During digestion, the energy in food is converted to energy the body can use. Scientists use calorimetry experiments to measure the calories, or energy, provided by food when it is digested or burned.

The relationship used to find the heat transferred energy is given by , where is the mass of the material, is the given specific heat capacity, and is the change in temperature of the material.

In this experiment, food was burned over a Bunsen burner under a can of 200 ml of water. The temperature change of the water and mass change of the food can be used to determine the calories in four different food items.

Table 1 shows the values of the change of mass of the food items, the change in temperature of the water and the energy. Table 2 shows the energy to mass ratio of three of those food items.

Table 1

Roasted Peanut Peanut Cracker Cheese Puff

Water Temp. Initial 23.9 °C 33.2 °C 40.3 °C 53.9 °C

Water Temp. Final 30.0 °C 40.9 °C 55.9 °C 62.8 °C

Food Mass Initial 0.69 g 0.61 g 3.21 g 1.22 g

Food Mass Final 0.38 g 0.21 g 0.91 g 0.48 g

Energy 1.22 Cal 1.54 Cal 3.12 Cal 1.78 Cal

Table 2

Sample Energy to Mass Ratio (Cal/g)

1 1.36

2 3.93

3 2.40

Based on the information in Table 1, what variables must be measured in order to calculate the energy of the food samples?

Kevin wants to know if a particular kind of chemical fertilizer will help or hinder the growth of his tomato plants. He decides to conduct an experiment in which he grows three plants, one left untreated, one treated with the chemical fertilizer RapidGro and one treated with an organic compost. He records his findings in the charts below, measuring plant height and number of tomatoes over a period of time.

Height of plant (inches):

Number of tomatoes:

On the fourteenth day Kevin picks the biggest tomato from each plant and record its dimensions, as well as other information, which is found below.

Tomato 1 (no fertilizer): in diameter, dull red, lumpy in shape, wormholes, flavorful.

Tomato 2 (RapidGro): in diameter, shiny red, round, somewhat tasteless.

Tomato 3 (compost): in diameter, deep red, lumpy shape, very flavorful.

Which plant is the control?

There are two types of forces that occur with all substances on Earth. Intramolecular forces occur between atoms in a molecule, while intermolecular forces occur between neighboring molecules. Intermolecular forces can be dipole-dipole forces, hydrogen bonding, or London dispersion forces.

Professor 1:

Water molecules represent an example of hydrogen bonding due to the attraction between the hydrogen atoms and the oxygen atoms in the molecule. This strong dipole-dipole occurs due to lone pairs present on such atoms as Fluorine, Nitrogen, and Oxygen, which are able to pair more closely to the hydrogen atom in another nearby molecule. Water can be present in a solid, liquid, or gaseous state on Earth depending on the competition between the strength of intermolecular bonds and the thermal energy of the system. In 1873, a Dutch scientist, Van der Waals derived an equation that included both the force of attraction between the particles of a gas and the volume of the particles at high pressures. This equation led to a better fit for experimental data than the Ideal Gas Law.

Professor 2:

Water is the only substance on Earth that we routinely encounter as a solid, liquid, and gas. At low temperatures, the water molecules lock into a rigid structure, but as the temperature increases, the average kinetic energy of the water molecules increases and the molecules are able to move more creating its other natural states of matter. The higher the temperature, the more likely water is to be a gas. Water is proof of the kinetic theory, which assumes that there is no force of attraction between the particles of the gas state. The best fit for experimental data involving water in a gaseous form is found by using the Ideal Gas Law, since there is no interaction between the gaseous molecules. This law accounts for all of the forces that occur with gases on Earth.

Which of the following experiments could solve the debate between the two professors?

Naturally occurring water in lakes and reservoirs used as sources for drinking water feature a variety of dissolved minerals such as magnesium, sodium, and calcium. Water treatment plants must closely monitor the levels of these minerals to ensure they do not exceed unsafe levels. An experiment carried out by a scientist at a water treatment plant are described below.

