Thursday, October 18, 2007
Thurs-Day 1
Bio- we did the NY Bio Regents required lab on diffusion of glucose and iodine (and lack of diffusion of starch). Our results were as expected for a "cell" membrane (the dialysis tubing) that is permeable to RELATIVELY small molecules such as glucose (though a glucose molecule is about 10 x larger than a water molecule) and iodine ( an iodine molecule is about 4 x larger than a water molecule). We observed that starch molecules, which are polymers of glucose i.e. they are polysaccharides, are too large (hundreds of times larger than water molecules) to go through the selectively permeable "cell" membrane.
We observed that the glucose did not rapidly diffuse through the cell membrane but we still observed, via the use of heated Benedict's solution, that some glucose did diffuse through the membrane and into the outer solution; the glucose and Benedict's solution, when heated, turned brick red.
Tomorrow, we will finish our unit on the cells and cellular transport (active and passive types)
and review for MONDAY'S exam (also, text 7.3 outline is due on Monday).
Test 1 (Scientific Method and Biology Lab Tools test) corrections are due on Friday.
Chem 7- we explained the other two factors that affect the rate of chemical reactions (i.e. the rate of EFFECTIVE collisions between reactant particles). The factors are:
3. SURFACE AREA (for solids, liquids, or aqueous solutions)- by "CHOPPING UP" or SPREADING OUT the reactant samples (i.e. INCREASING the SURFACE AREA of the reactants), more and more reactant particles are exposed/available for COLLISIONS. So the total number of collisions per second increases, which increases the total NUMBER of EFFECTIVE collisions per second.
Nothing happens to the FRACTION of molecules that have proper orientation and nothing happens to the FRACTION of molecules that have sufficient KE for an effective collision BUT, since the total number of collisions per second is increasing, so will the reaction rate!
4. CATALYST- a catalyst is the ONLY factor that affects the FRACTION of molecules that have proper geometric ORIENTATION/POSITION for an effective collision. Furthermore, catalysts TEMPORARILY bind the reactant particles in such a way that their bonds are WEAKENED so that LESS (activation!) ENERGY is required for an EFFECTIVE collision. So, WITHOUT SPEEDING UP THE PARTICLES or changing the sample temperature, a catalyst can still increase a reaction rate by INCREASING the FRACTION of particles that have proper orientation for an EFFECTIVE collision AND ALSO lowering the energy REQUIRED for an EFFECTIVE collision. Thus, even at the same temperature, a greater fraction of particles will have SUFFICIENT KE for bond breakage since the catalyst "strained/pre-weakened" the bonds that need to be broken for an effective collision.
Recall, a catalyst is like a big, brawny bouncer that almost breaks a person's arm; if someone else comes along and merely taps (a low energy collision) the arm , the arm (bonds) will break.
Chem 8/9-
To have an EFFECTIVE collision, particles must collide with SUFFICIENT kinetic energy for bond breakage (the minimum quantity of energy for bond breakage is the reaction's ACTIVATION ENERGY!) AND ALSO the particles must collide with the proper geometric spatial orientation (so that the "correct" bonds break to form the desired products).
We discussed TWO of the FOUR factors that will AFFECT the number of effective collisions that occur per second (the REACTION RATE, literally):
Temperature and Concentration!
Increasing temperature will increase the average kinetic energy of the particles so the particles will naturally collide more frequently (less time between collisions since the particles sped up).
ALSO, increasing the temperature (average KE) will increase the FRACTION of particles that have SUFFICIENT kinetic energy for an effective collision (e.g. from 1/20 to 1/3). Therefore, the number of effective collisions per second (the reaction rate) increases.
However, temperature has no EFFECT on the FRACTION of molecules that are colliding with proper orientation so that fraction remains the same (e.g. one-fourth before, one-fourth after); the other factor, though, is more than enough to increase the reaction rate.
Decreasing temperature has the opposite effect of what was described above, naturally.
Increasing concentration means that there are MORE PARTICLES per mL than before! Therefore, there will be MORE collisions between particles every second.
Even though increasing concentration has NO EFFECT on the SPEED/KINETIC ENERGY of the particles and NO EFFECT on the fraction of particles with proper orientation, the fact that there are more collisions per second (e.g. from 5 collisions per second to 1000 collisions per second) will lead more EFFECTIVE collisions per second.
Decreasing concentration has the opposite effect of what was described above.
We mentioned the other two factors that can be used to increase or decrease a reaction rate:
surface area (for solids, liquids, or aq) and catalysts. We will discuss those more, tomorrow.
