Chapter 16 Flashcards
What is the equation of the rate of reaction?
Change in concentration of prodcut / time
What is the reaction rate?
The change in the amount of reactants or products per unit time
How can you measure reaction rate using gas volume?
If a gas is given off, you can collect it in a gas syringe and record how much you have at regular time interva;s
When reacting an acid with a carbonate, which method can you use to measure the reaction rate and why?
You can measure the amount of volume of a gas given off as carbon dioxide is given off
If a gas is given off, what will happen to the mass of a system?
It will decrease
How can you measure reaction rate using mass?
If a gas is given off, you can measure how the mass changes in a given time period
What does a calorimeter do?
It measures absorbance
What is absorbance a measure off?
The amount of light given off by a system
How can you measure reaction rate using colour change?
You can track colour change using a calorimeter. You can plot a graph of concentration of e.g.iodine against absorbance value.
How can you measure reaction rate using a change in pH?
If H+ ions are given off, you can measure the pH of the solution at regular intervals
How can you measure reaction rate using a titration?
You can take small samples of a reaction at regular time intervals and titrate them using a standard solution. The rate can be found from measuring the change in concentration over time.
How can you measure reaction rate using electrical conductivity?
If the number of ions changes, so will the electrical conductivity
How can you work out the reaction rate from any point on a concentration-time graph?
Draw a tangent at time t and calculate the gradient
What do orders tell you?
How a reactants concentration affects the rate
What does the order of reaction with respect to a particular reactant tell you?
How the reactants concentration affects the rate
Where X is the concentration of a particular reactant, if X doubles and the rate stays the same, what will happen to the rate if X triples?
The rate will stay the same
If the rate is proportional to the (X), what is the order of reaction with respect to X?
First order
If the rate is proportional to (X squared), what is the order of reaction with respect to X?
Second order
What is the overall order of reaction?
The sum of the orders of all the reactants
What is the half-life?
The time it takes for half of the reactant to be used up
If the half-life is constant in a concentration-time graph, what order of reaction is the reaction?
The reaction is first order
What is the initial rate of reaction?
The rate of reaction at the start of the reaction
Describe how you can determine the order of reaction using the initial-rates method:
1-Carrying out seperate equations using different inital concentrations of one reactant, and seeing how the initial concentration affects the rate
2-You can figure out which order it is from this
What do you do in a clock reaction?
You measure how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants
In a clock reaction, as a limiting reactant is used up, what will there be a sudden increase in?
The concentration of a certain product
In a clock reaction, how do you usually know when the desired amount of a product is formed?
There is usually and observable end point
What is the relationship between how quick a clock reaction occurs and the rate of reaction?
The quicker the clock reaction finishes, the faster the rate of reaction
What three assumptions do you have to make when performing a clock reaction?
- Temp stays constant
- Conc of each reactant doesn’t change significantly
- When the end point is seen, the reaction has not proceeded too far
In an iodine clock reaction, what is the reaction you are monitoring?
H2O2 + 2I- + H+ —> 2H20 + I2
When does starch turn to a blue-black colour?
In the presence of iodine
When sodium thiosulfate is added to the iodine clock reaction mixture, what does it do?
It reacts instantaneously with any iodine that forms
What happens in the clock reaction when all the sodium thiosulfate is used up?
Any more iodine that forms will stay in solution, so the starch indicator will suddenly turn blue/black, marking the end of the clock reaction
How can the initial rate of the iodination of propanone be found??
You can monitor the reaction by taking samples at regular intervals. Firstly stop the reaction each time by adding sodium hydrocarbonate and neutralise the acid, then titrate each against sodium thiosulfate and starch to work out the concentration of iodine
What is k?
The rate constant
What does a large rate constant mean?
A high reaction rate
For a general reaction, A + C -> C + D, what is the rate equation?
Rate = k (A)^x (B)^y
In the rate equation, what do x and y represent?
The orders of reaction
When NO reacts with CO and O2, the reaction is second order with respect to NO and zero order with respect to CO and O2. What is the rate equation for the reaction?
rate = k (NO)^2
What is the rate determining step?
The slowest step in a multi-step reaction
What is the overall rate decided by?
The slowest rate-the rate determining step
What is the rate determining step also known as?
The rate limiting step
If a reactant appears in the rate equation, what must this mean?
It must affect the rate
If a reactant appears in the rate equation, is it involved in the rate determining step?
Yes, or something derived from it
If a reactant doesn’t appear in the rate equation, is it involved in the rate determining step?
No
What is nucleophilic substitution?
When a nucleophile attacks another molecule and is swapped for one of the attached groups
Why is the carbon-halogen bond polar?
Because most halogens are much more electronegative than the carbon atom
As the carbon atom is partially positive, what does this mean?
It is easily attacked by nucleophiles
In the Sn1 mechanism, how many molecules/ions are involved in the rate determining step?
1
In the Sn2 mechanism, how many molecules/ions are involved in the rate determining step?
1 molecule and 1 ion, or 2 ions in the rate determining step
What type of nucleophilic substitution mechanism can primary halogenoalkanes react by?
Only the Sn2 mechanism
What type of nucleophilic substitution mechanism can secondary halogenoalkanes react by?
Both the Sn1 and Sn2 mechanism
What type of nucleophilic substitution mechanism can tertiary halogenoalkanes react by?
Only the Sn1 mechanism
For the Sn2 mechanism, what does the rate equation involve?
Both reactants, for example (CH3CH2Br)(OH-)
In an Sn2 mechanism, what does the rate of the reaction depend on?
The concentration of each of the reactants
In an Sn1 mechanism, what is the rate of reaction dependant on?
The rate is only dependant concentration of the halogenoalkane. e.g. r = k(CH3Br)
In the Sn1 mechanism, the two step process, which step is the rate-determining step?
The first step
What does the Arrhenius equation link?
The rate constant(k) with the activation energy and temperature
As the activation energy gets bigger, what happens to the rate constant?
The rate constant gets smaller
What is the link between the size of the rate constant and the rate of reaction?
The larger the rate constant, the faster the rate of reaction
What is the Arrhenius equation?
k = Ae^(-Ea / RT)
What does lnk equal?
lnA -Ea/RT
When you plot lnk against 1/T, what is the gradient?
-Ea / R
When you plot lnk against 1/T, what does the gradient X 8.31 equal?
The acitvation energy
Why do we use catalysts?
In order to increase the rate of reaction, by providing an alternative pathway with lower activation energy
What are the 2 ways a catalyst can be classified as?
A homogenous catalsyst and a heterogenous catalyst
What are homogenous catalysts?
Catalysts which are in the same state as the reactant
What are heterogenous catalysts?
Catalysts that are in a different state from the reactants
What does a solid heterogenous catalyst do?
It provides a surface for the reaction to take place on
Can heterogenous catalysts be separated from the products and leftover reactants?
Yes, easily
How can a heterogenous catalyst be poisined?
The poisinous substance clings onto the catalysts surface more strongly than the reactant does, preventing the catalyst from getting involved in the reaction
Give an example of when a catalyst can be poisined?
Sulfur can poisin the iron catalyst used in the Haber Process
