Rates Flashcards
(52 cards)
1
Q
What is the rate of a reaction?
A
Change in concentration / change in time
2
Q
What is the order of a reaction?
A
- Changing the concentration often changes the rate of a reaction. The rate of a reaction is proportional to the concentration of a particular reactant raised to a power
- For each reactant the power is the order for that reactant. In a reaction, different reactants can have different orders and each may affect the rate in different ways
- zero order (0)
- frist order (1)
- second order (2)
3
Q
What is zero order?
A
- When the concentration fo a reactant has NO EFFECT on the rate, the reaction is zero order with respect to the reactant
1. Any number raised to the power of zero is 1
2. Concentration does not influence the rate
4
Q
What is first order?
A
- A reaction is first order with respect to a reactant then the rate depends on its concentration raised to the power of 1
1. If the concentration of A is doubled (x2), the reaction rate increases by a factor of 2^1 = 2
2. If the concentration of A is tripled (x3), the reaction rate increases by a factor of 3^1 = 3
5
Q
What is second order?
A
- A reaction is second order with respect to a reactant when the rate sends on its concentration raised to the power of two
1. If the concentration of A is doubled (x2), the reaction rate increases by a macro of 2^2 = 4
2. If the concentration of A is tripled (x3) the reaction rate increases by a factor of 3^2 = 9
6
Q
What is the rate equation and rate constant?
A
- The rate equations gives the mathematical relationship between the concentrations of the reactant and the reaction rate. For two reactants A and B, the rate equation is shown below
- The rate constant K is the proportionality constant. It is the number that mathematically converts between the rate of reaction and concentration and orders
7
Q
How do you figure out the overall order of the reaction?
A
- The overall order of reactions gives the overall effect of the concentrations of all the reactants on the rate of reaction
- Overall roder = sum of orders with respect to each reactant
8
Q
How do you work out the orders from experimental results?
A
- Orders of reaction must be determined experimentally by monitoring how a physical quantity changes over time
- Orders cannot be found directly from the chemical equation
- When comparing the effect of different concentrations of reactant on reaction rates, it is important that the rate is always measured after the same time, ideally as close to the start of the experiment as possible
- The initial rate is the instantaneous rate at the beginning of an experiment when t=0
9
Q
What is continuous monitoring of rate?
A
- Concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction and this is called continuous monitoring and different methods of continuous monitoring of reactions that produce gas as on of the products is:
- Monitoring by gas collection
- Monitoring by mass loss - Not all reactions produce gases so another property is needed that can be measured with time, and a useful property is a colour change which can be estimated by eye or monitored using a colorimeter
10
Q
How do you monitor with a colorimeter?
A
- In a colorimeter the wavelength of light passing through a coloured solution is controlled using a filter and the amount of light absorbed by a solution is measured
- A colorimeter measured the intensity of light passing through a sample The filter is chosen so that it is the complementary colour to the colour being absorbed in the reaction. Absorbance is recorded, which is directly linked to the concentration of the solution
11
Q
What are the shapes of different concentration time graphs?
A
- The gradient of a concentration-time graph is the rate of the reaction. The order with respect to a reactant can also be deduced from the shape of a concentration-time graph fro zero and first order reactions
- The order with respect to the reacts can only be obtained is all other recant concentration remain effectively constant
12
Q
What is a concentration-time graph for a zero order like?
A
- A zero order reaction produces a straight line with a negative gradient
- The reaction rate for not change at all during the course of the reaction
- The value of the gradient is equal to the rate constant, k
13
Q
What is a concentration-time graph for a first order like?
A
- A first order reaction produces a downward curve with a decreasing gradient over time
- As the gradient decreases with time, the reaction gradually slows down
- In a first order concentration-time graph, the time for the concentration of the react to halve is CONSTANT
- This time is called the half-life and the rate constant of a first order reaction can be determined using this value
14
Q
What is a concentration-time graph for a second order like?
A
- The graph for a second order is also a downward curve, (like first order), steeper at the start but tailing off more slowly
15
Q
What is the half life?
A
-The time taken for the concentration of a reptant to decreases to half of its original value
16
Q
What is a rate-concentration graph like?
A
- Rate concentration graphs can be plotted from measurements of the rate of reaction at different concentrations
- Rate concentration graphs are very important as they offer a route into the direct link between rate ad concentration is the rate equation
17
Q
What is a rate-concentration graph for a zero order like?
A
- A zero order reaction produces a horizontal straight-line with zero gradient
- The intercept on the y axis gives the rate constant, k
- The reaction rate does not change with increasing concentration
18
Q
What is a rate-concentration graph for a first order like?
A
- A first order reaction produces a straight-line graph through the origin
- Rate is directly proportional to concentration for a first order relationship
- The rate constant can be determined by measuring the gradient of the straight line of this graph
19
Q
What is a rate-concentration graph for a second order like?
A
- A second order reaction produces an upward curve with increasing gradient
- As this rate-concentration graph is a curve, the rate constant cannot be obtained directly from this graph
- By plotting a second graph of the rate against the concentration squared, the result is a straight line through the origin. The gradient of this straight line graph is equal to the rate constant k
20
Q
What is the initial rates method?
