Question 1

As we approach the upper left hand corner of the periodic table, which of the following is true?

Accepted Answer

The upper left hand corner of the periodic table contains the elements with the smallest atomic radius. As we approach that corner, the atomic radius becomes smaller and smaller due to fewer electrons present in the electron shell. For example, lithium (period 2) has 2 shells of electrons, and is much smaller than cesium (period 6) which has 6 shells of electrons.

Question 2

A student wishes to produce magnesium bromide and water using a reaction based around magnesium hydroxide. Which other reactant is he or she missing?

Accepted Answer

The reaction described can be written out as: Mg(OH)₂ + ___ ? MgBr2 + H?O. From this, we can see that we need a bromine reactant, but the bromine reactant can only have hydrogen or oxygen in the compound. From the list of answers, only answer C, (HBr) or hydrogen bromide fits this description.

Question 3

If a car burns one gallon of gasoline (C8H18) in a standard combustion reaction, how much water is produced? Gasoline weighs 2.75 kg per gallon.

Accepted Answer

If one gallon of gasoline is burned, 3.91 kg of water will be produced. The general equation for combustion of gasoline is C8H18 + 12.5O? ? 9CO? + 9H?O. It needs to be recognized that a combustion reaction uses oxygen and produces carbon dioxide and water. The mole ratio is 1:9 for gasoline to water. Gasoline is 114g/mol, so 2.75kg is 24 moles of gasoline. 24 x 9 x18g/mol = 3.91 kg of water produced.

Question 4

The reaction seen below can be categorized as which kind of reaction mechanism? NH?NO? N2O + H?O

Accepted Answer

A reaction in which a single reactant produces two products is a decomposition reaction. It is named decomposition because it is literally the breakdown of a larger molecule into two smaller molecules, usually accompanied by a release of energy.

Question 5

The rate of a reaction can usually be increased by all of the following except:

Accepted Answer

Increasing the amount of water in an aqueous reaction will dilute the solution, resulting in less contact between reacting species. This will slow down the reaction.

Question 6

Which of the following is an example of a heterogeneous reaction?

Accepted Answer

Iron rusting occurs between a solid reactant (iron) and a gaseous reactant (oxygen), and is thus a heterogeneous reaction. Methane and oxygen are both gases, and are thus a homogeneous reaction type.

Question 7

What will an exothermic reaction do?

Accepted Answer

An exothermic reaction will create energy or heat. An endothermic reaction consumes energy or heat. Note that a reaction with reduces entropy is not possible. All reactions, according to the 3rd law of thermodynamics, must increase entropy in some regard.

Question 8

Lillian has discovered a new 2-step reaction that can produce aluminum hydride. Reaction 1 operates very slowly however; about 5 times slower than Reaction 2. Reaction 1 likely has a:

Accepted Answer

Slower reactions typically have a higher activation energy, assuming the concentration of reactants isn't terribly low (but in this case, it wouldn't be, because it's the first reaction). Volatile intermediates should lead to a faster reaction, and answers C and E aren't real terms in chemistry.

Question 9

The reaction rate constant can be determined by:

Accepted Answer

Arrhenius' equation, k = Ae^-(E/RT), can be used to relate the rate constant with activation energy and temperature. Boltzmann's equation is a statistical model of a thermodynamic system in non-equilibrium.

Question 10

10 grams of potassium is placed into a solution of hydrochloric acid. After 20 seconds, 4.4 grams of potassium remained. What was the average rate of reaction?

Accepted Answer

If 4.4 grams of potassium remained, then 5.6 grams of potassium reacted. 5.6 grams divided by 20 seconds is an average rate of 0.28 grams per second.

Question 11

If the rate law of a reactant is zero, then this means that:

Accepted Answer

A reactant with a rate law of zero means that adding more of the reactant will not increase the reaction rate. A rate law of one means that adding more reactant will result in an equivalent increase in the rate of the reaction.

Question 12

In the following reaction, which chemical species is a reaction intermediate? Step 1: H?O? (aq) + Br- (aq) ? H?O (aq) + OBr- (aq)
Step 2: H?O? (aq) + OBr- (aq) ? H?O (aq) + Br- (aq) + O? (g)

Accepted Answer

In this 2-step reaction, OBr- is a reaction intermediate. Reaction intermediates are chemical species found as a product of the first reaction, and a reactant in the second reaction.

Question 13

Edward conducts the following set of experiments to determine the reaction order of hydrogen in the reaction. What is the reaction order of hydrogen in this reaction? Br Concentration, H? Concentration , Initial rate
0)10.1 0.10.1 2.38 x 10-2
0)20.2 0.10.1 2.38 x 10-2
0)10.1 0.20.2 4.41 x 10-2

Accepted Answer

The rate law of hydrogen in this reaction is 1. Doubling the hydrogen concentration resulted in roughly double the rate, meaning there is a linear relationship between the reactant and the rate. This corresponds to a reaction order of 1.

Question 14

The reaction rate constant is usually described using which of the following units in a first order reaction?

Accepted Answer

The reaction rate constant for a first order reaction has units of hertz, or s^-1.

Question 15

When a chemical reaction is at equilibrium, the reaction quotient is what percentage of the equilibrium constant?

Accepted Answer

When a reaction is at equilibrium, the reaction quotient (Q) is equal to the equilibrium constant (K), and thus is 100% of the equilibrium constant.

Question 16

Jenny has carried out a reaction in her lab to equilibrium. She tells her friend, Joe, that there are no longer any reactions occurring in the solution. Joe tells her she is wrong, because:

Accepted Answer

A reaction at equilibrium has equal forward and backward rates. This gives the appearance of the reaction having stopped, but there is still something going on at the molecular level.

Question 17

If the following reaction is in equilibrium, what would a graph representing the concentration of H? look like? 2NH?(g) ? N2(g) + 3H?(g)

Accepted Answer

In a reaction at equilibrium, there is no longer any change in the concentrations of products and reactants. Thus, all chemical species in the reaction, including H?, should have a concentration graph that is a flat, horizontal, line.

Question 18

If a reaction has proceeded to completion, then what does this mean?

Accepted Answer

If a reaction is completed, or proceeds to completion, then all the reactants have reacted. This means there are now zero reactants, and the concentration is all on the product side of the reaction equation.

Question 19

Kelley measured the concentrations of a reaction in progress, and stated them below. What is the current equilibrium constant for this reaction? 2S(s) + 3O?(g) 2SO?(g)
S: 1.5 mols
O?: 4 mols
SO?: 2.9 mols

Accepted Answer

The equilibrium constant for this reaction can be calculated by: (SO?)2/(O?), or 2.92/4. The sulfur solid is not incorporated in the equation, because the reaction is occurring in the gas phase.

Question 20

A catalyst will usually make a reaction proceed much faster. How do catalysts work? I. Puts reactants close together
II) Reduces activation energy
III) Increases the temperature

Accepted Answer

A catalyst can work through two primary methods - putting reactants close together so a reaction is more likely to occur, and reducing the activation energy. Catalysts do not increase the temperature.

Quiz 35: Praxis II Elementary Education Content Knowledge