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Chemistry

Important Terms and Definitions

Energy

The capacity to do work, generate heat, and/or generate electricity.
Kinetic energy

The energy of moving particles.  An increase in kinetic energy results in an increase in temperature. Kinetic energy results from vibrational motion, rotational motion, and translational motion of particles.
Heat

The transfer of thermal energy from one source to another.
Specific heat capacity

 The heat required to change the temperature of a unit mass of a substance by 1 degree celsius.

Enthalpy

The sum of a chemical system’s kinetic and potential energy under constant pressure and temperature.

Molar enthalpy
The enthalpy change per mole of a substance.

Calorimetry

A technological process of measuring the amount of heat released or absorbed in a chemical reaction. 
Chemical change

A chemical change happens when the composition of substances changes during a chemical reaction. Some of the indicators of chemical change can be temperature increase/decrease or color change.

Heat of reaction

Change in enthalpy of a chemical reaction that happens under constant pressure. It's the amount of heat that's either added to the system or removed from the system. 
Exothermic reaction

Exothermic reactions are those that release energy, usually in the form of either heat or light. 

Endothermic reaction

Endothermic reactions are those that absorb energy, usually in the form of either heat or light.

Bond energy

The energy required to break a chemical bond or the energy released when a bond is formed.
Activation energy

The energy required to break apart the atoms in reactants. Activation energy is always higher than the energy contained in the reactants and the products. A catalyst can be used to lower the activation energy. 
Hess's Law

This law states that the total enthalpy change of a reaction can be calculated from the sum of the energy changes that occur in the separate reactions comprising it. 
Heat of formation    

The changes in potential energy that occur at the time of compound formation from its elements (for example, the formation of water from hydrogen and oxygen).

Calculating Enthalpy Change Using Calorimeter Data

The calorimeter is utilized to measure the amount of heat released or absorbed during a chemical reaction. The components of a bomb calorimeter are shown in the images below:

Figure 1: Bomb calorimeter and diagram showing components. From LibreTexts Chemistry (2019, June 5), adapted from work by Harbor1 for Wikimedia Commons. https://chem.libretexts.org/Courses/University_of_Kentucky/UK%3A_General_Chemistry/05%3A_Thermochemistry/5.2%3A_Calorimetry

Using Calorimetry Data to Calculate the Enthalpy Change of a Reaction

For a chemical reaction that occurs in a calorimeter, the following assumptions are made:

  • no heat is lost to the environment from the calorimeter;
  • the amount of heat released or absorbed by the calorimeter material is minimal or negligible, and;
  • the diluted solution has the same density and specific heat capacity as water.

With the above assumptions, the amount of heat absorbed or released is calculated using the temperature change, specific heat capacity, and the mass of the solution, as shown in the formula below:

Heat released or absorbed by the chemical reaction = −heat taken in by water

Qcal = mcΔt

Where:

Q = heat in Joules (J)

m = mass in grams (g)

Δt = Change in temperature in degrees Celsius

Qcal = ΔrH

ΔrH = Standard enthalpy of reaction

Example 1:

What minimum mass of methane must be burned to warm 4.00 L of water from 22.4ºC to 87.6ºC, assuming no heat losses?

Given:

The molar enthalpy of combustion for methane is –890 kJ/mol

Q = ΔH

mcΔt= rHm

n =  mcΔt/ ΔrHm

n = (4.00 kg)(4.19kJ/kg°C)(87.6-22.4°C)/(890kJ/mol)

n = 1.2278 mol

m = nM

m = (1.2278 mol)(12.01 +4.04 g/mol) = 19.7 g

Calculating Enthalpy Change Using Hess's Law

Determine the heat of reaction for the following reaction:

4NH3(g)   +   5O2(g)    à    4NO(g)  +  6H2O(g)

Using the following sets of reactions:

N2(g)  +  O2(g)   à  2NO(g)                                            DH = 180.6 kJ

N2(g)  +  3H2(g)   à  2NH3(g)                        DH = -91.8 kJ

2H2(g)  + O2(g)   à  2H2O(g)                         DH = -483.7 kJ


4NH3 is needed on the reactant side in the final equation, so we reverse the second equation and multiply it by two:

Original equation: N2(g)  +  3H2(g)   à  2NH3(g)                   DH = -91.8 kJ

After reversing and multiplying by two: 4NH3 à 2N2 + 6H2     DH = +183.6 kJ

Note: Don’t forget to reverse the sign of DH and multiply the value by two as well.


5O2 is needed on the reactant side, but if it is found in more than one place, just skip it. It will take care of itself.


4 NO is needed on the product side in the final equation and it is found in the first equation. Multiply it by two to get 4 NO.

Original equation: N2(g)  +  O2(g)   à  2NO(g)                                      DH = 180.6 kJ

After multiplying by two gives:   2N2  +  2O2   à  4NO             DH = 361.2 kJ

Note: Don’t forget to multiply DH by two as well.


6H2O is needed on the product side and it is found in the third equation. To get 6H2O, multiply the equation by three.

Original equation: 2H2(g)  +    O2(g)   à  2H2O(g)                                DH = -483.7 kJ

After multiplying by three: 6H2  +    3O2   à  6H2O            DH = -1451.1 kJ

Note: Don’t forget to multiply DH by three as well.


Canceling and adding terms gives us the following:

***This is an exothermic reaction because the final DH is negative. 

Note: The number of oxygen took care of itself, leaving five oxygen (5O2) in the end. Therefore, if a specie appears in more than one place, skip it at the previous level.

Calculating Enthalpy Change Using Enthalpy of Formation

The enthalpy change of a reaction can be calculated using the tabulated values of enthalpy of formation with the help of the following formula:

ΔrH = [Sn ΔfHm ]prod[Sn ΔfHm ]reac

Example :

Calculate the enthalpy change of the combustion of liquid ethanol.

Step 1: Write the balanced equation.

C2H5OH(l)  +  3 O2(g)  à 3 H2O(g)  +   2 CO2(g)

Step 2: Calculate the enthalpy change using the above equation.

ΔrH =[ 3 * H2O+ 2 *CO2  ] – [1 *C2H5OH + 3 * O2]

ΔrH = [3*-285.8 kJ/mol + 2*-393.5 kJ/mol ] - [1*-277.6 kJ/mol + 3*0 kJ/mol]

ΔrH = -1366.8 kJ

Note: The enthalpy of formation for the elements found in the periodic table is zero. In this example, the enthalpy of formation for oxygen is zero.

Potential Energy Diagrams 

Figure 2: Energy diagrams of exothermic and endothermic chemical reactions. By Milton Beychok (2011, April 11) for Citizendium.org. https://en.citizendium.org/images/f/fb/ExoEndo_Reax.png

Summary

The enthalpy of a chemical reaction can be calculated in three different ways:

  • Using calorimetric data.
  • Using Hess's law.
  • Using enthalpy of formation.

You can choose which method to use based on the information given to you in the question. 

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