Thermochem Review

A student is trying to develop a model of an instant cold pack. What is the essential characteristic of the chemical reaction that the student should use to develop the cold pack?

• The standard enthalpy of reaction should be zero.
• The standard enthalpy of reaction should be negative.
• The standard enthalpy of reaction should be positive.
• The enthalpy of formation of products should be negative.

Which two statements about the standard enthalpy of formation of a compound are true?

• It is calculated when all substances are in their gaseous states.
• It is calculated when all substances are in their respective states at STP.
• It is the enthalpy change that occurs to form 1 gram of the compound.
• It is the enthalpy change that occurs to form 1 mole of the compound.

Calculate the standard enthalpy for the reaction based on the information shown. Round to the nearest whole number.

• -135 kJ/mol

• -1135 kJ/mol

• -825 kJ/mol

• -73 kJ/mol

The thermochemical equation for the formation of PCl₃ from its constituent elements is shown. How much energy is released when 82.1 g of PCl₃ form?

• -65.4 kJ
• -191.1 kJ
• -223.2 kJ
• -261.7 kJ

The assumption that the heat gained is equal to the heat lost in a calorimetry experiment is based on what concept?

• thermal expansion
• thermal compression
• thermal equilibrium
• thermal conductivity

An element in its standard state has a heat of formation of 0 kJ/mol. What two conditions define standard state?

• 0*C and 1 atmosphere
• 25*C and 1 atmosphere
• 100*C and 0 atmospheres
• 32*F and 0 atmospheres

Your fingers quickly begin to feel cold when you touch an ice cube. What important thermochemical principle does this change illustrate? Choose the best answer.

• the law of conservation of energy
• Hess's law of heat summation
• chemical bonds breaking when molecules collide
• the flow of energy from warmer to cooler objects

Which of the following statements are true regarding Hess’s law of summation? Select both statements that apply.

• It allows you to indirectly determine the enthalpy of a reaction.
• It requires the use of at least one thermochemical equation.
• The enthalpy change for a reaction equals the sum of the enthalpy changes in each step of a reaction.
• It measures the enthalpy change caused by the dissolution of one mole of a substance.

When reversing a reaction that originally has ΔH = -86 kJ, what change needs to be made to the ΔH value?

• Find its inverse.
• Multiply it by two.
• Make it positive.
• No changes are necessary.

Find the enthalpy of the red reaction. Round to the nearest whole number.