4.2.1
Enthalpy of Reaction
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Enthalpy of Reaction
The enthalpy change of a reaction tells us the amount of heat energy released into the surroundings in an exothermic reaction or absorbed from the surroundings in an endothermic reaction.

Enthalpy change
- An enthalpy change is a measure of the heat given out or taken in during a process.
- When objects are heated, they use energy to expand.
- Enthalpy takes into account the energy used in the expansion.
- The enthalpy change of a reaction is represented using ΔH.

Enthalpy vs energy
- We use enthalpy instead of energy because we cannot easily measure energy changes. This is because objects expand when heated.
- To measure an energy change, we would have to fix the volume of the object.
- Enthalpy is much easier to use as it allows for expansion.
- Enthalpy changes are instead measured under constant pressure.
- The atmosphere is at a constant pressure, so we require no extra equipment.

Endothermic vs exothermic
- A negative value of an enthalpy change, ΔH < 0, indicates an exothermic reaction.
- A positive value, ΔH > 0, indicates an endothermic reaction.
- If the direction of a chemical equation is reversed, the arithmetic sign of its ΔH is changed.
- So, a process that is endothermic in one direction is exothermic in the opposite direction.

Molar enthalpy changes
- The ΔH value indicates the amount of heat associated with the reaction involving the number of moles of reactants and products as shown in the chemical equation.
- For example, consider this equation:
- H2(g)+½O2(g)⟶H2O(l) Δ𝐻=−286kJ
- This equation indicates that when 1 mole of hydrogen gas and ½ mole of oxygen gas change to 1 mole of liquid water 286 kJ of heat are released to the surroundings.
- This is assuming constant temperature and pressure throughout the reaction.
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Changing coefficients
- If the coefficients of the chemical equation are multiplied by some factor, the enthalpy change must be multiplied by that same factor (ΔH is an extensive property).
- For example if we double the coefficients of the previous equations, the enthalpy change also doubles:
- 2H2(g)+ O2(g)⟶2H2O(l) Δ𝐻= (2 x −286kJ) = - 572 kJ

State symbols
- The enthalpy change of a reaction depends on the physical states of the reactants and products, so these must be shown.
For example, if gaseous rather than liquid water is formed in the reaction mentioned previously, only 242 kJ of heat are released:
- H2(g)+½O2(g)⟶H2O(g) Δ𝐻=−242kJ
1Structure - Models of the Particulate of Matter
1.1Introduction to the Particulate Model of Matter
1.2The Nuclear Atom
1.3Electron Configuration
1.4Counting Particles by Mass: The Mole
1.6Elements, Compounds & Mixtures
1.7States of Matter & Changes of State
1.8Reacting Masses &. Volumes
1.9Solutions
2Structure - Models of Bonding & Structure
2.1The Ionic Model
2.2The Covalent Model
2.3Covalent Structures
2.4The Metallic Model
2.5From Models to Materials
2.6Valence Electrons & Ionic Compounds
2.7Molecular Shape
3Structure - Classification of Matter
3.1The Periodic Table: Classification of Elements
3.2Periodic Trends
3.3Group 1 Alkali Metals
3.4Halogens
3.5Noble gases, group 18
3.6Functional Groups: Classification of Organic
3.7Functional Group Chemistry
3.8Alkanes
3.9Alcohols
4Reactivity - What Drives Chemical Reaction?
4.1Endothermic & Exothermic Reactions
4.2Enthalpy of Reaction, Formation, & Hess' Law
5Reactivity - How Much, How Fast & How Far?
5.1Kinetics
5.2Rates of Reaction
5.3Stoichometry
5.4Le Châtelier’s Principle
5.5Introduction to Equilibrium
5.6Equilibrium Constant
5.7Reaction Quotient & Equilibrium Constant
6Reactivity - The Mechanisms of Chemical Change
6.1Proton Transfer Reactions
6.2The pH Scale
6.3Strong & Weak Acids and Bases
6.4Acid Deposition
6.5Types of Organic Reactions
6.6Oxidation & Reduction
6.7Electrochemical Cells
6.9Acid-Base Titrations
6.9.1Titration Calculation Weak Acid & Strong Base6.9.2Titration Experimental Detail6.9.3Extended Response - Titration6.9.4Titration Calculations6.9.5Titration Curves6.9.6Titration Calculation Strong Acid & Weak Base6.9.7IB Multiple Choice - Titrations6.9.8Polyprotic Acids6.9.9Titration Calculations Strong Acid & Strong Base6.9.10Titrations Curves 2
7Measurement, Data Processing & Analysis
7.1Uncertainties & Errors in Measurements & Results
7.2Graphical Techniques
7.3Spectroscopic Identification of Organic Compounds
7.4Infrared Spectroscpy
Jump to other topics
1Structure - Models of the Particulate of Matter
1.1Introduction to the Particulate Model of Matter
1.2The Nuclear Atom
1.3Electron Configuration
1.4Counting Particles by Mass: The Mole
1.6Elements, Compounds & Mixtures
1.7States of Matter & Changes of State
1.8Reacting Masses &. Volumes
1.9Solutions
2Structure - Models of Bonding & Structure
2.1The Ionic Model
2.2The Covalent Model
2.3Covalent Structures
2.4The Metallic Model
2.5From Models to Materials
2.6Valence Electrons & Ionic Compounds
2.7Molecular Shape
3Structure - Classification of Matter
3.1The Periodic Table: Classification of Elements
3.2Periodic Trends
3.3Group 1 Alkali Metals
3.4Halogens
3.5Noble gases, group 18
3.6Functional Groups: Classification of Organic
3.7Functional Group Chemistry
3.8Alkanes
3.9Alcohols
4Reactivity - What Drives Chemical Reaction?
4.1Endothermic & Exothermic Reactions
4.2Enthalpy of Reaction, Formation, & Hess' Law
5Reactivity - How Much, How Fast & How Far?
5.1Kinetics
5.2Rates of Reaction
5.3Stoichometry
5.4Le Châtelier’s Principle
5.5Introduction to Equilibrium
5.6Equilibrium Constant
5.7Reaction Quotient & Equilibrium Constant
6Reactivity - The Mechanisms of Chemical Change
6.1Proton Transfer Reactions
6.2The pH Scale
6.3Strong & Weak Acids and Bases
6.4Acid Deposition
6.5Types of Organic Reactions
6.6Oxidation & Reduction
6.7Electrochemical Cells
6.9Acid-Base Titrations
6.9.1Titration Calculation Weak Acid & Strong Base6.9.2Titration Experimental Detail6.9.3Extended Response - Titration6.9.4Titration Calculations6.9.5Titration Curves6.9.6Titration Calculation Strong Acid & Weak Base6.9.7IB Multiple Choice - Titrations6.9.8Polyprotic Acids6.9.9Titration Calculations Strong Acid & Strong Base6.9.10Titrations Curves 2
7Measurement, Data Processing & Analysis
7.1Uncertainties & Errors in Measurements & Results
7.2Graphical Techniques
7.3Spectroscopic Identification of Organic Compounds
7.4Infrared Spectroscpy
Practice questions on Enthalpy of Reaction
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