Unit 4- How fast? Flashcards
Define the rate of reaction
The change in concentration over time with the units moldm-3s-1
Define the rate equation
An equation where the relationship between the concentration and rate is shown
Define the rate determining step
The slowest step of a reaction which controls the rate of reaction
Define heterogenous catalyst
catalyst which are in a different state to the reactant
define homogenous catalyst
catalyst which are in the same state as the reactants
How do catalyst work?
adsorbs the molecules on the surface to increase the rate of reaction. Normally a high surface area and a transitional metal used. Can become poisoned through impurities
What are the methods that can be used to measure the rate of reaction?
- Colorimetry
- conductimetry
- change in mass
- Volume produced
- Titrimetric analysis
How can measuring the volume of the gas produced be used to measure the rate of reaction?
- Simple but inaccurate method is to count the number of bubbles in a given time
- Measure the volume of the gas given through water displacement in a gas syrine or test tube. A graph can be produced of volume against time can show the change in rate
How to calculate the rate of reaction by measuring the change in mass?
- Measure the decrease in mass
- As the reaction proceeds
How to use colorimetry to calculate the rate of reaction?
- Involved when colour changes are used
- Can use simple observation or a colorimeter
- Appearance of colour or loss of colour is used
- Can change the reaction to measure the impact on rate
How can titrimetric analysis be used to measure the rate of reaction?
- remove aliquots of the reactive mixture at regular intervals
- Aliquots added to another reagent to quench the reaction or leave it in cold water to prevent the concentration changing
- Quenched aliquots titrated to find the concentration of known compounds
- If an acid is used, titrate can be carried out with sodium hydroxide
How is conductimetric analysis used to measure the rate of reaction?
- Number and type of ions can affect the elecrtrical conductivity
- A change in the ions will be reflected by a change in the conducitivty
- This is seen by changes in the ion
What are the reactants in the iodine clock reaction?
- hydrogen peroxide ( H2O2)
- I-
- H+
- I2
- S2O32-
What are the products in the iodine clock reactions?
- H2O
- I2
- I-
- S4O62-
How does the iodine clock reaction work?
- Potassium iodide is added to hydrogen peroxide
- Acidified dilute sulfuric acid, starch and sodium thiosulfate added
- Iodine produced which reacts with sodium thiosulphate to produce iodide ions
- When thiosulfate used up, the iodide reacts with starch to form a blue complex
- Excess can be used
What is the symbol equation for the iodine clock reaction?
- Step 1 - H2O2 + 2I- + 2H+ –> I2 + 2H2O
- Step 2- I2 + 2S2O32- –> 2I- + S4O62-
- Step 1 is the rate determining step
What is the rate of reaction proportional to?
eEa/RT
What is the rate of reaction equal to when referring to activation energy?
k=AeEa/RT
Where A is the arhennius equation, R is the gas constant, T is the absolute temperature
What is the equation when logarithm is used?
lnK = -Ea/RT
When an activation energy graph is drawn, what do the axis represent?
- Y axis is lnK
- x axis is 1/T
- Gradient is -Ea/R
- Y intercept is the arrhenius constant
When an SN1 reaction occurs, what does the rate equation look like?
- Only one substance involved in the rate determining step
- R = k[X] where X is a substance
- nucleophile not involved in the rate determining
What does the rate equation look like for an SN2 reaction look like?
- Both substances in the rate equation
- R=k[X][C] where X and C are two substances, the nucleophile generally involved in the rate determining step
If the order of reaction is a) 1 and b) 2 , what does the graph for half life like?
a) constant curve
b) as half life halves, the concentration doubles
if the order is a)0 b)1 c)2 what does the graph for concentration against time look life? what does the graph for rate against concentration look like?
a) for concentration against time - a y=x graph
for rate against concentration - straight line in x axis
b) for concentration against time- a curved graph
for rate against time - a y=x graph
c) for concentration against time - a steeper curved graph for rate against time- a curved graph
Explain the iodine-propanone reaction
- small aliquots removed
- sodiom hydrogencarbonate qunches the reaction
- aliquots titrated with standard solution of sodium thiosulfate -starch is indicator at end point
- rate=k[H][CH3COCH3]
- Iodine is 0 order therefore is in the reaction after RDS
- Excess proponone therefore concentration stays constant
- If experiment is repeated , use twice as much proponone, the slope is found to be twice as steep
Suggest the possible mechanism for the iodine- proanone reaction
- Protonation of Oxygen in propanone
- Electron in C=O shifts into carbon to form carbocation
- Carbocation loses proton
- CH bond broken
- iodine molecule acts as an electrophile
- electrophilic adddition occurs
what is the effect of temperature on rate?
increase in temperature increases the rate of reaction as the rate constant increases with temperature. Therefore smaller Ea means faster reaction
