Energetics

Welcome to Energetics! Understanding Energy in Chemistry

Hello future Chemist! This chapter, Energetics, is all about the energy changes that happen during chemical reactions. Every time you burn fuel, use a battery, or even dissolve a salt in water, energy is being transferred. Understanding energetics helps us explain why some reactions release heat and warm their surroundings, while others absorb heat and cool down.
We will explore exothermic and endothermic reactions, calorimetry experiments, energy level diagrams, and how to calculate enthalpy changes from experimental data and bond energies!

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Section 1: The Basics of Energy Flow and Enthalpy

What is Enthalpy Change (\(\Delta H\))?

Chemical reactions involve breaking existing bonds in reactants and forming new bonds in products:

• Breaking Bonds: Requires energy input (an endothermic process).
• Making Bonds: Releases energy to the surroundings (an exothermic process).

The overall energy change of a reaction at constant pressure is called the molar enthalpy change, denoted by the symbol \(\Delta H\) (expressed in \(\text{kJ/mol}\)).

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Section 2: Exothermic and Endothermic Reactions

Exothermic Reactions

An exothermic reaction releases heat energy to the surroundings.

• Temperature effect: The temperature of the reaction mixture and surroundings increases.
• Enthalpy change: \(\Delta H\) is negative (\(\Delta H < 0\)).
• Bond energy balance: Energy released making new bonds is greater than the energy required to break old bonds.
• Examples: Combustion of fuels, neutralisation reactions between acids and alkalis, displacement reactions of metals, and respiration.

Endothermic Reactions

An endothermic reaction absorbs heat energy from the surroundings.

• Temperature effect: The temperature of the reaction mixture and surroundings decreases.
• Enthalpy change: \(\Delta H\) is positive (\(\Delta H > 0\)).
• Bond energy balance: Energy required to break old bonds is greater than the energy released making new bonds.
• Examples: Thermal decomposition of carbonates (e.g. \(\text{CuCO}_3\) or \(\text{CaCO}_3\)), photosynthesis, and dissolving certain salts such as ammonium chloride in water.

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Section 3: Simple Calorimetry and Heat Calculations

Calorimetry is an experimental technique used to measure the heat energy transferred during chemical reactions such as combustion, displacement, dissolving, and neutralisation.

1. Solution Calorimetry (Dissolving, Neutralisation, Displacement)

A polystyrene cup is used as an insulator with a lid to minimise heat loss to the surroundings. A known volume of solution or water is measured, initial temperature is recorded, reactants are mixed, and the maximum or minimum temperature reached is noted to find the temperature change \(\Delta T\).

2. Combustion Calorimetry

A known mass of a fuel (e.g. in a spirit burner) is burned to heat a known volume of water in a metal copper calorimeter. The mass of fuel burned and the temperature rise of the water are recorded.

Calculating Heat Energy Change (\(Q\))

The heat energy transferred (\(Q\)) is calculated using:

\(Q = mc\Delta T\)

• \(Q\) = heat energy change (in Joules, \(\text{J}\))
• \(m\) = mass of the liquid being heated or cooled (in grams, \(\text{g}\); for aqueous solutions, \(1\text{ cm}^3 \approx 1\text{ g}\))
• \(c\) = specific heat capacity of water (typically \(4.18\text{ J/g}^{\circ}\text{C}\) or \(4.2\text{ J/g}^{\circ}\text{C}\), provided in questions)
• \(\Delta T\) = change in temperature (\(T_{\text{final}} - T_{\text{initial}}\) in \(^{\circ}\text{C}\))

Calculating Molar Enthalpy Change (\(\Delta H\))

1. Calculate the amount in moles (\(n\)) of the limiting reactant: \(n = \frac{\text{mass}}{M_r}\) or \(n = \text{concentration} \times \text{volume}\).
2. Convert heat energy \(Q\) from Joules to kilojoules: \(Q\text{ in kJ} = \frac{Q}{1000}\).
3. Calculate \(\Delta H = -\frac{Q\text{ (in kJ)}}{n}\). Assign a negative sign for temperature increases (exothermic) and a positive sign for temperature decreases (endothermic).

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Section 4: Energy Level and Reaction Profile Diagrams (Paper 2)

Energy level diagrams show the relative energy levels of reactants and products, the overall enthalpy change \(\Delta H\), and the activation energy (\(E_a\)), which is the minimum energy colliding particles must possess to react.

1. Exothermic Reaction Profile

• Reactants have higher chemical energy than products.
• The energy profile rises from reactants to the peak (activation energy \(E_a\)), then falls to the lower product level.
• \(\Delta H\) arrow points downwards from reactants to products (negative \(\Delta H\)).

2. Endothermic Reaction Profile

• Reactants have lower chemical energy than products.
• The curve rises to the peak (activation energy \(E_a\)) and finishes at a higher level than the reactants.
• \(\Delta H\) arrow points upwards from reactants to products (positive \(\Delta H\)).

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Section 5: Calculating Enthalpy Changes Using Bond Energies (Paper 2)

Bond energy is the amount of energy required to break one mole of a particular covalent bond in gaseous molecules.

Calculation Formula

\(\Delta H = \Sigma(\text{bond energies of bonds broken in reactants}) - \Sigma(\text{bond energies of bonds formed in products})\)

\(\Delta H = (\text{Energy In}) - (\text{Energy Out})\)

Step-by-Step Method

1. Draw the displayed formulae for all reactant and product molecules to see all bonds clearly.
2. Calculate the total energy needed to break all bonds in the reactants (Energy In).
3. Calculate the total energy released when all bonds form in the products (Energy Out).
4. Subtract Energy Out from Energy In to find \(\Delta H\).