Correct Answer:(3) The rate of the forward reaction is equal to the rate of the reverse reaction.

Why:

Chemical equilibrium is the state in which the chemical activity or the concentrations of the reactants and products have no net change over time.  This is the state in which the forward reaction goes at the same rate as the reverse reaction. Their rates are not zero, but simply equal. Therefore there are no net changes in the product or reactant concentrations.   Therefore (3) is the best answer.

Answering this question:

The key to answering this question correctly is knowing that at equilibrium the rates of the forward and reverse reactions are equal.  You can eliminate (1) because we know the reaction does not just simply stop it is always going forward and backwards.  You can eliminate (2) because we stated that the rates equalize making no net change in the overall products and reactants.  It does not state that there must be an equal amount of product and reactant. For the same reason you can eliminate (4), the forward and backward reactions have the same rates, but it does not state that the Moles are equal.  This leaves you with (3) as the only possible answer.

Correct Answer: (2) a decrease in activation energy and increase in the reaction rate.

Why:

A catalyst is a substance that accelerates a chemical reaction.  This is done by lowering the activation energy required to begin the reaction and allows it to run more efficiently.  The catalyst will often work by changing the structure of a molecule or by bonding to the reactant molecules and causing them to combine, react, and release a product or energy.  In the process the catalyst is unchanged and can often be reused over and over.

Answering this question:

The simplest way to tackle a catalyst question is to remember that catalysts always increase the rate of the reaction.  Realizing this will immediately eliminate answers (1) and (3).  To narrow down to the final answer you have to consider what the activation energy is and which would make a reaction faster.  Lowering the energy required for the reaction would make it easier and faster to perform and therefore a catalyst lowers the activation energy required and increases the rate of the reaction (2)

Correct answer: (2) lower energy and higher entropy

Why:

According to the Second law of thermodynamics, systems in nature tend to move towards lower energy and higher entropy.  Entropy is a measure of randomness or disorder of a system. Therefore a system with greater disorder has greater entropy.  Another way entropy can be described is the number of states available to a system at a given time, where states can be various things such as speed or position.

A simple way to imagine entropy is to think about probability.  Is it more likely to have a disordered system or a very ordered one?  For example imagine 10 pennies in a jar.  You can turn them all to heads up (low entropy ordered system) but shake the jar and  you can imagine that some of the pennies will now be head and some tails ( a high entropy less ordered system)  It certainly is possible that they'll all remain heads but its unlikely and we can see that the state favors higher entropy.

Answering this question:

The best way to answer this question is to simply memorize the facts. Systems in nature tend to go towards lower energy and higher entropy.   If you get caught and need to reason it out this question is still possible.  We know that the natural state of elements, compounds etc is their lowest energy state.  It would make sense to apply that same concept here and know that systems would tend to go to the lowest energy state.  We can also remember this by remembering hot water will inevitably cool unless more energy/heat is added to it.  This allows us to narrow our choices down to two possible answers.  Then we have to imagine what happens to entropy or the order of a system.  We know that water left out will eventually evaporate and doesn't stay in its ordered state.  We all realize that keeping anything structured and orderly requires effort and energy and left unattended will tend to favor disorder.  This leaves (2) as the only possible answer.

Correct answer: (1) sodium oxide

In order to determine the ratio of metal ions to nonmetal ions, it is important to breakdown the chemical formula into the ions present in the substance.  An ion is defined as a charged atom because there is disparity in the number of protons and electrons.   This variation in number occurs when there is a gain or a loss of electrons of an atom.

Answering the question:

Potassium sulfide is empirically denoted as K₂S.  The elements in the compound are Potassium, K and Sulfur, S.  K is the metal component of the substance while S is the nonmetal.  We can deduce from the periodic table that the oxidation number of K is 1+ while that for S is 2-. In order to produce the compound with a net charge of zero, two K ions are needed to cancel out one S ion.  Following this, we use a subscript of two after the K symbol to show the ration.  Therefore the metal:nonmetal ion ratio is 2:1.

Option no. 2, sodium chloride, empirically denoted as NaCl is eliminated because the ratio is 1:1.  One sodium ion is needed to balance out one chloride ion.

Option no. 3, magnesium oxide, empirically denoted as MgO is eliminated because the ratio is 2:2 or simply 1:1.  Two magnesium ions are needed to cancel out two oxide ions.

Option no. 4, magnesium chloride, empirically denoted as MgCl₂ is also eliminated because the ratio is 1:2.  This means two chloride ions are needed to balance out one magnesium ion.

This leaves us option no. 1, sodium oxide, Na₂O as the answer having a 2:1 metal:nonmetal ion ratio.  This means that two sodium ions are needed to cancel out one oxide ion.

