1. activation energy
2. heat of reaction
3. potential energy of the reactants
4. potential energy of the products

Correct Answer Number: 1

Explanation: A catalyst speeds up a chemical reaction by lowering (decreasing) the activation energy. See also question #8, this topic.

1. hydrogen fluoride
2. hydrogen iodide
3. ethene
4. ethyne

Correct Answer Number: 1

Explanation: Exothermic reactions have a negative H. Choices 2, 3, 4 have positive a H. Only choice 1 is negative. You could have also used Process of Elimination.

1. The sign of H is positive, and the products have less potential energy than the reactants.
2. The sign of H is positive, and the products have more potential energy than the reactants.
3. The sign of H is negative, and the products have less potential energy than the reactants.
4. The sign of H is negative, and the products have more potential energy than the reactants.

Correct Answer Number: 2

Explanation: Endothermic reactions absorb energy so there is more potential energy in the reactants than in the products. (Gain in Heat energy = increase in potential energy of reactants.) Table I notes endothermic reactions are positive. Eliminating choices 3 and 4, only choice 2 has a positive H and a gain in potential energy.

1. This change increases the density of the reactant particles.
2. This change increases the concentration of the reactant.
3. This change exposes more reactant particles to a possible collision.
4. This change alters the electrical conductivity of the reactant particles.

Correct Answer Number: 3

Explanation: Reaction rate is affected by nature and concentration of reactants, temperature, surface area and a catalyst. Increasing surface area exposes more particles to contact with reactants, increasing the number of particle collisions. Imagine a cube of chocolate 3 feet by 4 feet and 2 inches thick. Take the same block and make it into a 24 candy bars. A class of chemistry students could eat the bars faster than the single chocolate block.

1. decreased temperature and decreased concentration of reactants?
2. decreased temperature and increased concentration of reactants?
3. increased temperature and decreased concentration of reactants?
4. increased temperature and increased concentration of reactants?

Correct Answer Number: 4

Explanation: Rate or speed of reaction is influenced by temperature, concentration, surface area, nature of reactants, and presence of a catalyst. To speed up a reaction, increase the number of effective collisions between molecules. Temperature is a measure of the average kinetic energy, so increasing temperature speeds up a reaction. Increasing the concentration of the reactants adds more particles and this also increases the number of effective collisions.

1. potential energy of the products
2. potential energy of the reactants
3. heat of reaction
4. activation energy

Correct Answer Number: 4

Explanation: Activation energy is the minimum amount of energy needed to start a chemical reaction. (Sort of like the minimum amount you can pay on a credit card balance each month). The total $ due has not changed but you can get by paying the smaller amount. Activation energy does not change the potential energy of the reactants or products and it has no effect on the heat of reaction. A catalyst speeds up the reaction because it decreases the amount of energy needed to start the reaction (minimum balance payment). See also question #1, this topic.

1. The amount of products is greater than the amount of reactants.
2. The amount of products is equal to the amount of reactants.
3. The rate of the forward reaction is greater than the rate of the reverse reaction.
4. The rate of the forward reaction is equal to than the rate of the reverse reaction.

Correct Answer Number: 4

Explanation: Equilibrium means equal reaction rates. Choices 1 and 3 are "greater than", not "equal to" and choice #2 does not mention reaction rates. Only choice 4 states
" equal reaction rates." A system in equilibrium can have unequal amounts of product and reactants; or it could have equal amounts of product and reactants. Equal reaction rates, however, are necessary for dynamic equilibrium to result.

1. NH₄Cl
2. NH₃
3. HCl
4. KCl

Correct Answer Number: 2

Explanation: See Ref. Table G. Notice the curves for choices 1 and 4 increase. Choice 2 and 3 both decrease but choice 2 (NH₃) decreases more than 50 degrees, while HCl decreases only about 16 degrees.

1. stirring the solute and solvent mixture
2. increasing the surface area of the solute
3. raising the temperature of the solvent
4. increasing the pressure on the surface of the solvent

Correct Answer Number: 3

Explanation: This is an easy but tricky question. Choices 1 and 2 make KCl dissolve faster (rate of reaction) but do not increase the solubility of KCl. A glass of soda pop goes flat as it warms up on the counter; hot chocolate can dissolves more marshmallows than a cup of cold chocolate milk can. Temperature affects solubility.
In the case of HCl, increasing the temperature, increases the solubility. Look at Ref. Table G and notice how the solubility curve of KCl increases as the temperature increases.

