How is reaction rate determined

These factors include temperature and catalysts. When you are able to write a rate law equation for a certain reaction, you can determine the Reaction Order based on the values of s and t. The reaction rate for a given reaction is a crucial tool that enables us to calculate the specific order of a reaction. The order of a reaction is important in that it enables us to classify specific chemical reactions easily and efficiently. Knowledge of the reaction order quickly allows us to understand numerous factors within the reaction including the rate law, units of the rate constant, half life, and much more.

Reaction order can be calculated from the rate law by adding the exponential values of the reactants in the rate law. It is important to note that although the reaction order can be determined from the rate law, there is in general, no relationship between the reaction order and the stoichiometric coefficients in the chemical equation. NOTE: The rate of reaction must be a non-negative value. It can be zero and does not need to be an integer. Each variable represents the order of the reaction with respect to the reactant it is placed on.

In this certain situation, s is the order of the reaction with respect to [A] and t is the order of the reaction with respect to [B]. So if you have a reaction order of Zero i. You could remove or add reactants to the mixture but the rate will not change. This table also includes further equations that can be determine by this equation once the order of the reaction is known Half life, integrated rate law, etc.

Reaction Rate is the measure of the change in concentration of the disappearance of reactants or the change in concentration of the appearance of products per unit time. The rate constant is not dependant on the presence of a catalyst. Catalysts, however, can effect the total rate of a reaction. Chem1 Virtual Textbook.

Reaction rates were computed for each time interval by dividing the change in concentration by the corresponding time increment, as shown here for the first 6-hour period:. Notice that the reaction rates vary with time, decreasing as the reaction proceeds. Results for the last 6-hour period yield a reaction rate of:.

This behavior indicates the reaction continually slows with time. Using the concentrations at the beginning and end of a time period over which the reaction rate is changing results in the calculation of an average rate for the reaction over this time interval.

At any specific time, the rate at which a reaction is proceeding is known as its instantaneous rate. Consider the analogy of a car slowing down as it approaches a stop sign. A few moments later, the instantaneous rate at a specific moment—call it t 1 —would be somewhat slower, as indicated by the speedometer reading at that point in time. As time passes, the instantaneous rate will continue to fall until it reaches zero, when the car or reaction stops.

Like the decelerating car, the average rate of a chemical reaction will fall somewhere between its initial and final rates. The instantaneous rate of a reaction may be determined one of two ways. If experimental conditions permit the measurement of concentration changes over very short time intervals, then average rates computed as described earlier provide reasonably good approximations of instantaneous rates.

Alternatively, a graphical procedure may be used that, in effect, yields the results that would be obtained if short time interval measurements were possible. If we plot the concentration of hydrogen peroxide against time, the instantaneous rate of decomposition of H 2 O 2 at any time t is given by the slope of a straight line that is tangent to the curve at that time Figure 2.

We can use calculus to evaluating the slopes of such tangent lines, but the procedure for doing so is beyond the scope of this chapter. These test strips contain various chemical reagents, embedded in small pads at various locations along the strip, which undergo changes in color upon exposure to sufficient concentrations of specific substances. The usage instructions for test strips often stress that proper read time is critical for optimal results.

This emphasis on read time suggests that kinetic aspects of the chemical reactions occurring on the test strip are important considerations. This method can be used for reactions that produce carbon dioxide or oxygen, but are not very accurate for reactions that give off hydrogen because the mass is too low to be accurately measured. Measuring changes in mass may also be suitable for other types of reactions. In a reaction in which a precipitate is formed, the amount of precipitate formed in a period of time can be used as a measure of the reaction rate.

For example, when sodium thiosulphate reacts with an acid, a yellow precipitate of sulfur is formed. This reaction is written as follows:.

One way to estimate the rate of this reaction is to carry out the investigation in a conical flask and place a piece of paper with a black cross underneath the bottom of the flask. At the beginning of the reaction, the cross will be clearly visible when you look into the flask. However, as the reaction progresses and more precipitate is formed, the cross will gradually become less clear and will eventually disappear altogether.

To finish balancing the equation, we must add a coefficient of 2 in front of hydrogen gas:. Keep in mind, however, that in our calculations, we will often be working in moles, rather than in molecules. In our example here, we can see that the stoichiometric coefficient of H 2 g is 2, while for O 2 g it is 1, and for H 2 O l it is 2. Occasionally, you might come across the term stoichiometric number, which is related to the stoichiometric coefficient, but is not the same. Water is 2, hydrogen gas is 2, and oxygen gas is 1.

For reactants, the stoichiometric number is the negative of the stoichiometric coefficient, while for products, the stoichiometric number is simply equal to the stoichiometric coefficient, remaining positive.

Therefore, for our example here, the stoichiometric number for H 2 g is -2, and for O 2 g it is This is because in this reaction, H 2 g and O 2 g are reactants that are consumed, whereas water is a product that is produced.

Lastly, you might occasionally come across some chemical species that are present during a reaction, but that are neither consumed nor produced in the reaction. A catalyst is the most familiar example of this. For such species, their stoichiometric coefficients are always zero.

In our balanced chemical equation, the coefficient for H2 g is 1, and the coefficient for HCl g is 2. The molar ratio between these two compounds is therefore This tells us that for every 1 mole of H2 g that is consumed in the reaction, 2 moles of HCl g are produced.

Privacy Policy. Skip to main content. Chemical Kinetics. Search for:. Reaction Rates Measuring Reaction Rates Reaction rates are determined by observing the changes in the concentrations of reactants or products over a specific time frame. Learning Objectives Produce rate expressions when given chemical reactions and discuss methods for measuring those rates.