Enthalpy and Thermochemical Equations

Thermochemical Equations

By Carol Vaughn

Poll

If the change in enthalpy of the reaction is positive, the reaction must _____ heat.

• gain

• lose

​We can represent the change in enthalpy using diagrams, such as the graphs shown here...

(​activation energy = the energy needed for a chemical reaction to occur)

Labelling

Add labels to the appropriate diagram to identify which term best identifies the reactions illustrated.

Drag labels to their correct position on the image

endothermic

exothermic

isothermic

hypothermic

hyperthermic

​...or we can include it in the chemical equation itself, as seen in the two examples in the notes.

​Many students neglect this section of the notes and struggle on the 1-pt questions. It is important to remember that the terms enthalpy/heat/q/ΔH can be used nearly interchangeably so long as the pressure conditions are held constant. At this stage, this is a safe concept to assume.

Categorize

Options (8)

heat is a reactant

q > 0

energy is being released

energy is entering the surroundings

the temperature of the system will increase

the activation energy is relatively high for this reaction to occur

the reaction is endothermic

the system absorbs heat

Based on the thermochemical equation shown, identify each statement as true or false.

True

False

Multiple Choice

The chemical equation for the oxidation of iron(II) oxide is given. This oxidation is an exothermic reaction with an enthalpy change of 560.66 kJ.

If this is put into the equation as either a reactant or product, on which side of the reaction arrow would you place the "+560.66 kJ"?

• left

• right

Multiple Select

Which of the pictured chemical reactions will release energy? Select all that apply.

• A

• B

• C

• D

​If you're thinking this looks a whole lot like stoichiometry, you're right! But if that makes you nervous, just treat it as unit conversion. If you were somebody that liked proportions, don't worry, you can still use those, also.

← based on the coefficient from the balanced equation

Multiple Choice

The molar enthalpy of a substance in a chemical reaction is always based on the participation of one mole of that substance in the reaction.

• True

• False

Multiple Choice

For the same chemical reaction, the energy part of the molar enthalpy conversion factor (ex: kJ/mol) will be the same value no matter which substance is being used in the calculations.

• true

• false

Try to set up these equations based on the example in your notes before checking your answers based on the next Slide.

Multiple Select

Based on the thermochemical equation shown, select all of the options for molar enthalpy values that are written correctly.

• 84.5 kJ/mol C₂H₆

• 84.5 mol C₂H₆ / kJ

• 84.5 kJ / 2 mol C

• 84.5 kJ/mol C

Reorder

Put these options in the correct order to solve for how many grams of a substance were used in a chemical reaction.

• given

• molar enthalpy

• molar mass

1

2

3

• 1
• 2
• 3

Multiple Choice

Stoichiometric calculations must be compared as

• grams

• moles

• joules

• liters

Multiple Choice

Based on the thermochemical equation from our notes, how much heat is released when 320 g of oxygen is used to burn glucose?

• 4700 kJ

• 2300 kJ

• 9400 kJ

• 28,000 kJ

Multiple Choice

The chemical equation for the oxidation of iron(II) oxide is given. This oxidation is an exothermic reaction with an enthalpy change of 560.66 kJ.

How many grams of iron(II) oxide were consumed if 317 kJ were released? (M_FeO=71.84 g/mol)

• 162 g

• 40.6 g

• 2470 g

• 3100 g