Chemical Bond Energies (GCSE Chemistry)
Chemical Bond Energies
Bond Energies
- Energy is essential for reactions. In order for a chemical reaction to occur, energy is needed. The minimum amount of energy, the activation energy, is supplied to break the reactant bonds.
- Bonds are broken in reactants. In order for particles to react, existing bonds must be broken to release the atoms, so they are free to make new bonds.
- Bond breaking is endothermic. When bonds are broken, this process is endothermic. Energy is taken in from the surroundings in order to break old bonds.
- Bonds are formed in products. When a reaction has taken place, bonds are formed between atoms to create products. Energy is released when new bonds are made.
- Bond formation is exothermic. When new bonds are formed, this process is exothermic. Energy is released to the surroundings through the formation of new bonds.
Calculating Overall Energy Change in a Reaction
- Bonds have specific energies. Every chemical bond has it’s own ‘energy’. This is the amount of energy required to break the bond, which is the same as the amount of energy released to form the bond.
- Reactions have energy changes. Since bonds are broken and formed during a reaction, we can calculate the overall energy change of a reaction. To do this, we need to know the total bond energies of the reactants and also the total bond energies of the products. We can then use a simple equation to calculate the overall energy change of the reaction.
Exothermic Reactions
- Exothermic reactions have a negative energy value. If the overall energy change of a reaction is negative when a reaction is complete, then the reaction is exothermic. This is due to a low amount of energy being used to break bonds in the reactants, and a high amount of energy being released via formation of bonds in the products.
Endothermic Reactions
- Endothermic reactions have a positive energy value. If the overall energy change is positive when a reaction is complete, then the reaction is endothermic. This is due to a high amount of energy being used to break bonds in the reactants, and a low amount of energy being released via formation of bonds in the products.
Calculating Energy Transferred
Practice Question: Calculate the overall energy change for this reaction:
N2 + 3H2 → 2NH3
The bond energies for the molecules are:
N≡N: 941 kJ/mol; H-H: 436 kJ/mol; N-H: 391 kJ/mol
1.Draw out the bonds. For these bond energy questions, the way to approach them is to draw out the bonds on each side.
N2 + 3H2 → 2NH3
N≡N + 3 x (H-H) → 2 x (3 x N-H)
2. Write out the correct equation.
Overall energy change = Total energy from bonds broken – Total energy of bonds formed
3. Work out energy from bonds broken.
N≡N + [3 x (H-H)]
941 + [3 x 436] = 2249
4. Work out energy from bonds formed.
2 x (3 x N-H)
2 x (3 x 391) = 2346
5. Work out the overall energy change.
Overall energy change = Total energy from bonds broken – Total energy of bonds formed
= 2249 – 2346
= -97kJ mol-1
The negative sign on the overall energy change means that there was a greater amount of energy from the formation of bonds as compared to the breaking of bonds. Therefore, this is an exothermic reaction.
Chemical bond energies refer to the energy required to break a chemical bond between two atoms. This energy is released when a bond forms, and is measured in units of joules or kilojoules.
The factors that affect chemical bond energies include the type of bond, the size of the atoms involved, and the number of bonds between the atoms. Stronger bonds, such as those in covalent molecules, typically require more energy to break than weaker bonds, such as those in ionic compounds.
Chemical bond energies are important in chemistry because they help us understand the stability of different molecules and the energy involved in chemical reactions. By studying chemical bond energies, we can predict the products of reactions and the amount of energy released or absorbed during a reaction.
Chemical bond energies can be measured using a variety of techniques, including calorimetry, spectroscopy, and thermodynamics. Each of these methods provides different information about the bond energy and allows us to determine the strength and stability of different chemical bonds.
Bond enthalpy refers to the energy required to break a bond in one mole of a substance, whereas bond dissociation energy refers to the energy required to break one bond in a molecule. Bond enthalpy is typically used in thermochemistry to calculate the energy involved in chemical reactions, while bond dissociation energy is used to study the stability of individual bonds in a molecule.
Chemical bond energies play a crucial role in the study of chemistry, as they provide insight into the behavior of atoms and molecules and the energy involved in chemical reactions. Understanding chemical bond energies is essential for studying fields such as organic chemistry, biochemistry, and materials science.
Chemical bond energies can be used in a variety of practical applications, including the design of new drugs and materials, the development of renewable energy sources, and the optimization of industrial processes. By using knowledge of chemical bond energies, scientists and engineers can design more efficient and effective products and technologies.
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