Heat of Reaction
Definition: The heat of reaction is the amount of heat energy that is either taken in (absorbed) or given out (released) when a chemical reaction takes place. This tells us whether the reaction makes the surroundings hotter or colder.
Cause of Heat Change: The heat energy change happens because chemical bonds are being broken and new ones are being formed. Breaking bonds takes in energy, while forming new bonds gives out energy.
Alternative Name: The heat of reaction is also known by another name: enthalpy change. In science, we write this as the symbol ΔH (read as “delta H”).
Reaction Types: There are two main types of heat changes in reactions:
- Exothermic reactions: These give off heat to the surroundings (the temperature goes up).
- Endothermic reactions: These take in heat from the surroundings (the temperature goes down).
Measurement Units: We measure the heat of reaction using a unit called kilojoules per mole. This is written as kJ mol⁻¹. It tells us how much heat is transferred for each mole of substance involved.
Enthalpy Change (ΔH)
Definition: Enthalpy change (ΔH) is the amount of heat energy a chemical system gives off or takes in while the pressure stays the same. It shows us how much energy is involved in the reaction.
Energy Measure: ΔH helps us measure how much heat energy goes in or out during a chemical reaction. It’s a way of tracking energy changes in the system.
Formula: We calculate ΔH using the formula: ΔH = H(products) – H(reactants), which means we subtract the energy of the reactants from the energy of the products.
Exothermic Indicator: If the reaction gives out heat, we say it is exothermic. For these reactions, ΔH is a negative number (ΔH < 0).
Endothermic Indicator: If the reaction takes in heat, it is endothermic. For these reactions, ΔH is a positive number (ΔH > 0).
Magnitude Significance: The size (magnitude) of the ΔH value tells us how much heat is involved in the reaction—whether a little or a lot.
Specific Types of Heat of Reaction
Heat of Combustion
Definition: Heat of combustion is the total amount of heat energy that is given out when one mole (a fixed number of particles) of a substance burns completely in a large amount of oxygen gas. When something burns, it means it reacts with oxygen and gives off heat while forming new products, usually including carbon dioxide and water.
Reaction Nature: Every combustion reaction gives off heat energy to the surroundings. This means it is an exothermic reaction, which is a type of reaction that increases the temperature of the surroundings. Because energy is released, the ΔH value (which shows energy change) is written as a negative number.
Conditions: To make sure the heat of combustion is measured fairly and accurately, scientists carry out these experiments under standard conditions. These standard conditions include a temperature of 25°C (which is 298 K) and a pressure of 1 atmosphere (the average air pressure at sea level).
Examples: Fuels like methane (CH₄), ethanol (C₂H₅OH), and propane (C₃H₈) are used because they burn well and give off a lot of energy. Each of these fuels has a specific amount of heat energy that is released when 1 mole burns completely.
Sample Equation: A simple combustion reaction can be shown like this:
C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ mol⁻¹
This tells us that when 1 mole of carbon (a solid) reacts with 1 mole of oxygen gas, it forms carbon dioxide gas and releases 393.5 kJ of heat energy.
Molecular Structure Impact: The more atoms of carbon and hydrogen a fuel has, the more energy it can release during burning. That’s because more chemical bonds break and form, and this process involves energy being given out as heat.
Heat of Neutralisation
Definition: Heat of neutralisation is the heat energy that is given out when an acid reacts with a base (which is also called an alkali) to form one mole of water. At the same time, a salt is produced.
Reaction Nature: Just like burning fuels, this is an exothermic reaction. That means heat is released to the surroundings, and the ΔH value is negative.
Standard Value: When a strong acid such as hydrochloric acid reacts with a strong base such as sodium hydroxide, the reaction releases about -57 kJ of heat energy for each mole of water produced. This value is quite constant for most strong acid-base pairs.
Example Equation: An example is:
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) ΔH = -57 kJ mol⁻¹
This means that hydrochloric acid and sodium hydroxide react to make sodium chloride (a salt) and water, and they release 57 kJ of heat for every mole of water made.
