1.3 Rates of Reaction Flashcards
What factors affect rate?
Concentration
Surface area of reactants
Temperature
Catalysis
How does concentration affect rate?
- Increase in the concentration of a solution or the pressure of a gas, will result in an increase in the rate of reaction
- If the concentration of a solution or the pressure of a gas is increases, there are more particles in a given volume. Therefore there will be more collisions in a set amount of time and the probability of successful collisions becomes higher.
How does the surface area of reactants affect rate?
If the surface area of a solid is increases, there are more particles exposed to react. Therefore there will be more successful collisions in a set amount of time and the rate of reaction increases
How does temperature affect rate?
- The rates of most chemical reactions increase dramatically for only small increases in temperature
- For many gaseous reactions, the rate is approximately doubled by a rise in temp of about 10ºC
- An increase in temperature causes:
1. An increase in the average velocity of the particles, resulting in a greater number of collisions per second
2. An increased number of particles which possess the activation energy Ea.
Define activation energy (Ea)
The minimum amount of energy required for a reaction to occur.
Hoe do catalysts affect the rate?
Catalysts provide an alternative pathway with a lower activation energy, Therefore the proportion of particles with Ea is increased and the rate of reaction increases.
Define catalyst
a substance that increases the rate of a chemical reaction, without getting used up in the process
What are the two categories of catalyst?
homogenous and heterogenous
What is a homogenous catalyst?
catalyst is in the same physical state as the reactants and products
What is a heterogenous catalyst?
The catalyst is not in the same physical state as the reactants and products
Define rate of reaction
the change in concentration of a reactant or product with respect to time
What is the equation used to calculate rate?
Rate = change in concentration of reactant or product/time taken for change to take place
Describe the relative rate over the course of a normal reaction
Reactions are fastest at the start (greater likelihood of collisions), and get slower as the reactant concentration drops
What is the progressive rate method?
- Reaction is allowed to progress and take readings (absorbance/pH), or take samples at various times (sample should be quenched to stop reaction, and titrated.)
- Plot a graph of concentration against time for the reactant, shape of graph gives order with respect to the reactant monitored.
How do you quench a reaction?
Quenching can be carried out by rapid cooling, or adding large quantities of cold water/quenching chemicals
What is the initial rate method?
Method 1:
- Initial rate calculated by drawing a tangent to the curve at t=0 on the concentration/time graph and calculating the gradient
- As the reactants are consumed during the reaction, the rate and therefore gradient decreases
Method 2:
- Measure the time taken for a certain amount of product to be formed, eg. Clock reactions (measure the time it takes for eg. Appearance of a precipitate, disappearance in solid or change in colour)
- Initial rate taken as 1/t
- Measure the initial rate of several sections with varying initial concentrations. Plot graph and determine the order of reaction.
How do you calculate gradient?
Gradient = (y2 - y1)/(x2 - x1)
How can you measure the release of gas in a reaction?
A gas syringe can be used to measure gas volume (same as gas concentration)
The change in mass can be recorded using an electronic balance
How can you measure colour change in a reaction?
Colorimeter can be used to measure its concentration, as well as a calibration curve that relates absorbance values to known concentrations of solutions of the coloured substance
How can you measure concentration of H+ or OH- ions in a reaction?
- Change in pH measured by a pH meter
- Can be titrated with a standard solution of alkali for H+ ions or acid for OH- ions
What other reactions can titrations be used to measure that aren’t H+ or OH- reactions?
- iodine titrations
- eg. CH3COH3 + I2 -> ICH2COCH3 + H+ + I-
- Titrated in acidic conditions with sodium thiosulfate, using a starch indicator
- After titration, precipitate the iodide by asking acidified silver nitrate to form AgI(s) which can be dried and weighed
state the rate equation
Rate = k[A]^x[B]^y
define order of reaction
(with respect to a particular reactant) the power to which the concentration of a reactant is raised in the rate equation
define overall order of reaction
the sum of the powers to which the concentration terms are raised in the rate equation, ie. x + y
define rate constant
the proportionality constant that links the rate of the reaction to the concentrations in the rate equation
what does a zero order concentration-time graph look like?
straight line decrease or horizontal line
what does a first order concentration-time graph look like?
decreasing exponential
what does a second order concentration-time graph look like?
decreasing exponential (more extreme than first order)
what does a zero order rate-concentration graph look like?
horizontal line
what does a first order rate-concentration graph look like?
linear positive correlation (through 0)
what does a second order rate-concentration graph look like?
positive exponential through 0
What factors will affect the rate constant (k)?
temperature and adding a catalyst
how does temperature affect the rate constant (k)?
- Increasing temp gives more energy to the particles, resulting in more frequent collisions, and more collisions having sufficient activation energy. This leads to an increased rate of reaction
- If the rate increased with temperature when the concentrations remain the same, then therefore k increases with temperature
How does catalysis affect the rate constant (k)?
- Adding a catalyst lowers the activation energy at which a reaction can occur
- Adding a catalyst will lower Ea, which will increase the value of k.
- Some catalysts (eg. acidified reagents) may be involved in the mechanism at various steps, in which case the catalyst may appear in the rate equation.
define reaction mechanisms
a series of steps that together make up the overall reaction
define rate determining step
the slowest step in the mechanism of a reaction
What is the significance of the rate determining step?
- A reaction with 3+ reacting species is very likely to have multiple steps, as the probability of three different species colliding instantaneously and reacting is very small.
- How fast a reaction happens depends on the speed go the slowest step. In terms of reactions, only things which affect the slow step will have any significant effect on the rate of reaction.
- Therefore the rate determining step limits the overall rate of reaction.
- Any step that occurs after the RDS will not affect the rate, provided it is faster than the RDS. Therefore that species won’t appear in the rate equation.
- Any species before the RDS will appear in the rate equation.
give the general equation for the hydrolysis of alkyl halides
R-X + OH- -> R-OH + X-
what type of reaction is hydrolysis of an alkyl halide?
nucleophilic substitution
What type of hydrolysis do primary alkyl halides undergo?
SN2 hydrolysis
What type of hydrolysis do tertiary alkyl halides undergo?
SN1 hydrolysis
What is the rate equation for the reaction of primary alkyl halides?
Rate = k[R-X]OH-]
What is the rate equation for the reaction of tertiary alkyl halides?
Rate = k[RX]
Describe SN2 hydrolysis (in the case of hydrolysis of alkyl halides)
- lone pairs on OH- are attracted to delta +ve centre carbon in haloalkane (electrons in C go the Br)
- Transition state formed of OH group attached to the centre carbon with no other bond broken (negatively charged)
- Br leaves, forming an alcohol and free Br- ion
Describe SN1 hydrolysis (in the case of hydrolysis of alkyl halides)
- electrons go from carbon to bromine, causing it to leave (SLOW STEP)
- OH- is attracted to tertiary carbocation intermediate
- Forms alcohol and free Br- ion
What is the reactive intermediate in SN1 hydrolysis of an alkyl halide?
carbocation intermediate
What is transition state 1 in SN1 hydrolysis of an alkyl halide?
Br as a leaving group to the carbocation intermediate
What is transition state 2 in SN1 hydrolysis of an alkyl halide?
OH as a joining group to the carbocation intermediate
Define intermediate
a species formed in one step of a multistep reaction, that is used up in a subsequent step, and is not seen as either a reactant or product of the overall equation.