Experiment 1:

A common way to determine the concentration of a particular chemical is by titration. In this titration, 10mL of the treated water sample was placed in a flask as shown below in Figure 1. A buret, (a special funnel with volume markings on the side and a knob on the bottom to control how much of the substance in the buret is dispensed) was placed above the flask as shown in Figure 1. It was filled with 50mL of a 20ppm (parts per million) solution of EDTA, a chemical that can react with magnesium to chemically remove it from the water. An indicator (a substance that changes color to indicate a chemical change) was also placed into the flask; this indicator appears purple in water solutions containing magnesium, and blue in water solutions without magnesium. The buret was used to dispense EDTA solution until enough EDTA had been added to the purple magnesium-containing water solutions to remove all the magnesium and turn the solution blue. The volume, in milliliters, of EDTA solution added to each of five water samples is recorded in Table 1.

Figure 1

Why does the researcher use the buret?

Clock reactions are chemical interactions that exhibit a physical change periodically over a given time interval. Many of these reactions involve iodine, the most famous being the Chlorine Dioxide-Iodine-Malonic Acid reaction. These reactions can be quite startling as flasks of colorless liquid periodically turn dark blue and then resolve back to their original colorless state. Even more striking, they seem to alternate between being colorless and blue several times. The term "clock reaction" is derived from the fact that the time at which these sudden changes occur can be predicted.

Beyond performing these reactions in a well stirred beaker, there are two other notable ways to conduct experiments with clock reactions that demonstrate interesting properties of these reactions. The first is in a continuous flow stirred tank reactor (CSTR). In a CSTR, the reactants are introduced at a continuous rate while the volume of liquid in the reactor is kept constant by siphoning off excess fluid. The result of this process is that one can maintain the ideal conditions in which the reaction may occur over time and restricts the buildup of excess product or reactant that would otherwise make the oscillations of the reactions decay. In a CSTR, clock reactions can be maintained switching predictably from colorless to blue, for example, for far longer than in a simple beaker.

The second way to conduct a clock reaction experiment is in a tank with no stirring at all. This allows the reactants to interact heterogeneously, or without being thoroughly mixed. When this occurs, we can get some parts of the tank that are one color and other parts that are another color. This means that we can observe two different stages of the reaction in one vessel. The patterns that this makes are called Turing patterns, named by the great computer scientist Alan Turing. Turing predicted that the heterogeneous mixing of chemicals called morphogens in complex organisms were responsible for biological pattern formation like spots on a leopard, stripes on a zebra, or patterns on a tropical fish. The existence of such patterns and chemicals has since been confirmed and clock reactions are often used to study these types of Turing patterns.

Given that different patterns happen when different concentrations of the reactants interact with each other, which of the following would NOT be a useful experimental variable in an experiment designed to explore the mechanism and dynamics of different patterns of chemicals in clock reactions?

A brilliant but underpaid high school chemistry teacher, together with his former student, manufactures a chemical compound, which they sell for a large mark-up. Their set of reactions are given below:

(1) A + B + C + acid --> D

(2) D --> intermediate

(3) Intermediate --> E

(4) E + acid --> X

His student notes the following observations:

Observation 1) If a greater amount of B and C are used in the reaction, this yields no increased amount of product D.

Observation 2) At any given point, D, the intermediate, and E may co-exist in equal proportions. However, at higher temperatures, higher proportions of product E are noted.

Observation 3) The addition of a strong base can reverse reaction 4.

If reagent A was the limiting reagent, how might the student determine this?

Naturally occurring water in lakes and reservoirs used as sources for drinking water feature a variety of dissolved minerals such as magnesium, sodium, and calcium. Water treatment plants must closely monitor the levels of these minerals to ensure they do not exceed unsafe levels. An experiment carried out by a scientist at a water treatment plant are described below.

