Thermochemistry

Thermochemistry

presenter: Dr. Luis Bello

Thermochemistry is the study of the relationship between chemical reactions and energy changes involving heat.

It deals with the study of the energy changes that occur during chemical reactions or physical changes such as melting, freezing, and evaporation.

Energy:

Energy is the capacity to do work.

In thermochemistry, energy is expressed in joules or calories.

Heat: Heat is the energy transferred between objects due to a temperature difference.

Heat flows from a hotter object to a cooler object.

System and surroundings: In thermochemistry, a system is the part of the universe that is being studied, while the surroundings are everything else in the universe.

​Enthalpy: Enthalpy is a measure of the heat content of a system at constant pressure. It is represented by the symbol ΔH and has units of joules or calories.

​Endothermic and exothermic reactions: An endothermic reaction absorbs heat from the surroundings, while an exothermic reaction releases heat to the surroundings.

​Heat of reaction:

The heat of reaction, also known as the enthalpy change, is the amount of heat absorbed or released during a chemical reaction.

​Standard enthalpy of formation:

The standard enthalpy of formation is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states at a specified temperature and pressure. It is represented by the symbol ΔH°f.

​Bond Enthalpy: Bond enthalpy is the energy required to break a chemical bond in a molecule. It is measured in units of joules per mole (J/mol). The bond enthalpy is positive because energy is required to break the bond. Conversely, the bond enthalpy is negative because energy is released when the bond is formed.

​Hess's law: Hess's law states that the heat of a reaction is independent of the pathway taken to reach the final state.

This means that the total enthalpy change of a reaction is equal to the sum of the enthalpy changes of all the intermediate steps.

​Calorimetry: Calorimetry is the measurement of heat changes in a chemical reaction or physical process.

The most common method of calorimetry is to use a calorimeter, which is a device designed to measure heat changes.

​The heat of combustion is the amount of heat released when one mole of a substance is completely burned in excess oxygen under standard conditions (usually at 298 K and 1 atm pressure). It is typically denoted as ΔHcomb and expressed in units of energy per mole (kJ/mol).

Multiple Choice

For an endothermic reaction, the heat is on the ____ side.

• reactant

• product

• either side

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

Multiple Choice

For an exothermic reaction, the heat is on the ____ side

• reactant

• product

• either side

Multiple Choice

What type of reaction is shown in the picture?

• endothermic

• exothermic

Multiple Choice

The specific heat of aluminum is 0.21 J/g°C.  How much heat(Q) is released when a 10 g piece of aluminum foil is taken out of the oven and cools from 100° to 50°?

• 105 J

• 10.5 J

• 1.05 J

• 0.105 J

Multiple Choice

Using the equations below:

C_(s) + O_2(g) → CO_2(g) ∆H = –390 kJ

Mn_(s) + O_2(g) → MnO_2(s) ∆H = –520 kJ

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

MnO_2(s) + C_(s) → Mn_(s) + CO_2(g)

• 910

• 130

• -130

• -910

Multiple Choice

Consider the following equations.

Mg_(s) + O_2(g) → MgO_(s) ∆H = –602 kJ

H_2(g) + O_2(g) → H₂O_(g) ∆H = –242 kJ

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

MgO_(s) + H_2(g) → Mg_(s) + H₂O_(g)

• -844

• -360

• +360

• +844

Multiple Choice

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

C_(s) +O_2(g) → CO_2(g) ΔH = –x kJ mol^–1

CO_(g) + O_2(g) → CO_2(g) ΔH = –y kJ mol^–1

What is the enthalpy change, in kJ mol^–1, for the oxidation of carbon to carbon monoxide?

C_(s) + O_2(g) → CO_(g)

• x + y

• -x - y

• y - x

• x - y

Multiple Choice

• -296.1

• 226

• -11

• 255.95