ap chem unit 5

The first order decomposition of some radioactive isotope is 3 days. Approximately what percentage of the original substance will have decayed after 12 days have passed?

12 days is 4 half lives, the amount decayed will be 100>50>25>12.5>6.25% remains, thus 94% of the original material will have decayed.

How do homogeneous catalysts and heterogeneous catalyst differ?

A homogeneous catalyst is a catalyst that exists in the same phase (liquid or gas) as the reactants. A heterogeneous catalyst, on the other hand, is a catalyst that exists in a different phase form the reactants.

adding a catalyst

Adding a suitable catalyst lowers the energy demands of the reaction (the activation energy) by providing a less demanding pathway. It does this by forming a different activated complex with a lower activation energy.

How does a catalyst increase the rate of a chemical reaction?

By providing a pathway for the reaction that has a lower activation energy.

Is present at the start of a reaction and is also present at the end of a reaction

Catalyst

The rate constant for a gas phase reaction can be increased by doing which of the following? Increasing the concentration of reactants Increasing the temperature Decreasing the size of the container

Changing the temperature will change the rate constant. Changing concentration and size of container will change the rate, but not the rate constant.

For the chemical reaction H2 + Br2 → 2HBr, what can be said of the relative rates of consumption of the reactants when compared to the formation of product?

HBr will be produced at twice the rate that bromine is consumed

Collision theory predicts all of the following EXCEPT that a reaction will only occur if the collision geometry is correct a reaction will not occur if the collision occurs with energy that is less than the activation energy. a reaction will not occur if the reactants do not collide frequency of collisions will increase with increasing temperature. more successful collisions will occur for a reaction with a larger activation energy.

Higher activation energy is a higher hump, and is harder to get over,making for fewer successful collisions.

Which of the following statements is/are true in relation to rate laws? A rate law must be determined experimentally and cannot be deduced from a chemical equation alone. Zero-order have rate constants equal to rates A "straight line" graph can be obtained for a first-order reaction by plotting the reciprocal of the concentration of reactant (y-axis) against time (x-axis)

I and II only

increasing the concentration of the reactants

Increasing the concentration of a reactant increases the probability of collisions, and therefore increases the possibility of reaction, for the particles must collide to react. Note that not all collisions result in successful reactions.

increasing the temperature

Increasing the temperature increases the average kinetic energy of the molecules. However, what is most important here is that the fraction of molecules that have the energy necessary to react (the activation energy) is increased.

reaction mechanism

It is a list of steps that produce the overall chemical reaction.

Each of the following is true about heterogenous catalyst EXCEPT Its presence changes the rate of chemical change. It does not undergo a permanent change. It is in the same phase as the reacting particles. It's presence lowers the activation energy of the overall reaction. Its presence decreases the potential energy of the activated complex

It is in the same phase as the reacting particles. meaning liquid vs solid vs gas, heterogeneous catalyst are in a different phase than the reactants they are catalyzing.

The specific part of a chemical reaction that can be used to determine the rate equation

Rate-Determining step

rate constant for a zero-order rate law

Rate=k; k must have units of mol/L•s

rate constant for a first-order rate law

Rate=k[A], k must have units of s-1

rate constant for a second-order rate law

Rate=k[A]2, k must have units of L/mol•s

represents a comparison between a reaction with high activation energy to a reaction with lower activation energy but the same heat of reaction?

Reactant and products at same level on both graphs, but a lower hump to represent a lower activation energy.

Describes the series of steps that a chemical reaction is broken in to

Reaction Mechanism

represents a comparison between the same reactants where one of the pair of diagrams represents a situation that includes a catalyst?

Same ∆H, but a lower activation energy.

Which accounts for the increase in the rate of reaction when a catalyst is added to a reaction system?

The addition of a catalyst provides an alternative reaction pathway,with a modified or different activated complex that has lower potentialenergy. This lowers the activation energy, Ea, thus increasing thereaction rate.

Which energy distribution diagram represents an increase in temperature?

The curve shifts to the right and flattens out, making for more molecules under the curve that will have the minimum energy required for successful reaction collisions.

