Thermochemistry is the study of heat changes that accompany chemical reactions.
What are the two main types of reactions based on heat changes?
Exothermic reactions and endothermic reactions.
What happens to the temperature of the surroundings in an exothermic reaction?
The temperature of the surroundings increases because heat is released.
What is the sign of the enthalpy change (ΔH) for an exothermic reaction?
Negative (-ΔH).
Give an example of an exothermic reaction.
Combustion reactions, such as CH₄ + 2O₂ → CO₂ + 2H₂O.
What happens to the temperature of the surroundings in an endothermic reaction?
The temperature of the surroundings decreases because heat is absorbed.
What is the sign of the enthalpy change (ΔH) for an endothermic reaction?
Positive (+ΔH).
Give an example of an endothermic reaction.
Thermal decomposition of calcium carbonate: CaCO₃ → CaO + CO₂.
What does an energy level diagram show?
An energy level diagram shows the relative energy content of reactants and products, illustrating whether a reaction is exothermic or endothermic.
What does an exothermic energy level diagram look like?
The energy level of the products is lower than the reactants, with a downward energy change arrow.
What does an endothermic energy level diagram look like?
The energy level of the products is higher than the reactants, with an upward energy change arrow.
What does the law of conservation of energy state?
Energy cannot be created or destroyed, but it can be transformed from one form to another.
What is the formula for calculating heat change (Q)?
Q = mcθ, where Q = heat change (J), m = mass (g), c = specific heat capacity (J g⁻¹ °C⁻¹), and θ = temperature change (°C).
What is the formula for calculating enthalpy change (ΔH)?
ΔH = -Q / n, where ΔH = enthalpy change (kJ mol⁻¹), Q = heat change (kJ), and n = number of moles of reactants.
What is Hess’s Law?
Hess’s Law states that the total enthalpy change for a reaction is independent of the route taken.
How does the nature of reactants and products affect heat changes?
Different substances have different stored chemical energy. Reactions involving substances with stronger bonds release more heat.
Why does the state of reactants and products affect heat changes?
Reactions involving gaseous reactants or products usually have larger heat changes than those involving solids and liquids.
What happens in bond breaking and bond formation?
Energy is required to break bonds, and energy is released when new bonds form.
Why is water commonly used in calorimetry experiments?
Water has a high specific heat capacity (4.2 J g⁻¹ °C⁻¹), making it efficient at absorbing heat.
What is standard enthalpy change?
The enthalpy change when a reaction occurs under standard conditions (298 K and 1 atm pressure).
Explanation Questions
Why is combustion considered an exothermic reaction?
Combustion releases heat as it involves the breaking of bonds in fuel molecules and the formation of stronger bonds in carbon dioxide and water, releasing energy to the surroundings.
Why do endothermic reactions require heat input?
Endothermic reactions absorb heat because more energy is required to break bonds than is released when new bonds form.
Why does respiration release energy while photosynthesis absorbs energy?
Respiration is an exothermic process that releases energy from glucose, whereas photosynthesis is endothermic, absorbing energy from sunlight to produce glucose.
Why does breaking bonds require energy?
Breaking bonds requires energy because atoms are held together by strong electrostatic forces that need to be overcome.
Why do exothermic reactions occur spontaneously more often than endothermic reactions?
Exothermic reactions release energy, making them energetically favorable, while endothermic reactions require an energy input.
Why is heat change proportional to the amount of reactants used?
The more reactants available, the more bonds are broken and formed, leading to a greater overall heat change.
Why does water have a high heat capacity?
Water molecules form hydrogen bonds that require large amounts of energy to change temperature.
Why is Hess’s Law useful in thermochemistry?
It allows indirect calculation of enthalpy changes when direct measurements are difficult.
Why does an increase in temperature affect the rate of a reaction?
An increase in temperature provides more kinetic energy to reactant molecules, increasing the frequency and energy of collisions, which leads to a higher reaction rate.
Why does pressure have a greater effect on reactions involving gases than on those involving solids or liquids?
Gases are compressible, so increasing pressure reduces the volume, bringing gas molecules closer together and increasing the frequency of collisions, which speeds up the reaction. In contrast, solids and liquids have fixed volumes and are not significantly affected by pressure changes.