U2.2: How Fast? Flashcards
What are the conditions for particles to react
- Sufficient collision energy
- Correct orientation
State what the 5 factors affecting rate of reactions are
- Catalysts
- Pressure
- Temperature
- Concentration of substrate
- Surface Area
Explain how catalysts affect the rate of reactions
- Catalyst: Provides an alternate pathway with lower EA, more particles having greater collision energy than EA, increasing frequency of successful collisions between substrate particles
- Therefore, increasing rate of reaction
Explain how Surface Area affects the rate of reactions
- SA: As SA increases, Rate of reaction Increases as well as frequency of successful collisions between substrate particles using the same mass of solid
Explain how Temperature affects the rate of reactions
- Temperature: As temperature increases, there is increased kinetic energy of substrate particles, increased collision energy of substrate particles, greater number of particles have collision energy greater than EA thus, increased frequency of successful collisions between substrate particles therefore, rate of reaction increases
Explain how Concentration of Substrate affects the rate of reactions
- Concentration of S: As concentration of S increases, there is Increased number of substrate particles in the same volume, Increased frequency of successful collisions between substrate particles in the same volume
- Thus, rate of reaction increases
Explain how Pressure affect the rate of reactions
- Pressure (only gases): As pressure increases, decreased distance between substrate particles, increases the frequency of successful collisions between substrate particles. (This can be affected by decreasing the volume or increasing concentration of substrate
- Thus, increasing the rate of reaction
Does pressure affect liquids?
Pressure only affects gases because the particles are unable to get closer together.
How can pressure be changed
- decreasing volume
- increasing concentration of substrate particles
Outline the collision theory
- As temperature increases
- KE of particles increases
- Collision energy increases
- Collision energy gets greater than activation energy
- Frequency of collisions increases
- Probability of reaction between particles increases
Explain what the Rate of Reaction progress graph shows
- Graph levelling off: All substrates have been used up
- Gradient at the start: Reaction is the fastest
- Gradient declining: Concentration of substrates decreasing thus, reaction slows down
- Y axis: Concentration of substrate
- X axis: Time
What condition will give the fastest rate of reaction
- high concentration, low volume
- High temperature, low concentration
Using a catalyst, is it an endothermic reaction or exothermic reaction
exothermic reaction
For temperature, what is the rate of reaction formula
- Collision frequency x Fraction of successful collisions
For temperature, how does the graph look
- Two lines, one at high temperature and the other at low temperature
- X axis is time in seconds
- Y axis is concentration of substrate
- Low temperature is lower and levels of later than high temperature
For concentration, what is the rate of reaction formula
- Collusion frequency x Fraction of successful collisions
For concentration, when looking at HCl and CaCO3 graphs, what does the graph look like when limiting reactant is HCl
- X axis is time in s
- Y axis is volume of CO2 in cm^3
- Low conc and high conc line where low conc line levels off quicker and is lower than high con. line
- Both CO2 Yield and Rate of reaction increases
For concentration, when looking at HCl and CaCO3 graphs, what does the graph look like when limiting reactant is CaCO3
- X axis is time in s
- Y axis is volume of CO2 in cm^3
- Low conc and high conc line. low conc line is steeper than high conc line. Low conc line is lower than high conc line. High conc. line levels of earlier than low conc line. Both lines level off at the same volume of CO2
- Only Rate of reaction increases
Define what a catalyst is and its properties
- DEF: Increases rate of reaction without being used up
- Provides an alternative reaction pathway which is lower in activation energy
State 3 benefits of catalysts
- Low temperature and pressure required which reduces energy demand
- Different reactions can be used with a better atom economy which reduces waste
- Low production costs
Define Temperature
- A measure of av. KE of particles in a substance
Draw and Explain Maxwell-Boltzman Graph for catalysts
- Graph starts at zero
- Curved line going up then down but never touches the x-axis
- Area under the graph represents the number of particles
- EC (activation energy with catalyst) is drawn towards origin and vertical line drawn upwards shows a greater shaded region
- X-axis is energy
- Y-axis is number of particles
Define Activation Energy
The minimum energy required for a reaction to start
Name 4 methods of measuring the rate of a chemical reaction
- Change in volume of gas
- Change in Mass of reaction mixture (mass decreases as gas escapes)
- Time taken for the reactant solid to disappear; This gives values at any given point in time (instantaneous)
- Change in pH of solution
Draw and Explain the Maxwell Boltzmann graph for temperature
- starts from 0
- peak of T1 graph comes first then T2 graph peak comes
- T2 graph levels of higher than T1 graph
- Graph starts at 0
- The peak represents the max # of molecules that are having the stated energy
- T2 represents molecules that have more energy
THE NUMBER OF PARTICLES STAYS THE SAME
What is true about EA and it being affected?