We then did our Kinetics Lab in which we investigated the influence of changing concentration on the rate of a reaction ("the iodine clock reaction"). We also looked at the effect of changing temperature on the rate of a reaction. We will discuss our results on Monday.
We observed that the glucose did not rapidly diffuse through the cell membrane but we still observed, via the use of heated Benedict's solution, that some glucose did diffuse through the membrane and into the outer solution; the glucose and Benedict's solution, when heated, turned brick red.
Tomorrow, we will finish our unit on the cells and cellular transport (active and passive types)
and review for MONDAY'S exam (also, text 7.3 outline is due on Monday).
Test 1 (Scientific Method and Biology Lab Tools test) corrections are due on Friday.
Chem 7- we explained the other two factors that affect the rate of chemical reactions (i.e. the rate of EFFECTIVE collisions between reactant particles). The factors are:
3. SURFACE AREA (for solids, liquids, or aqueous solutions)- by "CHOPPING UP" or SPREADING OUT the reactant samples (i.e. INCREASING the SURFACE AREA of the reactants), more and more reactant particles are exposed/available for COLLISIONS. So the total number of collisions per second increases, which increases the total NUMBER of EFFECTIVE collisions per second.
Nothing happens to the FRACTION of molecules that have proper orientation and nothing happens to the FRACTION of molecules that have sufficient KE for an effective collision BUT, since the total number of collisions per second is increasing, so will the reaction rate!
4. CATALYST- a catalyst is the ONLY factor that affects the FRACTION of molecules that have proper geometric ORIENTATION/POSITION for an effective collision. Furthermore, catalysts TEMPORARILY bind the reactant particles in such a way that their bonds are WEAKENED so that LESS (activation!) ENERGY is required for an EFFECTIVE collision. So, WITHOUT SPEEDING UP THE PARTICLES or changing the sample temperature, a catalyst can still increase a reaction rate by INCREASING the FRACTION of particles that have proper orientation for an EFFECTIVE collision AND ALSO lowering the energy REQUIRED for an EFFECTIVE collision. Thus, even at the same temperature, a greater fraction of particles will have SUFFICIENT KE for bond breakage since the catalyst "strained/pre-weakened" the bonds that need to be broken for an effective collision.
Recall, a catalyst is like a big, brawny bouncer that almost breaks a person's arm; if someone else comes along and merely taps (a low energy collision) the arm , the arm (bonds) will break.
Chem 8/9-
To have an EFFECTIVE collision, particles must collide with SUFFICIENT kinetic energy for bond breakage (the minimum quantity of energy for bond breakage is the reaction's ACTIVATION ENERGY!) AND ALSO the particles must collide with the proper geometric spatial orientation (so that the "correct" bonds break to form the desired products).
We discussed TWO of the FOUR factors that will AFFECT the number of effective collisions that occur per second (the REACTION RATE, literally):
Temperature and Concentration!
Increasing temperature will increase the average kinetic energy of the particles so the particles will naturally collide more frequently (less time between collisions since the particles sped up).
ALSO, increasing the temperature (average KE) will increase the FRACTION of particles that have SUFFICIENT kinetic energy for an effective collision (e.g. from 1/20 to 1/3). Therefore, the number of effective collisions per second (the reaction rate) increases.
However, temperature has no EFFECT on the FRACTION of molecules that are colliding with proper orientation so that fraction remains the same (e.g. one-fourth before, one-fourth after); the other factor, though, is more than enough to increase the reaction rate.
Decreasing temperature has the opposite effect of what was described above, naturally.
Increasing concentration means that there are MORE PARTICLES per mL than before! Therefore, there will be MORE collisions between particles every second.
Even though increasing concentration has NO EFFECT on the SPEED/KINETIC ENERGY of the particles and NO EFFECT on the fraction of particles with proper orientation, the fact that there are more collisions per second (e.g. from 5 collisions per second to 1000 collisions per second) will lead more EFFECTIVE collisions per second.
Decreasing concentration has the opposite effect of what was described above.
We mentioned the other two factors that can be used to increase or decrease a reaction rate:
surface area (for solids, liquids, or aq) and catalysts. We will discuss those more, tomorrow.
We then did our Kinetics Lab in which we investigated the influence of changing concentration on the rate of a reaction ("the iodine clock reaction"). We also looked at the effect of changing temperature on the rate of a reaction. We will discuss our results on Monday.
# posted by C @ 11:43 AM 
