Unit 4.1 Review

Unit 4.1 - Thermochemistry Review

Unit 4.1: Thermochemistry

Introduction to thermochemistry
Endothermic vs exothermic reactions
Phase transitions
Specific heat
Potential energy diagrams
Hess's law

​Intro to thermochemistry

Heat flow
heat always flows from hotter to colder
Heat vs temperature
total thermal energy vs average kinetic energy
Law of conservation of energy (system vs surroundings)
reactions either take in (endothermic) or give off (exothermic) heat
Unit conversion (J to cal)
1 cal = 4.184 J

Multiple Choice

In an exothermic process, the surroundings are gaining energy.

• True
• False

Multiple Choice

What does temperature measure?

• Heat
• ºC
• Average Kinetic Energy

• Thermal Energy

Multiple Choice

If two objects have different temperatures, heat will flow from the warmer object to the cooler one UNTIL ____________

• one reaches a temperature of zero
• they both have an equal temperature
• one runs out of energy

​Endothermic vs Exothermic Reactions

Endothermic Reactions
Absorb heat from the surroundings (ΔH > 0)
Feel cold to the touch
Convert heat energy to bond energy
Exothermic Reactions
Release heat to the surroundings (ΔH < 0)
Feel hot to the touch
Convert bond energy to heat energy

Multiple Choice

In an exothermic process, the surroundings gain heat energy.

• True
• False

Multiple Choice

A reaction is performed in a beaker with a temperature probe recording the temperature. If the temperature began at 15.0 degrees Celsius and ended at 27.5 degrees Celsius, is the system endothermic or exothermic?

• Exothermic

• Endothermic

Multiple Choice

4 Fe (s) + 3 O₂ (g) → 2 Fe₂O₃ (s) + 1625 kJ

Is this reaction endothermic or exothermic?

• endothermic

• exothermic

​Phase changes

q=mΔH

Affect phase only, not temperature

Occur in pairs w/ equal magnitude, but opposite sign (e.g. ΔH_fus = -ΔH_solid)

Multiple Choice

Which of these phase changes requires energy to be released?

• boiling

• sublimation

• condensation

• melting

Multiple Choice

A sample of H₂O with a mass of 60 grams boils until it has completely changed to steam. How much heat was absorbed?

ΔH_fus = 334 J/g

ΔH_vap = 2260 J/g

• 7525 J

• 135,600 J

• 1112.1 J

• 20,040 J

​Specific Heat

q=mC_sΔT

Heating / cooling of a single phase without a phase change

Different specific heat for each phase

Multiple Choice

If 100 g of aluminum at 145ºC gains 6,800 J of heat, what is the final temperature (T_final) of the aluminum? Aluminum has a specific heat of 0.897 J/gºC. (Q=mcΔT)

• -69°C
• 0.52°C
• 221°C

Multiple Choice

A piece of aluminum with a mass of 20.0 grams is heated from 30 ^oC to 95 ^oC. If the specific heat of aluminum is 0.904 J/g^oC, how much heat is gained by the aluminum?

• 1717.60 J

• 1175.20 J

• 963.68 J

• 542.4 J

​Potential Energy Diagrams

Track potential energy during reaction

Additional activation energy necessary to form unstable activated complex

Shows energies and their differences (products, reactants, ΔH, E_a, etc.)

Multiple Choice

Is the reaction represented in this graph endothermic or exothermic?

• endothermic

• exothermic

Multiple Choice

What is the change in enthalpy $$\Delta H$$  for this reaction?

• -200 KJ

• 200 KJ

• 400 KJ

• -300 KJ

Multiple Choice

Which of the letters (A-D) represents the activation energy?

• A
• B
• C
• D

​Hess's Law and Enthalpy of Formation

ΔH = Σ ΔH_{f (products)} - Σ ΔH_{f (reactants)}

Any chemical equation can be written as a sum of standard enthalpy of formation equations.

Total enthalpy change equal to the sum of the enthalpies for each formation equation.

Multiple Choice

Which of the following enthalpy changes represent an enthalpy of formation reaction?

• Mg (s) + Cl₂ (g) → MgCl₂ (s)

• Mg²⁺ (g) + 2 Cl ^-(g) → MgCl₂ (aq)

• O₂ (g) → 2 O (g)

• 2 Mg (s) + O₂ (g) → 2 MgO (S)

Multiple Choice

What is ∆H for the following reaction (in kJ)?

MgO (s) + H₂ (g₎ → Mg (s) + H₂O (g)

Consider the following equations:

Mg (s) + O₂ (g) → MgO (s) ∆H _f= –602 kJ

H₂ (g) + O₂ (g) → H₂O (g) ∆H_f = –242 kJ

• -844

• -360

• +360

• +844