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Chemistry Regents June 2010 - Question 76 |
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Written by The Chemistry Wizard
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Answer: H2O(l) ↔ H+(aq) + OH-(aq)
Answering the Question: The question asks to write the balanced equation between H2O(l) and H+(aq) and OH-(aq). As these are the only species mentioned in the description, they would be the most appropriate answer. |
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Chemistry Regents June 2010 - Question 74 |
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Written by The Chemistry Wizard
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Answer: Two things indicate that the entropy of the product is greater than the reactant:
- Heat is applied to the reactant to produce the product which contains gases
- The reactant spontaneously decomposes to form the products that are less ordered.
Why? Entropy is a measure of the energy distribution of a system that cannot be used for work. This means it measures the degree of spread or sharing of heat energy within a system. When heat flows from a hot region to a cold region, then the cold region would have an increase in entropy. In the case of the above reaction, energy is supplied to the reactant; the reactant decomposes and produces not only a solid but also two gases. This gives the heat energy a greater area of distribution, as 2 moles of solid reactant will result in 1 mole of solid reactant and 2 moles of gaseous reactant. This means that the heat has more area for distribution, a factor influencing the entropy of a system |
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Chemistry Regents June 2010 - Question 71 |
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Written by The Chemistry Wizard
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Answer: Heat energy
Why? Heat energy is one other factor that would influence the rate of a chemical reaction. In general, heat will provide energy to the reactants, increasing their kinetic energy and increasing the rate of collisions as a result. However that is not the only impact of heat, it will also provide the reactants with more of the necessary energy needed for the reaction to proceed. In effect the addition of heat would provide more energy towards the activation energy.
Another factor that could impact the rate of the reaction is the presence of a catalyst. Catalysts help increase the rate of a reaction by either decreasing the activation energy required or allowing the reactants to better orient themselves. |
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Chemistry Regents June 2010 - Question 70 |
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Written by The Chemistry Wizard
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Answer: An increase in the number of drops of 0.02 M KIO3(aq), would result in an increase in the rate of the reaction because there is an increased chance for correct collisions between reactants.
Answering the Question:
A chemical reaction is dependent on a number of factors. One such factor is proper collision between the reactants. The greater the concentration of reactants, the greater the chance that the collision will occur and the faster the chemical reaction will proceed. In the case of KIO3(aq), the basis of the color change is the complex formed between starch and iodine as a result of the chemical reaction below.
4 KIO3 + 10 NaHSO3 → 2 I2 + 5 Na2SO4 + 3 H2SO4 + 2 K2SO4 + 2 H2O
In this reaction:
First, the iodate reacts with the bisulfite:
1) KIO3 + 3 NaHSO3 → KI + 3 NaHSO4
Iodate + Sodium bisulfite → Potassium Iodide + Sodium hydrogen sulfate
which can also be written as
IO3- + 3 HSO3- → I- + 3 SO42- + 3 H+
The formed iodide reacts with more iodate to get iodine (I2)
2) 5 I- + IO3- + 6 H+ → 3 I2 + 3 H2O
But, the iodine will not react with starch as long as bisulfite is present. The reaction with starch is slower than the reaction with bisulfite. So the iodine will react with bisulfite to form more sulfate, until there is no more bisulfite.
3) I2 + HSO3- + H2O → 2 I- + SO42- + 3 H+
when there is no more bisulfite, then the I2 produced in step 2 will then complex with the starch.
4) I2 + starch → iodine-starch complex (blue-black) |
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Last Updated on Tuesday, 07 June 2011 16:35 |
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Chemistry Regents June 2010 - Question 67 |
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Written by The Chemistry Wizard
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Answer: Heat will always flow from an area of higher energy to an area of lower energy. Temperature is a measure of heat energy, as such it can be inferred that since the solution has the higher temperature, that heat would flow from the solution of high temperature (60°C) to the room of lower temperature (22°C). The second law of thermodynamics suggests that isolated systems will always try to attain a state of equilibrium. In the case of differences in temperature, the area with the higher temperature will always transfer energy to the area of lower energy. In question 67, this means the transfer of heat from the solution with the higher temperature to the room with the lower temperature. |
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Chemistry Regents June 2010 - Question 61 |
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Written by The Chemistry Wizard
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Answer: Heat required to vaporize 200g of water is = mHv = 200 x 2260 = 452000J or 452KJ
The heat required to melt 200g of ice is = mHf = 200 x 334 = 66800J or 66.8KJ
Far more energy is required to vaporize a sample than to melt it.