Activation energy can’t be affected by temperature
What is meant by finding the instantaneous rate of reaction
Finding the rate at a specific point
What is true about the activation energy in an exothermic reaction
- The difference in the potential energy between the reactants and products
What are the 2 types of rates and how to measure the rate
- Average Rate: Total / Total Time Taken
- Instantaneous Rate: Plot the point at a specific time and mass then draw a tangent
Rate Law equation? and units
Rate = k[A]^a[B}^b
- k: rate constant
- rate: mol dm^-3 d^-1
- [conc]: mol dm^-3
- a: order of reaction w respect to A (same with b)
Define rate
Change in concentration over time
How is the order determined generally? What is another name for the overall order
- Experimentally: a+b (sum of all orders of reactants)
- Another name: “Molecularity”
Formula to find k?
k = (mol dm^-3)^1-n s^-1
- n: order number (1st, 2nd, 3rd order etc)
What is true about zero order reactions
- Rate will be equal to k as concentration doesn’t change (stays 1)
What are the steps in step equations added up all known as
Reaction mechanism
How to know which step to use in the reaction?
- Decide which is the slowest (takes the longest time) or requires the greatest Ea
- This is known as the RDS (Rate Determining Step)
Why is the RDS the slowest step?
The step delays the reaction
Draw energy profile diagram for a second order reaction
- Products more stable than reactants
- X-axis is reaction coordinate (seconds)
- Y-axis is Potential Energy (kJ)
- Two peaks drawn (like a Mcd Logo)
What is the intermediate and how to show this on an energy profile diagram? What is a feature of the intermediate?
- The products of the 1st step but used in the reactants in the 2nd step are known as the intermediate
- Shown as a temporary reactant higher than the “Reactants” base but it separates the two peaks
- The intermediate only remains stable for a short time as it’s used up
To get overall reaction, what happens to the intermediate when adding 2 steps together
- Intermediate cancels out as it is always used up
- This means when calculating the rate equation, the intermediate gets replaced by the reactant from the other reaction (which is in its place)
” Explain why step z is the RDS “ (modal answer)
- Rate equation involves x molecules of A and y molecules of B
- Thus, step z involves the same reactants as in rate equation
How to know if a substance is a catalysts from rate equations?
- Used in one step and produced in another
What is the initial rate method (graph and how to find instantaneous data)
- Y-axis Vol. of CO2 in cm^3
- X-axis Time (sec)
- Draw a linear gradient starting at 0 and this is the slope starting at point 0. Measure slope and values given from y-axis
How to determine concentration from a rate equation given concentration of two reactants and the rate? Provide an example
Finding concentration of B:
- See where the conc. of other reactant is kept constant
- Divide 2nd conc. of B with first then put to the power of n
- Divide the 2nd rate of B with first and equate to value for conc. to the power of n
- Find n and that is the order of B
NOTE: IF RATE AND CONC. MULTIPLIER IS SAME, IT IS A 1ST ORDER REACTION
Given concentration of two reactants and the rate, how to find if it is a Zero-Order Reaction
- Rate doesn’t change whilst the concentration of an element changes
Define “Order”
The power raised to the concentration of reactants in reaction
State the logarithmic Arrhenius Equation and the units
ln k = - Ea / RT + ln A
Where:
- A is Arrhenius constant related to the collision freq. and orientation of moles
- R is the gas constant (8.31 J K^-1 mol ^-1)
- k is the rate constant
- Ea is the activation energy in J mol^-1
- T is temperature in Kelvin
Explain how the Arrhenius equation is a straight line
- ln k = -Ea/R (1/T) + lnA
- y = mx + c
Where:
- -Ea/R(1/T) is the downwards slope
- ln k is y
- ln A is the y-intercept (c)
- X-AXIS IS 1/T
What is true about temperature dependence graph
- Rate doubles with a temp. increase of around 10*C
- Y-AXIS IS RELATIVE REACTION RATE
- X-AXIS IS TEMP (*C)
- straight line (not touching x-axis) up to 10*C then upwards exponential curve
Outline how the concentration v.s time graph looks like
- X-axis: Time (s)
- Y-axis: Conc. (mol dm^-3)
- Rate order 0 = straight line down (Demand curve)
- Rate order 1 and 2 = Exponential decrease (higher than rate order 2 - less step than rate order 2)
Outline how the rate v.s concentration graph looks like
- X-axis: Conc. (mol dm^-3)
- Y-axis: Rate
- Rate order 0 = straight line
- Rate order 1: Directly proportional (y-x line)
- Rate order 2: Exponential increase
Half life (t 1/2) definition
The time taken for the concentration of a reactant to become half of its initial value (mass, moles etc)
Describe the graphs for zero order, 1st order and 2nd order reactions in terms of how the length of half-life is affected. State what the x-axis and y-axis are too
- X-axis is time
- Y-axis is concentration
- 0 order: Length of half life decreases (directly proportional and negative slope)
- 1st order: Length of half life stays constant (sloping down)
- 2nd order: Length of half life increases (sloping even further down
State what “A” stands for in the Arrhenius Equation
This is a freq. factor telling us the number of molecular collisions per second w/ proper orientation
Explain why a single step mechanism is unlikely for a reaction of this kind
- Reaction involves x number of molecules
- Therefore, it is statistically unlikely
state the relationship between the rate constant and the activation energy
inversely proportional