A
- The initial rate is the instantaneous rate at the start of the reaction when the time t=0
- The initial rate can be found by measuring the gradient of a tangent drawn at t=0 on a concentration-time graph
21
Q
What is a clock reaction?
A
- A clock reaction is a more conveneint way of obtaining the initial rate of a reaction by taking a single measurement
- The time t from the start of an experiment is measured for a visual change to be observed, often a colour precipitate
- Provided that there is no significant change in rate during this time, it can be assumed that the average rate of reaction over time will be the same as the initial rate
- The initial rate is then proportional to 1/t
- The clock reaction is repeated several times with different concentrations, the values of 1/t are calculated for each experimental run
22
Q
What is an iodine clock?
A
- A common type of clock reaction relies on the formation of iodine, as aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured
- Starch is usually added since it forms a complex with iodine which is a an intense blue-black colour
23
Q
How accurate are clock reactions?
A
- In a clock reaction you are measuring the average rate during the first part of the reaction
- Over this time, you can assume that the average rate of reaction is constant and is the same as the initial rate
- In a clock reaction, you are measuring an average rate of change in reactant over time
- The shorter the period of time over which an average rare is measured, the less the rate changes over that time period
24
Q
What are multi-step reactions?
A
- An overall chemical equation compares the reactants and products and the balancing numbers give the stoichiometry, the relative amounts of the species in the reaction
- A reaction can only take place when particles collide. For the reaction above to take place in a single step, one molecule of H2O2, two I- ions and two H+ ions would have to collide together simultaneously, which is an extremely unlikely event
- Such reactions are much more likely to take apple in a series of steps and it is unlikely that more than two particles will collide together at the same time
- The series of steps that make up an overall reaction is called the reaction mechanism
25
What is the rate determining step?
- The steps in a multi-step reaction will take place at different rates
- The slowest step in the sequence is called the rate determining step
26
How do you predict reaction mechanisms?
- Chemists use their knowledge and understanding of chemical principles to propose possible mechanisms for reactions
1. reactants invokes in the rate determining step are always present in the rate equation
2. The order of reaction tells us the number of species in the rate determining step
- So the rate determining step prices important evidence in supporting or rejecting a proposed reaction mechanisms
27
What is the effect of temperature on rate constants?
- As temperature increases, two factors contribute to the increased rate and rate constant
1. Increasing the temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy Ea
2. As the temperature increases, particles move faster and collide more frequently
- To react, particles must also collide with the correct orientation
3. With increasing temperature, the increased frequency of collisions is comparatively small compared with he increase in the proportion of molecules that exceed Ea from the shift in the Boltzmann distribution and so the change in rate is mainly determined by Ea
28
What happens as temperature increases?
-As temperature increases, the rate increases and revalue of the rate constant k will also increase and for many reactions each 10 degrees Celsius rise in temperature doubles the rate constant and doubles the rate of the reaction
29
What is the Arrhenius equation?
1. You have seen that rate constants can be determined experimentally. By carrying out the same experiment at different temperatures, rate constants can be calculated at different temperatures
2. The Arrhenius equation, is an exponential relationship between the rate constant k and temperature T
30
What is the effect of concentration on the rate of the reaction?
1. Greater number of particles in a certain volume
2. Particles collide more frequently
3. Greater chance of successful collisions
4. Rate of reaction increases
31
What is the effect of pressure on the rate of the reaction?
1. There are the same number of gas particles but in a. smaller volume
2. Particles collide more frequently
3. Greater chance of successful collisions
4. Increase rate of reaction
32
What are catalysts and how do you they affect the rate of the reaction?
- Provide an alternative pathway for the reaction with a lower activation energy and the catalyst is not used up in the reaction
- They increase the reaction rate without being used up by the overall reaction
- They allow the reaction to proceed via a different route with a lower activation energy
33
What is collision theory?
- In order for particles to react they must collide:
1. With a certain minimum energy known as the activation energy (the particle must have sufficient energy to overcome the activation energy barrier of the reaction)
2. In the correct orientation relative to each other
34
How does surface area affect the rate of the reaction?
1. Greater number of particles exposed
2. Reactant particles collide more frequently
3. Greater chance of (successful?) collision
4. Rate of reaction increases
35
What are the two main types of catalyst and what is a definition and example of both?
1. Heterogeneous: catalyst is in a different phase to the reactants (has a different physical state from the reactants)
E.G. Catalytic converters in cars use palladium to catalyse the reaction between carbon monoxide and carbon dioxide
-They are usually solids in contact with gaseous reactants or reactants in solution
-reactant molecules are adsorbed (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
2. Homogeneous: catalyst is in the same phase to the reactants (has the same physical sate as the reactants
E.G. aqueous iron cations catalyse the redox reaction between iodide ions and peroxydisulfate ions
-The catalyst reacts with the the reactant to form an intermediate
-The intermediate then breaks down to give the product and regenerates the catalyst
36
What does the rate of a chemical reaction measure?