Why? The heat required for melting ice (solid to liquid) is far less than the energy required to vaporize the liquid to gas. The reason for this has nothing to do with the temperature of the substance, but the bonds keeping that substance in its current state. When a solid goes to liquid, the intermolecular forces of attraction holding the liquid together are still intact, however the particles are given sufficient energy to move in a disorganized manner. When the same substance has to change phase from liquid to gas, sufficient energy has to be introduced to the system to completely break the forces of attraction that were holding the substance together in a liquid state. Therefore, the transition from solid to liquid state is a "loosening" of the attractive forces, while going from liquid to gas is a "removal" of the attractive forces. |
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Chemistry Regents June 2010 - Question 60 |
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Written by The Chemistry Wizard
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Answer: q = mC∆T where;
q = the total amount of heat involved
m = mass
C = specific heat capacity
∆T = change in temperature
Therefore, q = mass of water x specific heat capacity of water x the change in temperature
q = 200 x 4.18 x (65-0)
q = 54340J or 54.34 KJ
Why? To determine the energy required to raise the temperature of a substance from one temperature to another, the equation q = mC∆T can be used. This allows for a change in temperature to impact the heat involved, unlike the heat of fusion. |
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Chemistry Regents June 2010 - Question 59 |
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Written by The Chemistry Wizard
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Answer: 66.8KJ
Why? The energy required to completely melt a 1g of a substance is called the heat of fusion. When a phase change occurs in a substance, the temperature of that substance will not change until the change is complete. This means that determining the heat energy required for the phase change cannot be calculated using a temperature change. The heat of fusion for a substance is constant for that substance and as a result can be used to determine the amount of energy required to melt or freeze a substance. Water is a well studied substance and is known to require 334J of energy per gram of ice to melt, or that much energy removed per gram of water to convert it to ice.
Answering the Question:
q = mHfus
q is the total amount of heat involved
Hfus is the symbol for the molar heat of fusion. This value is a constant for a given substance.
m is the mass of the substance being assessed
Therefore q = Δhfus(water) x (200/18)
q = 334 x (200)
q = 66800J or 66.8KJ |
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Chemistry Regents June 2010 - Question 47 |
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Written by The Chemistry Wizard
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Answer: (3)
Why? A voltaic cell or Galvanic cell (named after Luigi Galvani, who discovered it in 1780) is an electrical cell that produces electrical energy from chemical energy (reactions) and is considered the precursor to the modern battery. Within the voltaic cell are a number of chemical reactions resulting in the movement of electrons. The voltaic cell is composed two different metals connected via a salt bridge. Each metal is used to make a half-cell, where in one half electrons are released, and on the other half, electrons are accepted. In the voltaic cell, negatively charged species will move toward the anode and away from the cathode.
The voltaic half-cell is composed of a copper rod, in a solution of a copper salt, while the other half-cell is composed of a zinc rod in a zinc salt solution. Both half-cells would be connected using a sodium chloride salt bridge. The chemical process would involve the movement of electrons from the zinc metal rod into solution; this would cause the zinc metal to dissolve. Electrons would cross the sodium chloride salt bridge into the copper salt solution. This Cu2+ would then receive the electrons and form Cu metal on the copper rod. Therefore, the cathode side of the reaction (the copper side) is carrying out the reduction portion of the reaction as it is receiving electrons, while, the zinc side of the reaction or anode, is donating electrons.
Answering the Question:
To answer the question it is important to understand the process occurring within a voltaic cell, or at the very least that anions (negatively charged species) go to the anode while cations go to the cathode. Based on the chemical equation Zn will act as an electron donor and Cu2+ would be an electron acceptor. Zinc, as the electron donor would be the anode side of the reaction where oxidation is occurring. While, the copper side would be receiving electrons and would be responsible for the reduction portion of the reaction, as such it would be the cathode side. Therefore, answer (3) is the only possible answer. |
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Chemistry Regents June 2010 - Question 43 |
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Written by The Chemistry Wizard
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Answer: (4)
Why? Chemical reactions in equilibrium will always adjust to try and maintain that equilibrium. The above reaction would be categorized thermodynamically as exothermic. This means the forward reaction is producing heat as once of its products. If the temperature of the system is decreased, it means that the product in the form of energy is being removed. The forward reaction will, therefore, increase to compensate for the loss of heat, hence trying to place the system in a state of equilibrium. The reverse is also true, if the temperature is increased, then the equilibrium will shift towards the production of more reactant.
Answering the Question:
To answer the question, knowledge of how equilibrium reactions behave is important. Answer (1) suggests that a decrease in temperature (energy) would result in an increase of the reactants. As energy can be treated as a product in this situation, it could be equated to saying that if product is removed then the reactants would increase, and that is not possible. Answer (2) suggests that one reactant would increase (N2) while the other reactant would decrease (H2), which also seems unlikely. Answer (3) suggests that 1 product and 1 reactant would decrease, this also does not support the concept of equilibrium. Answer (4) suggests a decrease in reactant and an increase in product. Answer (3) is in agreement with the concept of equilibrium and is, therefore, the correct answer. |
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