Unit 6 check
Unit 6 check: Thermochemistry
10 questions from 9 lessons in Unit 6. Answer each one, and if you miss, follow the link back to where it is taught, then try again. Your score counts first tries.
Question 1 · from 6.1 · Decide and justify
A student drops magnesium ribbon into hydrochloric acid in a foam cup. A thermometer in the solution reads 21.0 °C before the reaction and 34.5 °C after it. Is the reaction of the magnesium and the acid exothermic or endothermic?
Which reason supports your decision?
Question 2 · from 6.2 · Fill the statement
A solid dissolves in water and the water cools. Complete the description of the energy diagram a student should draw for the dissolving.
The vertical axis is labeled . The water cooled, so the system took in energy and the products’ level is drawn the reactants’ level. The ΔH arrow points .
Question 3 · from 6.3 · Decide and justify
A warm metal block has sat in an insulated cup of cool water for an hour, and a thermometer shows the two at the same temperature. A student says the particles of the block and the water have stopped colliding with each other. Is the student right?
Which reason supports your decision?
Question 4 · from 6.4 · Decide and justify
A student dissolves a few grams of calcium chloride in 50.0 g of water in a coffee-cup calorimeter. The thermometer reads 21.0 °C before and 30.5 °C after the solid dissolves. Is dissolving calcium chloride exothermic or endothermic?
Which reason supports your decision?
Setup for question 5
Ice cools a drink by absorbing energy as it melts. The two items below use the molar enthalpies of fusion and vaporization of water.
Question 5 · from 6.5 · Calculate
A drink gives up 12.5 kJ of energy, all of it absorbed by ice at 0 °C as the ice melts completely to water at 0 °C. What mass of ice melts, in grams? The molar enthalpy of fusion of water is 6.01 kJ/mol and its molar mass is 18.02 g/mol. Give your answer to three significant figures.
Setup for question 6
A student runs three reactions, P, Q and R, one at a time. Each goes in an identical foam cup holding the same mass of water at the same starting temperature, and nothing else exchanges energy with the cup. The table gives each reaction's molar enthalpy of reaction, per mole of reactant A, and the moles of A that react. These numbers are a teaching example.
| Reaction | ΔH (kJ per mole of A) | Moles of A that react |
|---|---|---|
| P | −132 | 0.100 |
| Q | +58.5 | 0.200 |
| R | −75.6 | 0.300 |
Question 6 · from 6.6 · Calculate
Using the table: what is q for reaction Q, in kilojoules? Include its sign, and give your answer to three significant figures.
Setup for question 7
A student estimates the energy change for the reaction CH₄ + 2 Cl₂ → CH₂Cl₂ + 2 HCl, in which two of the hydrogen atoms of methane are replaced by chlorine atoms. CH₂Cl₂ has two C-H bonds and two C-Cl bonds. The table lists the average bond energies the student uses. These numbers are a teaching table.
| Bond | Average bond energy (kJ/mol) |
|---|---|
| C-H | 413 |
| Cl-Cl | 243 |
| C-Cl | 339 |
| H-Cl | 431 |
| H-H | 436 |
Question 7 · from 6.7 · Calculate
Using the table: what total energy, in kJ, is required to break all the bonds in the reactant molecules, one CH₄ and two Cl₂? Give your answer to the nearest kJ.
Setup for question 8
Nitrogen oxides form in hot engines and in industry, where burning ammonia is the first step in making nitric acid. A student looks up standard enthalpies of formation at 298 K and 1 atm for four compounds.
| Substance | ΔH°f (kJ/mol) |
|---|---|
| NH₃(g) | −45.9 |
| NO(g) | +90.3 |
| NO₂(g) | +33.2 |
| H₂O(g) | −241.8 |
Question 8 · from 6.8 · Multiple choice
Using the table: N₂(g) and O₂(g) have no row in it. A student says ΔH° for N₂(g) + O₂(g) → 2NO(g) cannot be found until those two are looked up. Which response is correct?
Question 9 · from 6.9 · Spot the error
The reaction 2NO(g) + O₂(g) → 2NO₂(g) has ΔH = −114.2 kJ. A student finds ΔH for 4NO₂(g) → 4NO(g) + 2O₂(g). Tap the line with the first mistake.
Question 10 · from 6.1 · Sort it
Each card names a change. Tap a card, then the bin that says whether the process is exothermic or endothermic for the substance or reacting species named.
Exothermic
Endothermic
0 of 10 answered.