Presentation on theme: "Chemical Kinetics Rates of chemical reactions. Every chemical reaction has its own signature rate Diamonds are made by converting Carbon from its graphite."— Presentation transcript:
Chemical Kinetics Rates of chemical reactions
Every chemical reaction has its own signature rate Diamonds are made by converting Carbon from its graphite state. takes millions years to consume the graphite (the reactant) to form a diamond (the product). this is why we place such a high value on diamonds the chemical reaction of photosynthesis (a plants ability to make its own food, which is the reaction's product along with Oxygen) takes place on a time scale as short as ^-12 seconds ( s)
Chemical Kinetics We can think of reaction rate in terms of the change of concentration with time. IF A→B, then... rate = - ΔA/Δt or rate = ΔB/Δt...wherein ΔA and ΔB are changes in the concentration (the molarity) over a time period.
The rate of reaction A→B, represented as the decrease of A molecules with time and as the increase of B molecules with time. B molecules A molecules
Deriving rates of reactions: you must first determine the concentration of the reactant at different intervals of time Average rate –the mean rate of change in the concentration of the reactant over the whole duration of the reaction - ΔA/Δt Instantaneous rate –a specific rate at a particular moment in time –If temperature is the same and the initial concentration of the reactant used in the reaction is the same, the rate will always be the same at that moment in time every time you perform the reaction Reactant concentration is proportional to the reaction rate...
rate is directly proportional to Br 2 concentration...
rate constant (k) a constant of proportionality between the reaction rate and the concentration of reactant. plot the data in a graph: rate vs. Br 2 concentration Is it a straight line or a curved line? The fact that the graph is a straight line shows that the rate is directly proportional to concentration. time (s)Br2 M reaction rate (M/s) x 10^ x 10^ x 10^ x 10^-5
units for rate constant Because reaction rate has the units M/s, and Br2 is in M, the unit of k is... 1/s or s^-1 k is not affected by the concentration of Br2. To be sure, the rate is greater at a higher concentration and smaller at a lower concentration of Br 2, but the ratio of rate/Br2 remains the same provided the temp. remains constant.
Find the rate constant for the following reaction time (s)Br2 M reaction rate (M/s) x 10^ x 10^ x 10^ x 10^-5 k = rate/Br 2 Use the chart to determine the rate constant 3.49 x 10^-3 s^-1
Reaction Rates and Stoichiometry STOICHIOMETRY - The quantitative study of reactants and products in a chemical reaction –Balancing equations and using molar masses to figure the amount of each substance being used as reactants to determine how much product is being converted during a reaction –Unbeknownst to you, we have been operating this concept, but we have yet to define it...
Reaction Rates and Stoichiometry I have introduced to you simple situations... A→B what if... 2A→B Uh-ohhhh... What to do? What to do? Well, this expression is stating that 2 moles of A disappear for each mole of B that forms. The rate of disappearance of A is twice as fast as the rate of appearance of B. How could we express this rate?
Reaction Rates and Stoichiometry How about like this... THE RULE: take the reciprocal of every product's coefficient and every negative reciprocal of every reactant's coefficient
The next step is to find the actual rate Molecular oxygen is reacting at the rate of M/s. At what rate is H 2 O 2 ?
HOMEWORK CHECK! NO + O 2 → 2NO 2 Suppose that at a particular moment during the reaction nitric oxide (NO) is reacting at the rate of M/s. a) At what rate is NO 2 being formed? b)At what rate is molecular oxygen reacting?
2NO + O 2 → 2NO 2 NO is reacting at the rate of M/s To express the rate of reaction we need to do three things... –Use proper sign (- or +) –Determine the reciprocal of the stoichiometric coefficient for each molecule in the reaction –Make all molecular rates equal to one another
2NO + O 2 → 2NO 2 NO is reacting at the rate of M/s At what rate is NO 2 being formed? GIVEN - _
2NO + O 2 → 2NO 2 NO is reacting at the rate of M/s At what rate is molecular oxygen reacting? GIVEN - _ _ _
N2 + 3H 2 → 2NH 3 Molecular hydrogen reacts at a rate of M/s 13.8 Suppose that at a particular moment during the reaction molecular hydrogen is reacting at the rate of M/s. a) At what rate is ammonia being formed? b) At what rate is molecular nitrogen reacting?
N2 + 3H 2 → 2NH 3 Molecular hydrogen reacts at a rate of M/s At what rate is ammonia being formed? _ - _ GIVEN
N2 + 3H 2 → 2NH 3 Molecular hydrogen reacts at a rate of M/s At what rate is molecular nitrogen reacting? - GIVEN -