1. decreasing the temperature to 15°C?
2. increasing the temperature to 35°C?
3. dissolving NaCl crystals in the equilibrium mixture?
4. dissolving NH₄NONH₃ crystals in the equilibrium mixture?

Correct Answer Number: 2

Explanation: Part 1: in the equation H = + 3.5 kcal/mol: the reaction is endothermic. If you forget that a positive H indicates an endothermic reaction, see Ref. Table I for this info. (Use the tables; they have lots of memory helps.)
Part 2: supplying heat (by increasing the temp.), favors the reaction that absorbs or uses up the extra heat (favors the endothermic reaction). Which one is that? The forward reaction or right side because H was +. This illustrates the Le Chatelier's principle: if a stress (like a change in temperature or concentration) is added to a system at equilibrium, the reaction is shifted in a way that uses up the stress.
Helpful memory hint: Increasing temperature of a system at equilibrium, favors the endothermic reaction. See also #15 and # 16 this topic.

1. decrease
2. increase
3. remains the same

Correct Answer Number: 1

Explanation: LeChatelier's principle: if a stress (like a change in temp. or concentration) is added to a system at equilibrium, the reaction is shifted in a way that uses up the stress. Increase the left side, the right side tries to use up the extra N₂(g). But to make more NO(g), you need to use some of the O₂(g) from the left side--remember you did not add any extra O₂(g)--so the amount of O₂(g) has to decrease.
See question #13 and #16 , this topic for other examples of LeChatelier's principle.

1. reactants, only
2. products, only
3. both reactants and products
4. neither reactants nor products

Correct Answer Number: 3

Explanation: This question can be solved several different ways. First by logic: pretend you become a new cable subscriber (a change or stress is added to your life). This will also affect the number of hours you watch TV (reaction). The reverse is also true: stop the cable and you probably will watch less TV. Only choice 3 makes sense. Remember the math rule? Whatever you do to one side of the equation, effects the other side.
Second way: Le Chatelier's principle applies here (if a stress like a change in temperature or concentration is added to a system at equilibrium, the reaction is shifted in a way that uses up the stress). If more CO(g) and O₂(g) were added, more CO₂(g) would result.
If more product, CO₂(g), was added, the concentration of the reactants would increase by using up the excess product and changing it back to CO(g) + O₂(g. See also question #13 and #15, this topic.

1. decreases
2. increases
3. remains the same

Correct Answer Number: 1

Explanation: Entropy is a measure of the disorder or randomness (lack of order) of a system. Molecules in gases have no definite volume and are more free to move than molecules in solids. As molecules change phase from a gas to a solid, the molecules are becoming more orderly and therefore have less disorder, less entropy. Compare the room of a teenage to the room of a parent. The teen has a more disorderly room (higher entropy). As the teen gets older and becomes a parent, the disorder in the room decreases.

1. C(s) + O₂(g) <-> CO₂(g)
2. CaCO₃(s) <-> CaO(s) + CO₂(g)
3. 2Mg(s) + O₂(g) <-> 2MgO(s)
4. 2H₂(g) + O₂(g) <-> 2H₂O(g)

Correct Answer Number: 2

Explanation: A change in pressure affects equilibrium when gases are involved. An increase in pressure favors the formation of the smaller number of moles of gas (shifts to the side with the smaller number of moles of gas).
If there is no change in the number of moles of gas, pressure has no effect on equilibrium.
There is no effect on Choice 1 because each side has only 1 mole of gas. Choice 3 has 1 mole of gas on the left, none on the right so increased pressure shifts it to the right.
Choice 4 has 3 moles of gas (2H and 1 O) on the left, 2 moles on the right, so increased pressure shifts it to the right. Choice 2 has no gas moles on the left and 1 mole of gas on the right, so increased pressure shifts equilibrium to the left. Voila: Choice 2.

1. aluminum
2. ammonia
3. nitric acid
4. sulfuric acid

Correct Answer Number: 2

Explanation: In the Haber process, ammonia is produced. Sulfuric acid is produced by the contact process. See also #14, this topic.

1. adding a catalyst
2. adding more O₂(g)
3. decreasing the pressure
4. increasing the temperature

Correct Answer Number: 2

Explanation: To get more product [right side, SO₃(g )], add more reactants (left side). As the reaction reaches equilibrium, the extra reactants are being converted into product [SO₃(g) + heat] and the reaction shifts to the right.