Reaction Rate and Rate Law - AP Chemistry

The rules for determing the reaction order with respect to inividual reactants are as follows.

If the concentration of a reactant is doubled and the rate of product formation is doubled then the reaction is first order with respect to this individual reactant.

If the concentration of a reactant is doubed and the rate of product formation is quadrupled then the reaction is second order with respect to this indivudal reactant

If the concentration of a reactant is tripled and the rate of product formation increases by nine times then the reaction is third order with respect to this indivudal reactant

If the concentration of a reactant is changed and the rate of product formation is not significantly changed then the reaction is zeroth order with respect to this individual reactant.

These rules also work in reverse.

That is if the concentration of a reactant is halved and the reaction rate decreases by a factor of 2, then the reaction is considered frst order with respect to this individual reactant.

The overall reaction rate is determined by adding the reaction orders of each indivual reactant together. So if the reaction was first order with respect to reactant \[X\] and first order with respect to \[Y\], it is third order overall

So the answer to the question is that the reaction is second order overall becasue the reaction is first order with respect to both reactants X and Y, which gives a second order reaction overall.

First order reactions have rates that are directly proportional to only 1 reactant. In radioactive decay, the rate of decrease of a radioactive material is proportional only to the amount of the material.

The rules for determing the reaction order with respect to individual reactants are as follows.

If the concentration of a reactant is doubled and the rate of product formation is doubled then the reaction is first order with respect to this individual reactant.

If the concentration of a reactant is doubled and the rate of product formation is quadrupled then the reaction is second order with respect to this indivudal reactant

If the concentration of a reactant is tripled and the rate of product formation increases by nine times then the reaction is third order with respect to this individual reactant

If the concentration of a reactant is changed and the rate of product formation is not significantly changed then the reaction is zeroth order with respect to this individual reactant.

These rules also work in reverse.

That is if the concentration of a reactant is halved and the reaction rate decreases by a factor of 2, then the reaction is considered frst order with respect to this individual reactant.

The overall reaction rate is determined by adding the reaction orders of each individual reactant together. So if the reaction was first order with respect to reactant \[X\] and first order with respect to \[Y\], it is third order overall.

So we know that the reaction is second order overall as defined by the question. It is first order with respect to Y because when Y was doubled the reaction rate was doubled so the reaction order is first order with respect to Y. So to ensure that the reaction is second order overall the reaction must be first order with respect to X. As compared to Experiment 1, Experiment 3 has the same concentration of reactant Y but the reaction rate is doubled, this must be due to a change in the concentration of reactant X. So if the \[X\] in experiment 3 is double the \[X\] in experiment 1 the reaction would be first order with respect to X and second order overall.

By the stoichiometry, since 1 mole of of H2 and 2 moles of ICl produces 2 moles of HCl
and 1 mole of I2 , we know that H2 is consume half as fast as ICl and produced at the same
rate as I2 .

The rate based on concentration is related to the coefficients in front of the compounds. Based on the reactants the rate should be negative (because change in concentration for the forward reaction will be negative) and based on the products should be positive. This means that II is incorrect. The rate for each compound in the reaction should be divided by it's coefficient to make it all related to 1M, showing that I and III are correct.

When the concentration of B doubles, the rate doubles. Making this reaction first order in regards to compound B. When the concentration of A doubles the rate is unaffected, making this reaction zero order in regards to compound A. This leaves a rate law of rate=k\[B\]

The rate is only based on the experimental values in the table. When B doubles and A stays the same the rate doubles, making it first order with respect to compound B. When compound A doubles and B stays the same, the rate quadruples, making it second order in regards to A.

A zero-order reaction has a rate of formation of product that is independent of changes in concentrations of any of the reactants; however, since the rate constant itself is dependent on temperture, changing the temperature can alter the rate.

Rate laws can rarely be determined from just the reaction; they usually require experimental data. However, both of the answer choices contain the word ALWAYS, which is too extreme. Rate laws can be independent of the reactants, these rate laws are known as zero-order.

Zero-order reactions are independent of changes in the concentration of any of the reactants. The rate constant itself is dependent on temperature, so changing the temp will change the rate of a zero-order reaction.

