Heats of Reaction

A student mixes a 10.0 mL sample of 1.0 M NaOH(aq) with a 10.0 mL sample of 1.0 M HCl(aq) in a polystyrene container. The temperature of the solutions before mixing was 20.0°C. If the final temperature of the mixture is 26.0°C, what is the experimental value of ? (Assume that the solution mixture has a specific heat of 4.2 J/(g•K) and a density of 1.0 g/mL.)

-50. kJ/molrxn

Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) The chemical equation shown above represents the reaction between Mg(s) and HCl(aq). When 12.15g of Mg(s) is added to 500.0mL of 4.0MHCl(aq), 95kJ of heat is released. The experiment is repeated with 24.30g of Mg(s) and 500.0mL of 4.0MHCl(aq). Which of the following gives the correct value for the amount of heat released by the reaction?

190kJ

How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from KCl(s)?

43.7 kJ is absorbed

Which of the following best describes the flow of heat when 1.0 mol of XY2 decomposes?

50 kJ of heat is transferred from the surroundings.

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxn The chemical equation above represents the reaction between HCl(aq) and NaOH(aq). When equal volumes of 1.00MHCl(aq) and 1.00MNaOH(aq) are mixed, 57.1kJ of heat is released. If the experiment is repeated with 2.00MHCl(aq), how much heat would be released?

57.1kJ

For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students' observation?

An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.

Which of the following best helps to explain why the value of ΔH° for the dissolving of CaF2 in water is positive?

Ca2+ ions have very strong ion-ion interactions with F- ions in the crystal lattice.

Two trials are run, using excess water. In the first trial, 7.8 g of Na2O2(s) (molar mass 78 g/mol) is mixed with 3.2 g of S(s). In the second trial, 7.8 g of Na2O2(s) is mixed with 6.4 g of S(s). The Na2O2(s) and S(s) react as completely as possible. Both trials yield the same amount of SO2(aq). Which of the following identifies the limiting reactant and the heat released, q, for the two trials at 298 K?

Limiting Reactant Na2O2. q 30. kJ

Which of the following phase changes involves the transfer of heat from the surroundings to the system?

NH3(l)→NH3(g), because NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

The dissolution of an ionic solute in a polar solvent can be imagined as occurring in three steps, as shown in the figure above. In step 1, the separation between ions in the solute is greatly increased, just as will occur when the solute dissolves in the polar solvent. In step 2, the polar solvent is expanded to make spaces that the ions will occupy. In the last step, the ions are inserted into the spaces in the polar solvent. Which of the following best describes the enthalpy change, ΔH, for each step?

Steps 1 and 2 are endothermic, and the final step is exothermic.

In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C. When the forecast calls for air temperatures to be below −5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below −2°C?

The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C.

In an experiment, 30.0g of ethane and 30.0g of propanol are placed in separate reaction vessels. Each compound undergoes complete combustion with excess O2(g). Which of the following best compares the quantity of heat released in each combustion reaction?

qethane>qpropanol

2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1120kJ/molrxn Based on the reaction represented by the chemical equation shown above, what is the amount of heat released when 4.00mol of H2S(g) reacts with 9.00mol of O2(g)?

−2240kJ