Experiment 1:

A common way to determine the concentration of a particular chemical is by titration. In this titration, 10mL of the treated water sample was placed in a flask as shown below in Figure 1. A buret, (a special funnel with volume markings on the side and a knob on the bottom to control how much of the substance in the buret is dispensed) was placed above the flask as shown in Figure 1. It was filled with 50mL of a 20ppm (parts per million) solution of EDTA, a chemical that can react with magnesium to chemically remove it from the water. An indicator (a substance that changes color to indicate a chemical change) was also placed into the flask; this indicator appears purple in water solutions containing magnesium, and blue in water solutions without magnesium. The buret was used to dispense EDTA solution until enough EDTA had been added to the purple magnesium-containing water solutions to remove all the magnesium and turn the solution blue. The volume, in milliliters, of EDTA solution added to each of five water samples is recorded in Table 1.

Figure 1

A new EDTA titration solution is prepared with 40ppm EDTA. Approximately how much EDTA titration solution should the scientist expect to use to titrate a 10mL sample of the same water?

The rate of a reaction is defined as the rate at which the products of a reaction are produced. A researcher wants to investigate the rate of reaction of the combustion of theoretical compound . The researcher combusts different amounts of starting material (the theoretical compound and oxygen gas ) and measures how the rate of reaction is affected by changing the amounts of starting material. Because the researcher is concerned about how the conditions of his lab may change, the researcher repeats trial 1. The researcher finds that the results of repeating trial 1 were identical each time.

The results of the experiment are shown in the table below. The brackets (\[\]) indicate concentration measured in mol/L.

Which of the following is the best explanation for the repetition of trial 1 during the experiment?

A student wanted to study the kinetics, or rates of a chemical reaction based on the concentrations of its reactants and products, of the reaction shown below.

This reaction is easy to monitor using a spectrophotometer, which measures how much light of a particular wavelength is absorbed by a solution. The deep purple potassium permanganate, or , absorbs light of a 550 nm wavelength in proportion to its concentration in the reaction solution. Manganese sulfate, or , is pale pink and absorbs light of a 500 nm wavelength in proportion to its concentration in the reaction solution. All other reactants and products are colorless and do not absorb visible light and thus cannot be monitored using the spectrophotometer.

Experiment 1:

The student constructed a standard curve, or a graph of the absorbance of solutions of varying concentrations of potassium permanganate, to quantify the relationship between concentration and absorbance. To prepare five sample of increasing concentration, he labeled five test tubes A, B, C, D, and E, weighed out 0.1, 0.2, 0.3, 0.4, and 0.5 grams of potassium permanganate into each, respectively, and added 1 milliliter (mL) of water to each test tube to dissolve. Then, he used the spectrophotometer to determine the absorbance at 550 nm of each sample. The data is graphed in Figure 1 below.

Figure 1

Experiment 2:

The student then studied potassium permanganate in the presence of oxalic acid, , to observe the reaction. Monitoring both the absorbances of potassium permanganate and manganese sulfate, he was able to determine the reaction rate using a special setting on the spectrophotometer. The reaction rate at various concentrations of reactants is shown below in Table 1.

A scientific relationship, known as Beer's Law, mathematically correlates absorbance to concentration via the following equation:

where is the absorbance, is the concentration of the sample, and is a constant specific for the chemical in the sample. According to Figure 1, what is the value of , in mL/g, for potassium permanganate?

The Environmental Protection Agency compiled the following information about heavy metals in drinking water. Heavy metals are toxic, and thus must be monitored very closely. All amounts are given in parts per billion (ppb), a common measurement of concentration for trace contaminants. Figure 1 shows total heavy metal contamination in each city as a makeup of various percentages of specific contaminants. Figure 2 shows specific amounts of contaminants (with the same color coding) in ppb as well as total contamination level in ppb.

Figure 1

Figure 2

A scientist wants to publish a paper about water treatment standards in the United States. Which figure is a better figure to include in her report?