Correct answer:

The rate of reaction would decrease if the temperature of the reaction mixture was lowered because the molecules will have lower kinetic energy necessary for productive collisions.

Why?:

Collision theory models chemical reaction as being the result of reactant molecules colliding with each other. These collisions facilitate reactions by bringing reactants together and by providing kinetic energy that can serve as a source of activation energy for the reaction.  Higher temperatures increase the speed at which molecules move, making collisions more energetic and more likely to have sufficient kinetic energy to meet the reaction's activation energy. Lower temperatures at constant pressure might not decrease the number of collisions, but lower temperatures would lower the number of collisions that provide enough energy for the reaction to overcome the barrier of activation energy. Lower temperatures will lower the number of productive collisions.

Answering this question:

Always remember that for a product to form, the reactants must collide in such a way as to produce the right product. This is facilitated by proper orientation of colliding reactants and the efficiency of their collisions. A higher temperature increases the kinetic energy of molecules. In effect, there would be much more aggressive collisions which could form the products at a faster rate. If temperature is lowered, the molecules will be moving slower. Thus, it would take some time for a molecule to collide another molecule because of the slower speed. Formation of the products would also be slower.

So, the rate of reaction would decrease if the temperature of the reaction mixture was lowered because the molecules will have lower kinetic energy necessary for productive collisions.

Correct answer: Increase in pressure will favor the forward reaction because the product side contains the lesser number of moles of gas.

Why?:

The left side of the reaction lists two moles of reactants for every mole of product listed on the right side. Higher pressures will favor the forward reaction since converting two molecules of reactants into one molecule of product will help relieve this pressure. It is as if pushing together the 2 molecules of NO₂ to unite and produce a more massive species. This concept of a reaction's equilibrium shifting to counteract a stress to the system in called Le Chatelier's principle.

Answering this question:

Gases are the ones most affected by changes in pressure. In solids, changes in pressure have very little effect.

In a chemical reaction, an increase in pressure will favor the side of the reaction with the lesser number of moles. When two things are pushed hard towards each other, they become very close to each other. In gases, this closeness could result to the formation of one whole mass of a new substance. This way, the increase in the pressure is relieved and equilibrium is restored.

So the forward reaction is favored when pressure is increased because the product side contains the lesser number of moles of gas.

Correct answer: The forward reaction is exothermic because energy is released.

Why?:

An endothermic reaction is one that absorbs energy. If this energy will be shown in the chemical equation, it will be written on the reactant side. It is as if one of the reactants. When a reaction is exothermic, energy is released. When shown on a chemical equation, it will be written on the reactant side as one of the products.

Answering this question:

The forward reaction is depicted as generating 55.3 kJ of energy. This indicates that the reaction is exothermic. It generates heat rather than absorbing heat from its surroundings.

Correct answer: The heat flowed from the hand to the test tube.

Why?:

Any reaction is accompanied by energy changes. This energy can be in the form of heat. When heat is absorbed in a given reaction, it is said to be endothermic. The direction of the heat flow is from the surroundings to the system. When heat is released, it is exothermic. The direction of the heat flow is from the system to the surroundings.

An exothermic reaction produces heat. When the reaction vessel of an exothermic reaction is touched, heat will flow from the vessel to the hand. The inside of the vessel is the system, and the hand is the surroundings. When the reaction vessel in an endothermic reaction is touched, a cold sensation will be felt. Here, the direction of the heat flow is from the hand to the system. As it is held by the hand longer, it can be noticed that the coldness gradually lessens as the hand transfers heat to the system.

Answering this question:

Just imagine when a hand gets burned by something hot, say a fire. The fire produces heat. It is an exothermic process. The heat produced by the fire is felt by the hand because there is transfer of that heat from the fire to the hand. Use a similar analogy in the above question. If the hand felt something hot, then the heat flow must have been from the test tube going to the hand. But since a cold sensation is felt, it is the other way around.

Therefore the direction of heat flow is from the hand going to the test tube.

Correct answer: (2) 20.0 mL

Why?:

This is an example of a neutralization reaction. HNO₃ is a strong acid and will completely ionize into the solution forming H⁺ and NO₃^- ions. KOH is a strong base and will completely ionize into K⁺ and OH^- in the solution. The balanced chemical reaction is

HNO₃ + KOH à KNO₃ + H₂O

Since there is a 1:1 ratio of acid to base in this reaction, we need the same number of moles of HNO₃ as there is KOH to neutralize all of the base.

Answering this question:

The following calculation is done to solve the problem.

moles HNO₃ = moles KOH

volume HNO₃ x molarity HNO₃ =  volume KOH x molarity KOH

volume HNO₃ =  (volume KOH x molarity KOH) / molarity HNO₃

volume HNO₃ = (100.0 ml x 0.100 M) / 0.500 M

volume HNO₃ = 20.0 ml

The correct answer is (2).

Correct answer: (2) the rates of the forward and reverse reactions

Why?:

Equilibrium is defined as the state in a reaction wherein the forward and the reverse reactions occur at equal rates. At equilibrium, the amount of the products and the reactants remain constant. But this does not necessarily mean that they are the same. This is because the forward and the reverse processes occur at the same rate. This simply means that at equilibrium, products are converted back to reactants and reactants are converted to products. But since their rates are equal, there is no net effect.

Answering this question:

In all the sciences, it is all about stability. And stability is associated with low energy just as how hyperacitivism (of high energy) is associated with instability. This implies that for the products to be spontaneously formed, the product side of the reaction must have lower energy than the reactant side. From this, it can be deduced that the reactant side of a chemical reaction has higher energies compared to the product side. Therefore (1) and (4) can be eliminated.

Regarding choice (3), concentrations of the product could be the same as the concentrations of the product but this is not always the case. Some products have greater concentration than the reactants since the reactants are the ones used to form the products.

So this leaves us with (2) as the correct answer.

Correct answer: (3) N_2(g) +  O_2(g) →  2NH_3(g)

Why?:

Endothermic reactions involve the absorption of energy. Absorption of energy can be observed in bond breaking. Bond breaking requires energy since a bond means that two atoms are held by each other. To break the bond that binds them together, an effort is needed. This effort is in the form of energy.

Combustion reactions are typically exothermic. This means that they release heat energy. For example, burning wood is a combustion reaction that releases heat energy for such applications as cooking food or heating a house. One way of determining if a given chemical reaction is a combustion process is through the presence of oxygen as a reactant. This is because oxygen supports combustion.

Answering this question:

All of the above reactions are combustion reactions, except for (3). Those combustion reactions all have oxygen as one of the reactants. Since these three reactions are all exothermic, they can't be an endothermic (heat absorbing) reaction which is demanded by the question.

The answer must be (3) by elimination.

Correct answer: (2) PE of products - PE of reactants

Why?:

The expression ΔH is an expression of the heat absorbed or evolved in a given reaction. This heat is also a measure of the potential energy of the system because heat is energy. Since heat cannot be directly measured, it is just expressed as the change in amount of heat involved after the products have formed. So ΔH is a measure of the potential energy involved in the reaction.

A change in the potential energy can be measured by comparing the "before" and the "after". In a reaction, it is comparing the difference between the potential energy of the products and the reactants. And speaking of difference, it means subtraction.

Answering this question:

Since a difference means subtraction, all the other choices except for (2) can be eliminated because they do not contain the operator for subtraction.

So the correct answer is (2).