EXPERIMENT: Factors Affecting Rate of Reaction
Reaction Equation: Replacement
Zn(s) + 2 HCl (aq) à ZnCl2(aq) + H2 (g)
Mg(s) + 2 HCl (aq) à MgCl2(aq) + H2 (g)
Net Ionic Equation:
Problems:
To properly design and perform experiments that demonstrate the factors that affect the rate of this reaction. (stoichiometry calculations using 0.01 mol metal with 2 times the needed acid to provide excess))
To fully explain as part of your conclusions how varying these factors affected the reaction rate ( on the particle level)
To pay attention to the controls and the variable in these experiments
To estimate the rate of increase in the reaction rate due to each factor (variable). i.e. 2x 2 times the rate
Record the times
Materials Available:
Mg and Zn in various sizes
HCl solutions of various concentrations
glassware and thermometers
Rates of Chemical Reactions
Not all matter reacts at the same rate
ex. Newspaper yellows ( slow oxidation ) over a number of years.
Newspaper burns ( rapid oxidation ) in a few seconds
Conditions Necessary for a Chemical Reaction to occur:
1.
2.
3.
Explanation of the Factors Affecting Rate of Reaction
1) Chemical Nature of the Reactants
Therefore the reaction rate will depend on:
The Forces of AttractionRepulsive Forces
Therefore the rate of reaction decreases as the force of attraction within the reactants increases because :
2) Ability of the Reactants to Come Into Contact with Each Other
Collision Theory
Reactant molecules translating toward each other will gradually slow down as the repulsive forces increase ( p-p or e-e ). The molecule then collides with the other reacting molecule. The molecules stop for an instant and then fly apart. If they react, the particles will break their existing bonds and chemically bond in a different pattern.
Within the reacting medium ( liquid or gas ) there will be an enormous number of collisions, but very few actual reactions. ( there are estimated to be 1036 collisions per second in a room )
There are two basic reasons for the lack of reactions:
A. The orientation of the molecules at collision ( p755)
B. Activation Energy ( Ea )
Is the minimum EK
3) Concentration of the reactants
As the concentration [mol/L] of the reactants decreases
As the slope of the reactant concentration curve decreases,
4) Effect of Temperature
The rate of Reaction increase as the temperature increases. The reaction rate generally
Temperature ( K) is the measure of the
There is a certain amount of energy necessary to cause the breaking of bonds in a collision. This energy is called the ACTIVATION ENERGY Ea
1.
2.
Conservation of Energy
All energy put into a reaction must be accounted for.
Therefore; ET = EP + EK
As the particles approach collision
Draw the EP vs. reaction path for reversible reactions
5) Effect of Catalysts of Rate of Reaction
Reaction
+ Catalyst
Catalyst:
2. Reaction Mechanism with catalyst
Slow reaction A-A + C à A2C
Fast reaction A2C + B-D à A2B + D + C
Reactions and Energy
1. Complete the chart below from the data on the above graph
Equation / Type of Reaction / DHrxn / EaA+ B à C + D
C+ D à A + B
2. Does the temperature of a system affect the Activation Energy? Explain your answer.
3. Describe significance of an activated complex.
4. Complete the chart below
Most likely to React ExplainLeast Likely to React. Explain
Produces most energy? Consumes most energy
Give an example of each type of reaction
Experiment: A Study of Reaction Rates ( Iodine Clock )
Purpose: to determine the extent that concentration ( Part I ) and temperature control ( Part II ) the rate of a chemical reaction ( Iodine Clock )
Background Information on this Reaction
The clock reaction is performed by mixing the two solutions described below.
Solution A : is a dilute solution of potassium iodate, KIO3 , which is the source of the reacting species, the iodate ion , IO3-(aq) .
Solution B: contains some starch and the other reacting species, the hydrogen sulphite ion , HSO32-(aq)
The initial step in the reaction is represented by the equation:
When the hydrogen sulphite ions (HSO32-(aq)) are consumed , the iodide ions (I-(aq)), react with the remaining iodate ions (IO3-(aq)) to produce iodine, I2(s)
The molecular iodine reacts with the starch to produce a blue substance, which indicates the completion of the reaction.
Apparatus: test tubes , stopwatch, burette , and beaker
Part I : Effect of Concentration Changes on Reactivity
Method:
Dilute solutions of solution A will be prepared to vary the [IO3-] while keeping the [HSO32-] constant. The temperature of all the solutions should be kept at room temperature.
1. Use a burette to measure 10.0 mL of solution A and pour it into a clean test tube. Use a volumetric pipet to place 10.0 mL of solution B into a second clean test tube.
2.Pour solution A into the test tube containing solution B, then pour the mixture back and forth 3 times to obtain uniform mixing. Time the reaction from the instant that the two solutions first make contact. Record.
3.Prepare different concentrations of the solution A (KIO3) . Do the dilutions as directed by your teacher.
Solution A volume (mL) / 9.0 / 8.0 / 7.0 / 6.0 / 5.0Plus Distilled water (mL) / 1.0 / 2.0 / 3.0 / 4.0 / 5.0
Note: the total volume is always 10.0 mL. Mix the solutions well.
4.Prepare 5 test tubes each containing 10.0 mL solution B. Repeat step #2 for each solution in #3. Record the times of reaction.
Observations: two groups results
Soln Aà / 10.0 mL / 9.0 mL / 8.0 mL / 7.0 mL / 6.0 mL / 5.0 mLTime of reaction #1
Time of reaction # 2
Calculations:
1.The initial concentration of KIO3 (A) is 0.020 mol/L. Calculate the number of moles per millilitre of KIO3 in the initial solution A.
2. Calculate the initial concentration of [ KIO3 ] in each of the 6 solutions.
3. Pot a graph of concentration [KIO3 ] Vs time of reaction.
Conclusions:
1. What generalizations can you make concerning the affect of varying the concentration on the time of reaction.
2. How is the TIME of the reaction related to the RATE of the reaction?
3. Why is it important to keep the total volume of 10.0 mL during the dilutions of solution A?
Part II: Affect of Temperature
Method:
1.Place 10.0 mL of solution A into a clean test tube and place 10.0 mL of solution B into a second clean test tube. Place the two solutions in the water bath and allow the contents to reach the temperature of the water bath. The water bath must be at the temperature indicated by you teacher for your group.
2. Mix the two solutions (3 times) and record the time for the reaction.
The test tube should be put back into the water bath after mixing.
3. Obtain the other temperature results from the remainder of the class.
Observations:
Temperature / 5oC / 10oC / 15oC / room oC / 30oC / 35oC / 40oCTime #1
Time #2
Plot the graph of time Vs temperature for this reaction
. Allow for extrapolation to 0 oC and 50 oC.
Conclusions:
1. What generalizations can you derive from the graph of time Vs temperature?
2. What are the extrapolated time values for 0 oC and 50 oC?
Rate of Chemical Reactions
ex.
ex.
Graphing Concentration Vs Time to Determine Reaction Rate
Rate of reaction can be determined by taking the slope of the curve
Average Rate (slope of a secant) and Instantaneous Rate ( slope of tangent )