Weak Acid/Base Effect: If the acid or base is weak (for example, ethanoic acid or ammonia), the heat released will be less. This is because part of the energy is used to help break them into ions before they can fully react.
Constancy for Strong Reagents: When both the acid and base are strong, the amount of heat released is almost always the same. This makes it easy for scientists to predict and compare.
Relative Heat Output: Strong acid and strong base reactions release more heat than reactions with weak acids or bases, since less energy is wasted getting the weak acid or base ready to react.
Heat of Precipitation
Definition: Heat of precipitation is the amount of heat energy given out when two solutions are mixed and a solid forms. This solid is called a precipitate.
Formation Source: This happens when ions (tiny charged particles in solution) from different compounds meet and react. If they form a solid that cannot stay dissolved in water, the solid sinks and forms a new substance.
Reaction Type: Some precipitation reactions give off heat (which means they are exothermic), but others take in heat (which means they are endothermic). The type of heat change depends on what ions are reacting.
Example (AgCl): For example:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
This makes a white solid called silver chloride, and heat is released as the solid forms.
Example (PbSO₄): Another example is:
Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s)
This reaction forms a white solid called lead(II) sulfate, and it also gives off heat.
ΔH Example (AgCl): When silver ions and chloride ions form silver chloride, the reaction releases 65.5 kJ of heat for every mole formed:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) ΔH = -65.5 kJ mol⁻¹
Heat of Displacement
Definition: Heat of displacement is the amount of heat released when a more reactive metal pushes out (displaces) a less reactive metal from a solution of its salt.
Reaction Type: These displacement reactions are exothermic, so they give off heat energy to the surroundings.
Example (Zn + Cu²⁺): Zinc is more reactive than copper, so it takes copper’s place in the solution:
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) ΔH = -210 kJ mol⁻¹
This reaction gives off 210 kJ of heat for every mole of zinc used.
Example (Mg + Cu²⁺): Magnesium is also more reactive than copper:
Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s) ΔH = -92.4 kJ mol⁻¹
Reactivity Rule: A more reactive metal can always replace a less reactive one from a solution. When this happens, energy is given out as heat. The bigger the difference in reactivity, the more heat is released.
Thermochemical Equations
Definition: Thermochemical equations are special chemical equations that show not only which substances react and what they form, but also how much heat is involved in the reaction.
State Symbols: These equations include small labels called state symbols that tell us the physical state of each substance: (s) for solid, (l) for liquid, (g) for gas, and (aq) for dissolved in water.
Heat Direction Indicator: The ΔH value in the equation tells us whether heat is given out or taken in. If ΔH is negative, the reaction is exothermic (heat is released). If ΔH is positive, the reaction is endothermic (heat is absorbed).
ΔH Significance: The number after ΔH shows how much heat energy is gained or lost in the reaction for every mole of substance used. It helps us measure energy changes accurately.
Magnitude Meaning: A large ΔH value means the reaction involves a big energy change. A small ΔH means a smaller energy change. The size of the number tells us how strong the heat effect is.
Mole Representation: The big numbers in front of the chemical formulas (called coefficients) tell us how many moles of each substance are used or produced. These numbers help us work out the total energy change in the whole reaction.
Examples of Thermochemical Equations:
- Combustion: C(p) + O₂(g) → CO₂(g) ΔH = -393.5 kJ mol⁻¹
- Neutralisation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(ce) ΔH = -57 kJ mol⁻¹
- Precipitation: Ag⁺(aq) + Cl⁻(aq) → AgCl(p) ΔH = -65.5 kJ mol⁻¹
- Displacement: Zn(p) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(p) ΔH = -210 kJ mol⁻¹
- Decomposition: CaCO₃(p) → CaO(p) + CO₂(g) ΔH = +178.3 kJ mol⁻¹
Purpose: These equations are helpful because they let us understand not just what substances react and what is formed, but also how much energy is involved in the process.