AP Chemistry Unit 5

Determine the enthalpy of the reaction given the following data.

• A. 352.6 kJ/mol

• B. -352.6 kJ/mol

• C. 176.3 kJ/mol

• D. -176.3 kJ/mol

Answer explanation

Given the heats of formation we can use the "products minus reactants" equation. ΔH = [2x-268.6]-[2x-92.3]

Tags

NGSS.HS-PS1-4

What is the enthalpy of reaction depicted in the reaction below?

• -299.4 kJ/mol

• 149.7 kJ/mol

• -149.7 kJ/mol

• 299.4 kJ/mol

Answer explanation

In order to determine the enthalpy of reaction, one must use Hess's Law. Both reactions with given enthalpies must be halved. The second reaction also must be flipped. Once this is done, add up the enthalpies to determine the new one.

Tags

NGSS.HS-PS1-4

Which of the following process is exothermic?

• CH4 (g) + 2O2(g) -> 2H2O(g) + CO2 (g)

• H2O (l) -> H2O (g)

• CO2 (s) -> CO2 (g)

• CaCO3 (s) -> CaO (s) + CO2(g)

Answer explanation

All combustion reactions are exothermic. Decomposition reactions are endothermic. Melting, evaporating, and sublimation are endothermic.

Tags

NGSS.HS-PS1-4

A student uses a calorimeter to determine the enthalpy of combustion of methane. Which of the following statements is true?

• After the combustion the temperature of the calorimeter will decrease because combustion reactions are exothermic.

• After the combustion the temperature of the calorimeter will increase because combustion reactions are exothermic.

• After the combustion the temperature of the calorimeter will increase because combustion reactions are endothermic.

• After the combustion the temperature of the calorimeter will decrease because combustion reactions are endothermic.

Answer explanation

All combustion reactions are exothermic. This causes the surrounding, the calorimeter, to increase in temperature.

Tags

NGSS.HS-PS1-4

Consider the following reaction. Which of the following statements is true?

• More energy is released when the bonds form than energy is required to break the bonds, because the reaction is exothermic.

• More energy is released when the bonds form than energy is required to break the bonds, because the reaction is endothermic.

• Less energy is released when the bonds form than energy is required to break the bonds, because the reaction is exothermic.

• Less energy is released when the bonds form than energy is required to break the bonds, because the reaction is endothermic.

Answer explanation

Using Q=mCΔT, one finds the change of temperature to be 36°C. Add the initial temperature to find the final temperature of 51°C.

Tags

NGSS.HS-PS1-4

Which of the following is defined by the measure of the average kinetic energy?

• Potential Energy

• Velocity

• Heat

• Temperature

Answer explanation

Temperature is defined as the measure of the average kinetic energy of the sample.

Tags

NGSS.MS-PS1-4

NGSS.MS-PS3-4

What is the amount of energy is required to raise the temperature of 43 g of liquid water from 25°C to 55°C? Water's specific heat is 4.18 J/g°C.

• A. 5.39 kJ

• B. 5390 kJ

• C. 4.49 kJ

• D. 9.88 kJ

Answer explanation

Use the equation Q=mCΔT. The change in temperature (ΔT) is 30°C, the mass is 43 g, and the specific heat of liquid water is 4.18 J/g°C. There are 1000J for every kJ. Therefore, the hear is 5.39 kJ.

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-4

What is the amount of energy required to melt 7.2 g of water at 0°C? Water's enthalpy of fusion is 6.01 kJ/mol and water's enthalpy of vaporization is 40.7 kJ/mol.

• A. 16.3 kJ

• B. 43.3 kJ

• C. 293 kJ

• D. 2.40 kJ

Answer explanation

We must use water's enthalpy of fusion because that refers to the amount of energy required to melt 1 mol of water. The heat of vaporization refers to vaporizing 1 mol of water.

Tags

NGSS.MS-PS1-4

NGSS.MS-PS3-4

When this reaction occurs, 75.0 kJ of energy is released. What mass of copper is produced?

• 0.588 g

• 36.7 g

• 110 g

• 1.73 g

Answer explanation

1 mol of copper is produced for every 129.7 kJ of energy that is released by this reaction. Using this as a conversion factor, one finds that 0.588 mol of copper should be produced. Multiply by 63.55 g/mol, copper's molar mass, to find 36.7 g of copper is produced.

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Which of the following is an endothermic process?

• Combusting methane

• Freezing benzene

• Condensing water

• Melting Wax

Answer explanation

Consider the relative energy of the states of matter. Since solid has less energy than liquid, melting is the only endothermic process listed in the options.

Tags

NGSS.MS-PS1-4

NGSS.MS-PS3-4