Thermochemistry: Chemical Reactions and Energy

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Questions and Answers

What is the First Law of Thermodynamics?

The energy of the universe is constant.

What is the equation for internal energy?

E = q + w

What is enthalpy?

Enthalpy is a measure of the heat content of a substance at constant pressure.

What is the equation for enthalpy change?

ΔH° = Hproducts – Hreactants Signup and view all the answers

Identify the following reaction as exothermic or endothermic: Production of quicklime CaO + H₂O -> Ca(OH)₂ ΔH₁ = −63.7 kJ/mol

Exothermic (B) Signup and view all the answers

Identify the following reaction as exothermic or endothermic: Thermal decomposition of limestone CaCO₃ → CaO + CO₂ (Requires temperatures > 900 °C)

Endothermic (B) Signup and view all the answers

What is the standard enthalpy of formation?

The change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states. Signup and view all the answers

The standard enthalpy of formation of a pure element in its standard state is defined as zero.

True (A) Signup and view all the answers

What is calorimetry?

Calorimetry is a technique used to measure the heat energy released or absorbed during a chemical reaction. Signup and view all the answers

What is the equation used to calculate the heat energy released or absorbed during a chemical reaction in calorimetry?

q = mCΔT Signup and view all the answers

What is Hess's Law?

The overall enthalpy change accompanying a chemical reaction is independent of the route taken in going from reactants to products. Signup and view all the answers

How can enthalpy changes be calculated?

Enthalpy changes can be calculated using calorimetry, bond energies, and Hess's Law. Signup and view all the answers

What are some examples of increases in entropy?

Increases in entropy occur when solids melt, liquids boil, ionic solids dissolve, or the temperature increases. Signup and view all the answers

What is Gibbs free energy?

Gibbs free energy is the energy from a reaction free to do work. Signup and view all the answers

What is the equation for Gibbs free energy?

ΔG = ΔH – TΔS Signup and view all the answers

What does a negative ΔG indicate?

Exergonic reaction, which is spontaneous (A) Signup and view all the answers

What does a ΔG = 0 indicate?

Equilibrium reaction, which is balanced (C) Signup and view all the answers

Study Notes

Chemical Reactions and Energy (Thermochemistry)

Thermochemistry studies energy changes in chemical reactions.
Energy is the ability to do work.
Work = Force x Distance, measured in joules (J).
1 Joule = the energy required to raise a 1kg substance 10cm against gravity.
1 calorie = the amount of heat necessary to raise the temperature of 1 gram of water by 1°C.
1 "food calorie" = 1 kcal = 1000 calories

Types of Energy

Potential energy (E = mgh) - energy due to position or condition.
Kinetic energy (E = ½mv²) - energy due to motion.
Electromagnetic energy
Nuclear energy
Chemical energy

Laws of Thermodynamics

First Law: The energy of the universe is constant (energy can't be created or destroyed, only converted).
Internal Energy (E): E = q + w (q = heat, w = work)
change in internal energy = energy added to the system + work done on the system

Enthalpy

Enthalpy (H) is a measure of the heat content of a substance at constant pressure.
Enthalpy change (ΔH°) represents heat released or absorbed during a chemical process.
ΔH°= H_products – H_reactants
Standard states: Define conditions for enthalpy values.
- Elements are in the form they exist at 1 atm and 25°C.
- For gaseous compounds, pressure is 1 atm.
- For pure liquids and solids, state at 1 atm.
- For solutions, concentration is 1 M.

Enthalpy Changes

Exothermic reactions release heat (ΔH negative) visualized on an energy diagram where products have lower energy than reactants. Energy is given out.
Endothermic reactions absorb heat (ΔH positive) visualized on an energy diagram where products have higher energy than reactants. Energy is absorbed.

Hess's Law

Overall enthalpy change is independent of the reaction path.
A route to calculate enthalpy change by applying successive reactions.

Measuring Enthalpy Changes (Calorimetry)

Calorimetry measures heat transfer.
q = mCΔT (q = heat transferred, m = mass , C = specific heat, ΔT = change in temperature).
Bomb calorimeters measure heat of combustion.

Standard Enthalpy of Formation

The enthalpy change when one mole of a compound is formed from its elements in their standard states; ΔH_f°.
Enthalpy of formation for elements in their standard state is defined as zero.

Entropy (S)

Entropy measures disorder or randomness in a system.
ΔS = S_final – S_initial (S= entropy).
Entropy is temperature dependent.
ΔS_total (total entropy change)= ΔS_system + ΔS_surroundings.
In a spontaneous process, ΔS_total is positive.

Gibbs Free Energy (ΔG)

Gibbs free energy (ΔG) determines the spontaneity of a reaction.
ΔG= ΔH-TΔS
ΔG < 0: Exergonic (spontaneous reaction, reaction will go forward).
ΔG > 0: Endergonic; reaction requires energy input to go forward.
ΔG=0: System in equilibrium (at a stop)

Coupling of Reactions in Metabolism

Cells couple reactions to drive endergonic processes (reactions that require energy).

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Description

Explore the fascinating world of thermochemistry, focusing on energy changes in chemical reactions. Learn about different forms of energy, the laws of thermodynamics, and the concept of enthalpy. This quiz will test your understanding of these essential topics in chemistry.