Measurement: Introduction

All science is based on analyzing data. There are two types of data in chemistry. Qualitative data is based on descriptions such as color, state and luster. Quantitative data is based on numerical measurements.

Chemistry represents its quantitative data using the metric system.  Mass is measured in grams, volume in liters, length in meters, and temperature in Celsius or the Kelvin scale. EVERY QUANTITATIVE MEASUREMENT MUST HAVE BOTH A QUANTITY AND A UNIT.

Numbers mean nothing without a unit!

Final Exam Discusion Page

Here are some suggestions for your notecard-
polyatomic ions you tend to forget
special reactions- HIGHLIGHT METATHESIS!
concentration formulas
gas law formulas
values for R and STP
pH & pOH formulas
reminders about how to do:
-rate law
-equilibrium
-redox

As you are working through the review, keep a sheet to write down formulas, constants, ideas & processes that you tend to forget or confuse, THEN make out your notecard from that sheet. Don't overfill the notecard. It is there as a reminder and security blanket. It doesn't help you if you can't find anything on it.

Chemical Kinetics II

Collision Theory

Collision Theory attempts to explain reaction rates.  It is based on the basic ideas:

• Molecules must collide to react.
• Concentration affects the rate.  A higher concentration allows for more particles to have a chance of colliding, therefore increasing the rate.  Fewer particles would lessen the chance of collision, therefore slowing the rate.
• Molecules must collide hard enough (but not too hard) to cause a reaction.
• Temperature and rate are related.
• Only a small number of collisions actually result in a reaction.

Activation Energy is the energy needed to form an activated complex or transition state so a reaction can occur.  In other words, how much energy is needed to get the reaction started.  Think of it as the amount of energy it takes to cut up all the vegetables you need to prepare a meal.

Take the synthesis reaction that forms hydrogen iodide gas.

Before the hydrogen and iodine can combine, the diatomic molecules must be broken apart in a redox reaction.  After the positive hydrogen ions and the iodine ions have been formed, the electrostatic force (attraction of a + for a -) will pull the ions together.

This reaction needs energy to break up the diatomic molecules and form ions.  This is the activation energy.  The H+ and I- are the activated complex (the transition state or intermediate step) in the mechanism of the reaction.

Chemical Kinetics

Chemical Kinetics

The Rate of Reaction is measured by the change in molarity over time.  By convention, we use the decreasing molarity of the reactants instead of the increasing molarity of the products.  The rate of a reaction depends upon many factors: the number of molecules, how fast they are moving, how they are colliding, …  Because of this, reaction rate is not linear, it is an exponential decay.

Instantaneous rate is the rate at a specific moment in time.  It is determined by the slope of the tangent line to the curve.  The only way to determine this without calculus is to graph the data and manually approximate a tangent line.

Average rate is the rate between 2 specific times.  It is determined by the slope of the line between the 2 times on the curve.  You do not need an actually graph the data to determine average rate, just use the (time, molarity) as (x,y) and determine the slope by ∆x/∆y.

The formula for the rate law for an equation is always    rate=k[reactant]^order

The power or order of the reaction for each reactant must be determined from experimental data.  The total order for a reaction is equal to the sum of the orders for each reactant. 

• This method requires that a reaction be run several times.
• The initial concentrations of the reactants are varied.
• The reaction rate is measured just after the reactants are mixed.
• Eliminates the effect of the reverse reaction.

Gases: Partial Pressure

Pressure is the force applied by the particles of a gas colliding with a surface such as the walls of a container.  If there is a mixture of gases, each individual type of gas particle will exert a pressure.  This is partial pressure, or the pressure exerted by a specific component of a gas mixture.

Dalton's Law of Partial Pressure states that the total pressure exerted by a mixture of gases will be the sum of the individual pressures of each of its components.

This concept can be combined with the ideal gas law to solve for the total pressure in a gas mixture.

Gases: Avogadro's Principle

Avogadro's Principle states that equal volumes of gases at equal temperature and pressure, will contain the same number of particles.  Therefore now we can have a molar volume, just as we have a molar mass.

The molar volume of any gas at STP is 22.4 L.  This can be determined by using the ideal gas law.
REMEMBER! This only applies to a GAS at STP.

We can now apply this concept to chemical reactions.  If a reaction takes place at constant temperature and pressure, the ratio of all the gases (reactants and products) will be the same as the mole ratio.

Gases: Ideal Gas Law

In Ideal Gas Law calculations there is only one set of conditions (P,V,T), but we've added the idea of mass.