Based on the question, you can tell that the rate is first order with respect to the first reactant. So if the overall reaction is third-order, that means that the exponents must add to 3. We know that the exponent of the first reactant is 1, so that must mean that the exponent of the second reactant is 2. Thus, the concentration will be squared when you account for how it contributes to the rate, so the rate increases by a facotor of 4.

An increase in temperature leads to an increase in reaction rate for endothermic reactions, while a decrease in temperature will slow down their rate.

For zero order reactions, reaction rate is independent of reactant concentrations; therefore, changing \[A\] and \[B\] will have no effect on reaction rate.

The rate law for this reaction would simply be , where k is the rate constant at a given temperature.

Keep in mind that the chemical equation and its coefficients have NOTHING to do with the rate law. The order of each reactant must be determined by experiment.

To find the order of each reactant, compare the initial reaction rates of two trials in which only one of the three reactants' concentrations is altered. For example, trials 1 and 4 keep A and B equal, but C is doubled. When C is doubled, we see that the initial reaction rate is quadrupled. As a result, we determine that the order of reactant C is 2. When the reactant is altered, but the initial reaction rate is kept constant, as seen in trials 1 and 3 with respect to A, the order of that reactant is 0. Finally, when the reactant is multiplied by the same factor that the initial reaction rate is multiplied, as seen in trials 1 and 2 with respect to B, the order of the reactant is 1.

Putting the data together: A is zeroth order, B is first order, and C is second order. Our rate law can thus be written .

When determining the rate law for a reaction, you need to determine the order for each reactant in the reaction. This can only be accomplished by performing an experiment where different trials show how the initial reaction rate changes based on the initial concentrations of the reactants. By keeping one initial reactant concentration constant and changing the concentration of the other reactant, we can see how the initial rate changes for each reactant.

For reactant A, we notice that the rate quadruples when its concentration is doubled by comparing trials 2 and 3. The concentration of A increases from 0.02 to 0.04, causing the rate to change from 0.005 to 0.020. This means that the order for reactant A is 2.

For reactant B, we see that the rate does not change when the initial concentration of B is tripled by comparing trials 1 and 2. As a result, the reaction is 0 order wih respect to B.

Now that we have the orders for each reactant, we can write the rate law accordingly.

Since anything to the 0 power is 1, B is omitted from the rate law and can be considered to be equal to 1.

Since the rate law is provided for the reaction, we can plug in the values that are relevant to the rate of the reaction. This will allow us to determine the rate constant.

Note that HF is not included; the reaction must be zero order with respect to HF.

Intermediate is created and destroyed, and therefore does not appear on the net equation, which is . Thus, the intermediate is . Note that when asked for an intermediate, the coefficient in front of it is not used, rather we are looking for the species that is a product of one reaction and a reactant in a subsequent step.

The reactant is not included in the rate law expression, and therefore altering its concentration does not affect the rate of the reaction. Catalysts always increase the rate of reactions by lowering its activation energy. Increasing temperature (average kinetic energy of the molecules) increases the frequency of collisions, and increases the proportion of collisions that have enough energy to overcome the activation energy and undergo a chemical reaction. Increasing the concentration of will increase the rate of the reaction as indicated by the rate law.

For this question, we're asked to identify something that will always increase the rate of a reaction. Notice that the question specifically states "always."

Let's go through each answer choice and see how it affects the reaction rate.

When the concentration of reactants is increased, this may increase the reaction rate, but not always. For example, if a reaction is zero-order with respect to its reactants, then changing the reactant concentration will have no effect on the rate.

Just like with reactants, decreasing the concentration of products may or may not change the reaction rate. If the reaction rate is zero-order with respect to the products, then a change in their concentration will have no effect on the reaction rate.

A change in temperature is the only thing that is guaranteed to change the reaction rate. This is because changing the temperature will directly change the rate constant of the reaction. Increasing the temperature will increase the rate constant, and hence the reaction rate.

When the concentration of A stays the same but the concentration of B doubles, the rate quadruples, showing a second order rate law based on B. When the concentration of A doubles and B stays the same, the rate also doubles, showing a first order rate law based on A.