Calculating Reaction Rates

Define Reaction Rates.

change in concentration of reactant/product over a certain unit of time

Define Species

any distinct type of atom, molecule, ion, or compound that participates in chemical processes

What are Overall Reaction Rates?

the rate of a particular species (particles) divided by its stoichiometric coeffient (moles)

1. the left graph communicates that as the reaction occurs, the number of product molecules increases

2. the right graph communicates that as the reaction occurs, the number of reactant molecules decreases

3. They are opposites of each other

What is the equation for Average Reaction Rate?

rate_A = [A] at time t₂ - [A] at time t₁ / t₂ - t₁

In a reaction, what happens to the concentration of the reactants? What equation can we create from this fact?

c_reactants gradually decrease

rate_r = -∆[R] / ∆t

In a reaction, what happens to the concentration of the products? What equation can we create from this fact?

c_products gradually increase

rate_P = ∆[P] / ∆t

What is important to note about reaction rates?

they cannot be negative so you must use absolute value

What does [x] represent?

concentration in mol dm^-3

What is the unit for rate?

mol dm^-3 s^-1

What is the Instantaneous Reaction Rate? How do you find it on a graph?

• reaction rate at a certain moment

• 1. From a concentration vs. time graph, if you draw a TANGENT to the curve at desired time, t, the slope of the line = the reaction rate at that time

• once you get the tangent, take the mol dm at t=0 and divide it by the t when mol dm = 0

What’s the average rate of NO₂

How do chemists track the progress of a reaction?

by appearance of a product/ disappearance of a reactant

What signal of progression you track depends on…

the reaction itself

What are the methods used to track change in a reaction?

1. change in gas volume

2. change in mass

3. change in light transmission

4. change in concentration via titration

5. change in concentration by conductivity

Tell me about Reaction Progress measured through Change in Gas Volume

• useful if gas is a product

• uses gas syringe or inverted burette for water displacement

• collect gas at regular intervals and plot a volume vs. time graph

Tell me about Reaction Progress measured through Change in Mass

• applicable for most reactions

• changes in mass could be continuous if reaction occurs on a scale

• if reaction gives off gas, decrease in mass is measured

Tell me about Reaction Progress measured through Change in Light Transmission

• useful is substance is coloured

• continuous method that uses absorbance vs. time graph

• uses colorimeter/spectrometer to shine light through substance and absorb light

  • less light detected = more light absorbed

Tell me about Reaction Progress measured through Change in Concentration using Titration

titration - mixing your sample solution with a known standard which results in a chemical reaction making new products

• therefore, samples must be drawn intermittently to get concentration

• quenching is used to stop reaction at point of withdrawal

  • quenching is when you freeze the flask in cold water which slows particles

What does Electrical Conductivity depend on?

concentration of ions and their charges

Tell me about Reaction Progress measured through Change in Conductivity

if electrical conductivity changes during a reaction, you can track this change using a calibrated conductivity meter containing inert electrodes into the solution