Introduction to Thermochemistry
Definition: Thermochemistry is a special part of chemistry that focuses on studying the heat (a form of energy) that is given out or taken in during chemical reactions. This helps us understand how much energy is involved when substances change.
Focus Area: It mainly looks at how heat energy moves in and out of substances when they react with each other. This can help explain why some reactions feel hot and others feel cold.
Types of Reactions Based on Heat Changes
Exothermic Reactions
Heat Release: In exothermic reactions, heat energy is released or given out to the surroundings. That’s why things around it may feel warm or hot.
Temperature Effect: Because heat is released, the temperature of the surroundings (like air or water around the reaction) goes up, making them feel warmer.
Energy Comparison: The products (new substances formed) have less energy stored in them than the reactants (original substances). That’s because some energy has been lost as heat.
ΔH Value: The change in heat energy, which is written as ΔH (delta H), is negative. This means heat is released into the environment.
Examples of Exothermic Reactions:
- Combustion: When fuels like petrol or gas burn in air (with oxygen), they produce carbon dioxide and water and release heat.
- Neutralisation: When an acid (like HCl) reacts with an alkali (like NaOH), they make salt and water and give off heat.
- Metal-Acid Reactions: When metals like zinc react with acids like hydrochloric acid, they produce a salt and hydrogen gas with heat.
- Precipitation: When two clear solutions mix and form a solid (like AgCl), heat may be released.
- Displacement: One metal replaces another in a solution, and this can also give off heat.
- Respiration: This is a process in our bodies where food is broken down to release energy.
Endothermic Reactions
Heat Absorption: In endothermic reactions, heat is taken in or absorbed from the surroundings. That’s why these reactions feel cold.
Temperature Effect: Because heat is absorbed, the surroundings become cooler, and the temperature around the reaction goes down.
Energy Comparison: The products of the reaction have more energy than the reactants because they have taken in extra heat.
ΔH Value: The ΔH value is positive, which means heat has been absorbed from the surroundings.
Examples of Endothermic Reactions:
- Thermal Decomposition: A substance like calcium carbonate breaks down when heated to form simpler products and absorbs heat.
- Salt-Water Reactions: Some salts like ammonium chloride absorb heat when they dissolve in water.
- Photosynthesis: Plants use sunlight energy to make food (glucose). The light energy is absorbed.
- Melting Ice: Ice absorbs heat from the surroundings to melt into water.
- Evaporation: Water takes in heat to become water vapour (gas).
Energy Level Diagrams
Purpose: These diagrams are used to show the difference in energy levels between the reactants and the products in a reaction. They help us understand if heat is taken in or given out.
Reaction Illustration: The diagrams give a clear picture of what happens to energy during a chemical reaction. They usually include arrows to show if energy goes up or down.
Exothermic Representation: In an exothermic reaction diagram, the line for the products is lower than the line for the reactants. This shows energy is lost or released.
Heat Loss: The space between the two lines (reactants and products) represents the heat that is released during the reaction.
ΔH Value (Exothermic): Because energy is lost, the ΔH value is negative, which is a signal that the reaction gives off heat.
Endothermic Representation: In an endothermic diagram, the line for the products is higher than the reactants. This shows energy is gained.
Heat Gain: The gap between the reactants and products shows the amount of heat absorbed to make the reaction happen.
ΔH Value (Endothermic): Since the energy increases, the ΔH is a positive number.
Factors Affecting Heat Changes
Reactant/Product Nature: Different substances have different amounts of stored chemical energy, so they release or absorb different amounts of heat.
Bond Strength Impact: Some bonds between atoms are stronger and need more energy to break. Breaking weak bonds releases less energy than breaking strong ones.
Temperature Influence: The starting and ending temperatures help us measure how much heat was given out or taken in during a reaction.
Reaction Conditions: Even though many reactions happen at room temperature, changes in temperature can make the reactions go faster or slower and affect heat changes.
Pressure Effect: Pressure changes don’t affect heat change much, especially when the substances are solids or liquids.
States of Matter: Gases usually show bigger heat changes compared to solids or liquids because gas particles have more space and energy to move.
Amount of Reactants: If you use more of the starting materials (reactants), more heat will be involved because the reaction is larger.
Bond Energy Dynamics: To start a reaction, bonds in the reactants need to be broken (which uses energy), and new bonds are made in the products (which gives off energy).
Exothermic Mechanism: If the energy released by making new bonds is more than the energy used to break old bonds, the reaction releases heat (exothermic).
Endothermic Mechanism: If more energy is needed to break the old bonds than is released by forming new ones, the reaction absorbs heat (endothermic).
Law of Conservation of Energy
Energy Principle: The law of conservation of energy says that energy can’t be made or destroyed; it can only change from one form to another, like from chemical energy to heat energy.
Chemical Energy Balance: In chemical reactions, the total amount of energy before and after the reaction stays the same, even if it changes form.
Energy Tracking: The energy might move around or change form (like into heat), but the overall amount of energy doesn’t change.
Thermochemistry Role: This idea is very important in thermochemistry because it helps us do calculations about heat in chemical reactions.
Key Equations and Formulae
Heat Change Formula: The formula Q = mcθ helps us calculate how much heat has been gained or lost.
- Q is the heat change in joules (J).
- m is the mass of the substance in grams (g).
- c is the specific heat capacity, which tells us how much heat is needed to raise 1 gram of a substance by 1°C (J g⁻¹ °C⁻¹).
- θ is the temperature change in degrees Celsius (°C).
Enthalpy Change Formula (per mole): ΔH = -Q / n is used to find the enthalpy change per mole of a substance.
- ΔH is the enthalpy change (kJ/mol).
- Q is the total heat change in kilojoules (kJ).
- n is the number of moles of the substance that reacted.
Enthalpy Comparison Formula: ΔH = Hproducts – Hreactants means we find the difference between the heat content (enthalpy) of products and reactants to know if heat was released or absorbed.
Additional Points
Specific Heat Capacity: This is how much heat is needed to raise the temperature of 1 gram of a substance by 1°C. For water, the value is 4.2 J g⁻¹ °C⁻¹.
Standard Enthalpy Change: This is the enthalpy change measured when the reaction happens under standard conditions, which means at 298 K (25°C) and 1 atmosphere of pressure.
Hess’s Law: This law says that the total enthalpy change for a chemical reaction is always the same, no matter which route the reaction takes. This helps us calculate ΔH when the direct path is hard to measure.