- The rate of a chemical reaction measures how fast a reactant is being used up or how fast product is being formed
- The rate of a reaction can be defined as the change concentration of a reactant or a product in a given time
37
How does rate of reaction change over time?
1. The rate of a reaction is fastest at the start of the reaction, as each reactant is at its highest concentration
2. The rate of a reaction slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease
3. Once one of the reactants has been completely used up, the concentration stops changing and the rate of reaction is zero
38
What are the methods for following the progress of a reaction?
1. Monitoring the removal (decrease in concentration) of a reactant
2. Following the formation (increase in concentration) of a product
- The method chosen will depend on the properties and physical states of the reactant and products in the reaction. In addition to concentration, measurable properties that might change as the reaction proceeds include gas volume, mass of reactants or products, and colour
39
What do you do for reactions that produce gases?
- If a reaction produces a gas two methods than can be used to determine the rate of the reaction are
1. Monitoring the volume of gas produced at regular time intervals using gas collection
2. Monitoring the loss of mass of reactants using a balance
- Volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or product. So the change in volume with time or the mass loss with time both give a measure of the rate of reaction
40
How do you monitor the loss of mass of reactants using a balance?
1. The rate of reaction between calcium carbonate and hydrochloric acid can also be determined by monitoring the loss in mass of the reactants over a period of time
2. The carbonate and the acid are added Roa. conical flask on a balance
3. The mass of the flask and contents is recorded initially and at regular time intervals
4. The reaction is complete when no more gas is produced so no more mass is then lost
5. A graph of mass lost against time is plotted
41
What is a catalyst?
- A catalyst is a substance that change the rate of a chemical reaction without undergoing any permanent change itself
1. The catalyst is not used up in the chemical reaction
2. The catalyst may react with a reactant to form an intermediate or may prove a surface on which the reaction can take place
- A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway of lower activation energy
42
Describe the energy of moving particles
1. In a gas, a liquid or a solution, some molecules move slowly with low energy and some molecules move fast with high energy
2. Most molecules move close to the average speed and have close to the average energy
3. This spread of the molecular energy in gases is known as the Boltzmann distribtuon
- The graph is marked with a line Ea, that represents the activation energy of a reaction and only a small proportion of the molecules have more energy than Ea, that is enough energy to react
43
What are some of the features of the Boltzmann distribution?
1. No molecules have zero energy - the curve starts at the origin
2. The area under the curve is equal to the total number of molecules
3. There is no maximum energy for a molecule - the curve does not meet the x-axis at high energy. the curve would need to teach incite energy to meet the x - axis
44
What is the Boltzmann distribution like with temperature?
1. As the temperature increases, the average energy of the molecules also increases
2. A small proportion of the molecules will still have low energy, but more molecules have higher energy
3. The graph is now stretched over a greater range of energy values
4. The peak of the graph is lower not he y -axis and further along the x -axis - the peak is at a higher energy
5. The number of molecules is the same, so the area under the curve remains the same
45
What is the Boltzmann distribution like at higher temperatures?
- At higher temperature:
1. More Molecules age an energy greater than or equal to the activation energy
2. Therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
3. 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
46
What is the Boltzmann distribution like with catalysts?
1. A catalyst provide an alternative reaction route with a lower activation energy (Ec on the graph)
2. Compared to Ea, a greater privation of molecules now have an energy equal to, or greater than the lower activation energy, Ec
3. On collision, more molecules will react to form products. The result is an increase in the rate of reaction
47
How do you work out rate on a first order concentration time graph?
Change in y / change in x
48
What are the two ways to work out k on a first order concentration time graph?
1. K = rate / [A]
| 2. K = ln2 / t1/2
49
What is the Arrhenius equation? What is the graph?
K = Ae^-Ea / RT
- Where A is the prexponential factor (frequency of collisions)
- Ea is the activation energy (Jmol^-1)
- R is the gas constant (Jmol-1K-1)
- T is temperature in Kelvin
2. Plot lnK against 1/t where -Ea/R will be the gradient and lnA will be the y intercept
50
How do you seduce a reaction mechanism form a rate equation and an overall equation?
1. Make step one the rate determining step
2. All reactants present in the rate equation go in the rate determining step
3. Make one of the products and an intermediate in the first step
4. In the second step use up the intermediate and any remaining reactants that are not present in the rate equation but are involved in the overall equation. This will hopefully leave you with the products!
51
How do you determine the activation energy of the reaction graphically?
1. Measure the rate of reaction at a range of different temperatures
2. Calculate the natural logarithm if each rate to give ln(rate)
3. Convert each C degrees temperature to Kelvin by adding 273 and then finding 1/T (K^-1)
4. Plot a graph of ln(rate) (on the y axis) against 1/T (on the x axis) and draw a kind of best fit
5. Usually ln(rate) will have negative values, so the x acid will be across the top of the graph paper
52
How do you find the initial rate of reaction?
1. Find gradient at t=0 of concentration-time graph