However, even if you did not know the empirical formulae for the molecules, comparing the groups of the elements in the period table would give an indication. Na is a group 1 metal, so is K which means they have similar properties when it comes to ionic state. Sulfur and oxygen are also are also in the same group and again have similar properties in terms of ionic state. Therefore a good guess would be (1) as none of the other combinations have elements in the same group as K and S.

Endothermic process is a type of reaction that absorbs energy in the form of heat.  When you break a chemical bond, energy is required, therefore, the process is said to be endothermic.  On the other hand, when a system releases energy in the form of heat, light or sound, the process is said to be exothermic.

Entropy is defined as the tendency of a system to shift from a higher level to a lower level in terms of molecular structure.

Answering the question:

As seen from the given balanced equation, 34.89 kJ of heat was added to KNO₃ which resulted in the breakdown of chemical bonds forming K⁺ and NO₃^- ions.  Therefore, this process of introducing heat to break chemical bonds is called an endothermic process.  It follows that when heat is added to a system, the entropy is sure to increase as well.

Therefore, option no. 1 is the answer.  Option nos. 2, 3 and 4 are all eliminated.

Zinc is a metal which is reactive with aqueous acids according to the equation below.

Zn_(s) + 2HCl_(aq) à ZnCl_2(aq) + H_2(g)

Zinc is not a highly reactive metal and requires two HCl molecules for every atom of zinc. If a higher concentration of HCl is used, then according to collision theory, there is a higher chance that zinc will collide with it, which is essential for the reaction to occur. This increased proper contact will improve the reaction rate. The same holds true for an increase in temperature. An increase in temperature leads to more excited molecules and a greater chance of collision. But more importantly it provides the reactants with energy helping them attain the required activation energy to react when they do collide.

Answering the question:

When answering this question there are a few general rules to note;

1)      When two reactants are mixed, the higher the concentration, the greater the rate of reaction. You have a greater chance of stepping on someone toes in a crowded room than you do in an empty one.

2)      Generally speaking for every 10°C increase in temperature, there is a doubling in the reaction rate.

Therefore when answering the question it is safe to assume that the greater the concentration and temperature the faster the reaction rate. Answer (4) has both the greater concentration and higher temperature.

Which leaves (4) as the correct answer

Correct Answer:

Answering the Question:

The above diagram is a potential energy diagram showing the forward reaction of the equilibrium equation. In order to draw the diagram correctly it is important to remember a few points. Firstly, whether the reaction is going in the forward or reverse direction, the activation energy must be overcome for the reactants to form product. Secondly, the above diagram is for an exothermic (gives off heat) reaction, an endothermic (requires heat) would be the same diagram moving in the reverse.

Therefore, it is important to remember the flow of the diagram, and if heat is given off, as is the case here, then the products will have a lower potential energy than the reactants. If heat is required then the products will have a higher potential energy than the reactants.

Correct Answer:

The concentration of NH₃(g) will decrease with increasing temperature because the reversible reaction to produce NH₃(g) is exothermic, that is it produces heat. According to Le Chatelier's principle if a chemical system at equilibrium experiences a change in concentration, temperature, volume, or pressure, then the equilibrium shifts to counteract the imposed change. In this case 1127kj is a measure of the heat energy resulting from the production of NH₃(g). Another way to interpret it is that heat is a product of the reaction. If heat is added to the reaction then less "heat product" is going to result as the equilibrium shift to counteract the added heat. This results in all the other products on the right of the reaction being reduced, hence less NH₃(g) being produced.

Answering the question:

It is important know and understand what Le Chatelier's principle states. The principle operates on the basis that reversible reactions are always seeking equilibrium and will do what is necessary to achieve it. This is often manipulated by scientist to obtain better yields of a product made from a reversible reaction. In the above case heat is treated as a product, if more product is formed then the system will try and correct itself by creating less product and once again achieving equilibrium.

Example Answer:

During the first three minutes the temperature is increasing. Temperature is a measure of the average energy of the particles making up a thermodynamic system. If kinetic energy is being added to a system and temperature is a measure of the average energy then it an indicator of the amount of kinetic energy present in the system at a specific point in time.

Correct Answer: 915 joules

The heat of fusion is the thermal energy required for 1 mole of substance to change state from a solid to a liquid. This energy is the extra energy that is required when melting is occurring and is responsible for the temperature remaining constant during this period.

Answering the question:

To answer this question you do not require any knowledge of chemistry but instead basic mathematics. Since heat of fusion is 122 joules per gram and you are melting 7.5 grams then the answer would be 7.5g x 122j/g = 915 joules.