Enthalpies of Formation and Reaction 10.2

Multiple Choice

Enthalpy (H)

• The heat content of a system at constant pressure.

• A chemical equation that includes the amount of heat released or absorbed during the reaction.

• The heat released or absorbed during a chemical reaction.

• The heat released after burning 1 mole of a substance.

Multiple Choice

When finding the enthalpy change from enthalpies of formation, the formula is...

• $$\Delta H\ =\ \sum_{ }^{ }\Delta H_{products\ }-\ \sum_{ }^{ }\Delta H_{reac\tan ts\ }$$

• $$\Delta H\ =\ \sum_{ }^{ }\Delta H_{reac\tan ts\ }-\ \sum_{ }^{ }\Delta H_{products\ }$$

Did you watch the video yet?

Posted in google classroom in section 10.2 is a video on how to solve Hess's Law. If you have not yet watched it check it out now before moving on if you missed class Thursday. (sorry can't post it here...)

Fill in the Blanks

Find the ΔH for the reaction: H₂SO₄ (L) --> SO₃ (g) + H₂O (g) using the following:

(As you add the lines, treating the --> like = things on either side can be canceled to get to the needed equation)

H₂SO₄ (L) --> H₂S (g) + 2O₂ (g) ΔH = 235.5 kJ

H₂S (g) + 2O₂ (g) --> SO₃ (g) + H₂O (L) ΔH = -207.0 kJ

H₂O (L) --> H₂O (g) ΔH = 44.0 kJ

Multiple Choice

What is ∆H (in kJ) for the following reaction? MnO₂(s) + C(s) → Mn_(s) + CO₂(g)

Using the equations below (remember if an equation needs to flip change the sign of the ∆H ):

C(s) + O₂(g) → CO₂(g) ∆H = –390 kJ

Mn(s) + O₂(g) → MnO₂(s) ∆H = –520 kJ

• 910

• 130

• -130

• -910

Multiple Choice

The following equations show the oxidation of carbon and carbon monoxide to carbon dioxide.

C(s) +O₂(g) → CO₂(g) ΔH = –x kJ/mol

CO(g) + O₂(g) → CO₂(g) ΔH = –y kJ/mol

What is the enthalpy change, in kJ/mol, for the oxidation of carbon to carbon monoxide?

C(s) + O₂(g) → CO(g)

• x + y

• -x - y

• y - x

• x - y

Multiple Choice

What is the ∆H value (in kJ) for the following reaction: MgO(s) + H₂(g₎ → Mg(s) + H₂O(g)

Consider the following equations.

Mg(s) + O₂(g) → MgO(s) ∆H = –602 kJ

H₂(g) + O₂(g) → H₂O(g) ∆H = –242 kJ

• -844

• -360

• +360

• +844

Multiple Choice

What is the phase of hydrogen and oxygen at standard state? (25°C is about room temperature)

• Solid

• Liquid

• gas

• any phase

Multiple Choice

Which type of reaction is referred to in the definition of standard enthalpy change of formation?

• the formation of a compound from its elements

• the formation of a crystal from its ions

• the formation of a molecule from its atoms

• the formation of a compound from other compounds

Multiple Choice

What is the standard heat of formation of 4 moles of SO₂ at 296.8 kJ/mol?

• 1187.2 kJ

• 1500 kJ

• 800 kJ

• 1000 kJ

Multiple Choice

If the energy of formation of the reactants is 297 kJ and the energy of formation of the products is 233 kJ,
what is the standard enthalpy of the reaction?

• 50 kJ

• 64 kJ

• 80 kJ

• 100 kJ

Multiple Choice

How much energy is required to turn 1 mole of N₂O₄ (g) into
2 moles N (g) & 4 moles O (g)? [Read the arrows]

• 1933 kJ

• -1875 kJ

• - 1933 kJ

• 1875 kJ

Multiple Choice

If N₂ (g) + 2O₂ (g) $$\longrightarrow$$   2NO₂(g) has a $$\Delta H_{rxn\ }$$  = 68,
then what is the $$\Delta H_{rxn}$$  if you reverse the reaction?

• 86 kJ

• - 86 kJ

• 68 kJ

• -68 kJ

Multiple Choice

Calculate the following enthalpy of the following reaction: 2Mg + O₂ --> 2MgO

Mg : ΔHf = 0kJ/mol

O₂ : ΔHf = 0J/mol

MgO : ΔHf =-601.6 kJ/mol

• -350.0 kJ/mol

• 0kJ/mol

• -601.6 kJ/mol

• -1203.2 kJ/mol

Multiple Choice

What is the enthalpy change for the reaction:   4C(g) + 5H₂(l) + ½O₂(g)   --->   C₄H₉OH(L)

The enthalpies of combustion of C(s), H₂(g) and C4H9OH(l) (in kJ/mol) are as follows:

C(s) + O₂(g)   --->  CO₂(g)    ∆H=a

H₂(g) + ½O₂(g)   --->   H₂O(L)   ∆H=b

C₄H₉OH(l) + 6O₂(g)   --->   4CO₂(g) + 5H₂O(L)  ∆H=c

[You can multiply or divide reactions by a value as long as you do the same to ∆H]

• c – 4a – 5b
• 2a + 10b - c
• 4a + 5b - c
• 2a + 5b + c

Multiple Choice

Which of the following represent enthalpy change of solution?

• Cl₂(g) ---> 2Cl^-(aq) .

• Mg²⁺(g) ---> Mg²⁺(aq)

• MgCl₂(s) --->

  Mg²⁺(aq) + 2Cl^-(aq)

• Mg²⁺(g) + 2Cl^-(g) ---> Mg²⁺(aq) + 2Cl^-(aq)

Multiple Choice

What is the enthalpy change, in kJ mol^–1, for the reaction: P₄O_6(s) + 2O₂(g) → P₄O₁₀(s)

The standard enthalpy change of formation values of two oxides of phosphorus are:

[Yes, you can cancel part of the exponent based on how many are across the -->]

P₄(s) + 3O₂(g₎ → P₄O₆(s) ΔH_f = –1600 kJ mol^–1

P₄(s) + 5O₂(g) → P₄O₁₀(s) ΔH_f = –3000 kJ mol^–1

• +4600

• +1400

• –1400

• –4600

Multiple Choice

In an endothermic reaction the system is releasing energy.

• True
• False
• Sometimes

Multiple Choice

In an exothermic reaction, heat is ...

• taken in
• given out
• nothing happens
• it gets colder

Multiple Choice

A chemical reaction that requires energy is a(n)

• exothermic reaction 
• endothermic reaction 
• over reaction 
• redox reaction

Multiple Choice

• -804.6
• -202.3
• -296.1
• -233.0

Multiple Choice

• -233
• -11.3
• -805
• -226

Multiple Choice

• 226
• 255
• 233
• 11.3

Multiple Choice

• -296.1
• 226
• -11
• 255.95