction. for this reason the reaction rate is dependent upon the activation energy, the resistance of an activation energy for a reaction means that collisions between reactants don’t always result in a chemical change
why is the orientation of colliding molecules important?
for a reaction to occur, reactants need to collide with enough energy to provide the activation energy. reacting molecules must also collide with each other in the correct orientation in such a way that particular bonds in the reactants are broken + new bonds are formed in the products. if the collision orientation is incorrect, the particles simply bounce off each other so no reaction occurs. so if there is favourable orientant then a reaction is likely
what change can alter the rate of a reaction?
a change in surface area of a solid reactant, concentration of reactants in a solution, pressure of any gaseous reactants, temperature of the reaction, presence of a catalyst
how much energy does colliding particles have in any given reaction mixture?
in any given reaction mixture, only a certain proportion of collision between reactant particles have energy that is greater than or equal to the activation energy and the correct orientation for reaction (like ones that are successful)
what to increase in order to increase the rate of the reaction?
the frequency of successful collisions by increasing the number of collisions that can occur in a given time and the proportion of collisions that have energy that is greater than or equal to the activation energy by increasing the energy of all collisions
what does a successful collision look like?
in a successful collision, the energy must be equal to or greater than the activation energy and the reactants must collide with the correct orientation to break their bonds, allowing the formation of new products
how to increase the frequency of collisions?
the rate of reaction increases as the frequency of collisions increases. furthermore, this can occur through increasing the concentration or pressure of the reactants (since collisions occur more frequently when particles are closer together) and through increasing the surface area of a solid reactant
what is the link between rate of reaction and increasing concentration?
the rate of a reaction increases when the frequency of collisions between reactants increases. when the concentration of a solution increases, there are more reactant particles moving randomly in a given volume of solution hence the frequency of collisions + successful collisions increases
what is the link between rate of reaction and increasing pressure?
for a reaction in the gas phase, the pressure of the gases can be increased either by adding more of the reacting gas to a fixed-volume container or by decreasing the volume of a container with a variable volume, like a gas syringe. increasing the pressure increases the concentration of gas molecules, causing more frequent collisions and increasing the number of successful collisions (frequency of collisions with energy equal/greater than the activation energy) in a given time
impact of increasing surface area to the rate of a reaction?
a greater number of exposed solid reactant particles results in a greater frequency of collisions between reactant particles therefore the rate of the reaction increases as well
what is the surface area of solids?
when a solid is involved in a reaction, only the particles at the surface of the solid participate in the reaction. the number of particles at the surface depend on the SA of the substance
