Energy Changes in Chemistry
Welcome to Energy Changes in Chemistry! Whenever a chemical reaction happens, energy is transferred to or from the surroundings. This is why some reactions feel hot to the touch while others feel ice-cold. Understanding energy changes helps us explain everything from how hand warmers and instant ice packs work to how rocket fuel powers spacecraft.
Don't worry if this topic seems a bit math-heavy at first. We will break it down into simple, step-by-step stages so you feel completely confident for your exam!
1. Exothermic and Endothermic Reactions
During any chemical reaction, energy (usually in the form of heat energy) is transferred between the reacting chemicals and their surroundings.
Exothermic Reactions
An exothermic reaction is one that gives out heat energy to the surroundings.
• What happens to the temperature? The temperature of the surroundings increases (it gets hotter).
• Key Memory Trick: Think of EXothermic as energy EXiting the reaction.
• Everyday examples: Self-heating drink cans, disposable hand warmers, burning fuels (combustion).
• Chemical examples: Combustion of methane (\(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\)), neutralisation reactions between acids and alkalis, and respiration in living cells.
Endothermic Reactions
An endothermic reaction is one that takes in heat energy from the surroundings.
• What happens to the temperature? The temperature of the surroundings decreases (it gets colder).
• Key Memory Trick: Think of ENdothermic as energy going IN to the reaction.
• Everyday examples: Instant cold packs used for sports injuries.
• Chemical examples: Thermal decomposition (such as heating calcium carbonate: \(\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2\)), photosynthesis, and the reaction between citric acid and sodium hydrogencarbonate.
Did you know? In an instant cold pack, a bag of water inside the pack is broken to dissolve a chemical like ammonium nitrate. This physical/chemical dissolving process is endothermic, rapidly cooling the pack without needing a freezer!
Quick Summary:
• Exothermic: Heat exits \(\rightarrow\) surroundings get hotter.
• Endothermic: Heat enters \(\rightarrow\) surroundings get colder.
2. Reaction Profiles (Energy Level Diagrams)
A reaction profile is a simple graph showing how the energy of chemicals changes as a reaction proceeds from reactants to products.
Key Terms to Know:
• Activation Energy (\(E_a\)): The minimum amount of energy that colliding reactant particles must have for a reaction to take place. Think of it as the "energy barrier" you must push a boulder over before it rolls down a hill.
• Overall Energy Change (\(\Delta H\)): The difference in energy between the reactants and the products.
Profile of an Exothermic Reaction
• Reactants have more chemical energy than the products.
• Because energy is given out to the surroundings, the products end up at a lower energy level than the reactants.
• The arrow for the overall energy change (\(\Delta H\)) points downwards, which means \(\Delta H\) has a negative value (\(- \text{ve}\)).
Profile of an Endothermic Reaction
• Reactants have less chemical energy than the products.
• Because energy is taken in from the surroundings, the products end up at a higher energy level than the reactants.
• The arrow for the overall energy change (\(\Delta H\)) points upwards, which means \(\Delta H\) has a positive value (\(+ \text{ve}\)).
Common Mistake to Avoid:
When drawing or labelling reaction profiles, always make sure the Activation Energy (\(E_a\)) arrow starts at the reactant level and points all the way to the very top of the curve (the peak).
3. Bond Breaking and Bond Making
Why do chemical reactions give out or take in energy? It all comes down to chemical bonds!
• Breaking bonds: Requires energy to be absorbed from the surroundings. Therefore, bond breaking is ENDOTHERMIC.
Analogy: Think of two strong magnets stuck together. You have to put effort (energy) in to pull them apart.
• Making bonds: Releases energy to the surroundings. Therefore, bond making is EXOTHERMIC.
Analogy: When two strong magnets snap together, they release energy (sound and kinetic energy).
Mnemonic to Remember:
"BENDO MEXO"
• Breaking bonds is ENDOthermic.
• Making bonds is EXOthermic.
Why is a reaction overall Exothermic or Endothermic?
• A reaction is exothermic if the energy released making new bonds is greater than the energy required to break existing bonds.
• A reaction is endothermic if the energy required to break existing bonds is greater than the energy released making new bonds.
4. Bond Energy Calculations
Bond energy is the amount of energy (measured in kilojoules per mole, \(\text{kJ/mol}\)) needed to break one mole of a particular chemical bond.
The Golden Formula:
\(\text{Overall Energy Change } (\Delta H) = \text{Energy needed to break bonds} - \text{Energy released making bonds}\)
Or simply: \(\Delta H = \text{In} - \text{Out}\)
Step-by-Step Method for Calculations
Step 1: Write out the balanced equation showing all the bonds (displayed formulas are very helpful).
Step 2: Calculate the total energy needed to break all the bonds in the reactants (Left-hand side).
Step 3: Calculate the total energy released when making all the bonds in the products (Right-hand side).
Step 4: Subtract the products total from the reactants total (\(\text{Reactants} - \text{Products}\)).
Step 5: Include the correct sign: a minus (\(-\)) sign for exothermic, or a plus (\(+\)) sign for endothermic.
Worked Example:
Calculate the overall energy change for the reaction between hydrogen and chlorine to produce hydrogen chloride:
\(\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}\)
Given bond energies:
• \(\text{H}-\text{H} = 436\text{ kJ/mol}\)
• \(\text{Cl}-\text{Cl} = 242\text{ kJ/mol}\)
• \(\text{H}-\text{Cl} = 431\text{ kJ/mol}\)
Step 1: Identify the bonds
• Reactants: \(1 \times (\text{H}-\text{H})\) bond and \(1 \times (\text{Cl}-\text{Cl})\) bond
• Products: \(2 \times (\text{H}-\text{Cl})\) bonds (notice the balancing number \(2\)!)
Step 2: Energy to BREAK bonds (Reactants):
\(\text{Energy in} = 436 + 242 = 678\text{ kJ/mol}\)
Step 3: Energy released MAKING bonds (Products):
\(\text{Energy out} = 2 \times 431 = 862\text{ kJ/mol}\)
Step 4: Overall energy change (\(\Delta H\)):
\(\Delta H = \text{Energy in} - \text{Energy out}\)
\(\Delta H = 678 - 862 = -184\text{ kJ/mol}\)
Conclusion:
Because the value is \(-184\text{ kJ/mol}\) (negative), the reaction is exothermic.
5. Quick Review & Top Exam Tips
• Check the signs: Negative \(\Delta H\) means Exothermic; Positive \(\Delta H\) means Endothermic.
• Don't forget balancing numbers: If the equation says \(2\text{H}_2\text{O}\), each water molecule has \(2\) \(\text{O}-\text{H}\) bonds, so that is \(2 \times 2 = 4\) \(\text{O}-\text{H}\) bonds in total!
• Double-check your subtraction: Always do \(\text{Reactants (Break)} - \text{Products (Make)}\), never the other way around.
• State units: Always write \(\text{kJ/mol}\) or \(\text{kJ}\) if required by the question.