Which of the following is true of all catalysts? They are used up in chemical reactions They are always transition metals They do not take part in chemical reactions. They work by increasing the activation energy of a reaction. They are present at the beginning of a a reaction and are unchanged at the end.

The definition of a catalyst, unlike a reactant, the catalyst does not get used up.

A(g) + B(g) → C(g) All of the following apply to the reaction above as it is carried out at constant temperature in a sealed rigid container EXCEPT The total pressure decreases. The rate of the reaction decreases. The entropy of the system decreases The number of molecules of C decreases. The frequency of collisions between molecules A and B decreases.

The pressure decreases because two molecules are converted into one. The rate of all reactions decrease over time. The entropy decreases because fewer molecules result from the reaction. Of course the number of C molecules will increase during the reaction. Since the concentration of A and B will decrease over the course of the reaction, their collisions will decrease.

Why do large crystals of sugar burn more slowly than finely ground sugar?

The rate of combustion is proportional to the surface area of sugar exposed to oxygen. Smaller crystals have more surface area and burn faster.

A sealed rigid container holds two first order reactant gases, A and B. If additional moles of gas B are added to the container while keeping the temperature constant, what can be said of the reaction under these conditions. a. The rate of the reaction will increase, as will the rate constant, k. b. the rate of the reaction will remain unchanged as will the rate constant, k. c. the rate of the reaction will increase but the rate constant, k, will remain unchanged. d. the rate of the reaction will remain unchanged but the rate constant, k, will increase. e. the rate of reaction will decrease as will the rate constant, k.

The rate will increase doe to the concentration increase

For a mechanism to be feasible, which of the following criteria must be filled? The sum of the overall steps must add up to the overall stoichiometry of the chemical equation The rate determining (slow step) must be the last step in the mechanism. The rate determining (slow step) must include all of the reactants that appear in the rate equation where the stoichiometric coefficients match the powers.

The three criteria are (1) the sum of the steps must add to the total reaction, like Hess' Law, (2) the molecularity must be reasonable unimolecular or bimolecular and rarely termolecular, and (3) the mechanism must correlate with the experimentally observed rate law.

The rate constantrate of a chemical reaction

The units for rate are always mol/L•s.

The reaction X + Y→Z has the rate law, Rate = k[X]2[Y]. If the concentration of X is doubled and the concentration of Y is tripled, the rate would be increased by a factor of...

[2]2[3]=12

As the concentrations of the reactants decrease over the course of a reaction, the frequency of collisions will...... but the effectiveness of the collisions will remain the same, as that is a function of the molecules themselves, the temperature, and the presence of any catalyst.

decrease

Concentration of reactants always ..... over the course of all reactions.

decrease

Shorter half life indicates....

faster reactions. Half life is an indication of reaction rate.

The rate constant changes at different temperatures. It increases at .... temps and .... at lower temps.

higher, decreases

A catalyst

is a chemical that gets used up during an early step, but returns as a product during a later step.

If its a ...... it will have no effect on the reaction rate since the concentration will remain constant for a pure liquid regardless of how much is in the container.

liquid

Which of the following plots will yield a straight line for a first order reaction where the slope= −k ?

ln [A] versus time

Rate constants only change when...

temperature changes or in the presence of a catalyst. Rate constants do not change with concentration or pressure changes.

The rate of a chemical reaction will be increased by all of the following factors EXCEPT the presence of a high activation energy the presence of a catalyst increasing the surface area of a reactant an increase in temperature increasing the concentration of the reactants

the presence of a high activation energy

overall order

the sum of the orders of all reactants in a chemical reaction

Heating a material in a sealed container, .....; change the number of molecules present.

would not

A rate law for which the units of rate are equal to the units of the rate constant (A) Rate= k [A]0(B) Rate= k [A]2(C) Rate= k [A] [B]3(D) Rate= k [A] [B]2 [C]2 (E) Rate= k [A]2 [B]

(A) Rate= k [A]0

A rate law for a reaction that is 3rd order overall (A) Rate= k [A]0(B) Rate= k [A]2(C) Rate= k [A] [B]3(D) Rate= k [A] [B]2 [C]2 (E) Rate= k [A]2 [B]

(E) Rate= k [A]2 [B]