12.1 Chemical Reaction Rates

12.1 Chemical Reaction Rates

• The rates of chemical reactions, the mechanisms by which reactions proceed, and the quantitative techniques used to determine and describe the rate at which reactions occur are covered in Chapter 12.

• The distance traveled by an object in a given amount of time is known as speed.
• A wage is the amount of money earned by a person working for a certain amount of time.
• The rate of a chemical reaction is a measure of how much reactant is consumed or how much product is produced in a given amount of time.

• The time dependence of some property is used to determine reaction rates.

• Volume or pressure can be used to determine the rates of reactions that consume or produce gaseous substances.
• Measurement of light absorption may be used for reactions involving one or more colored substances.
• Changes in a solution's conductivity can be used to measure rates.

• The relative amounts of reactants and products are used to express reaction rates.

• D[H2O2] is a negative quantity because the reactant concentration decreases as the reaction proceeds.

• As the concentration of H2O2 decreases, the rate of decomposition decreases.

• The concentration of hydrogen peroxide was measured every 6 hours over the course of a day at a constant temperature.

• The reaction rates vary with time, decreasing as the reaction proceeds.

• The reaction slows with time.
• Think of a car slowing down as it approaches a stop sign.
• The speedo reading at that point in time indicates that it would be somewhat slower.
• When the car stops, the instantaneous rate will fall until it reaches zero.
• The average rate of a chemical reaction will be somewhere between the initial and final rates.

• There are two ways in which the instantaneous rate of a reaction can be determined.
• Average rates computed as described earlier provide good approximations of instantaneous rates if experimental conditions permit the measurement of concentration changes over very short time intervals.
• If short time interval measurements were possible, a graphical procedure may be used that yields the results that would be obtained.

• The rate at any instant is the same as the slope of the line at that time.

• The test strips contain various chemical reagents, embedded in small pads at various locations along the strip, which change in color when exposed to sufficient concentrations of specific substances.
• Proper read time is important for optimal results according to the usage instructions for test strips.

• The emphasis on read time suggests that the chemical reactions occurring on the test strip are important considerations.

• The first equation shows the oxidation of the urine to produce glucolactone and hydrogen peroxide.

• The oxidizer of the iodide ion is hydrogen peroxide, which can be seen in the picture.
• Some strips have a substance that reacts with iodine to produce a different color.

• The two test reactions shown above are very slow, but their rates are increased by special enzymes embedded in the test strip pad.
• A typical test strip for use with urine requires 30 seconds to complete the color-forming reactions.

• Test strips can be used to detect substances in a person's urine.
• Many test strips have multiple pads with different reagents that can be used to detect multiple substances on a single strip.
• The rate of a reaction may be expressed in terms of the change in the amount of reactant or product.

• A negative sign has been added to account for the opposite signs of the two amount changes.

• The figure shows the change in concentrations over time for the decomposition of ammonia into nitrogen and hydrogen.
• The rates of change in the concentrations of the reactants and products are related to the factors that make up their chemistry.

• N2 and 3H2.

• The rates of consumption of the reactants and the rates of formation of the products are related.