4.1.1
Endothermic & Exothermic Reactions
Enthalpy Changes
Enthalpy Changes
For a given process or reaction, we can measure the enthalpy change.
Definition of enthalpy change
Definition of 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.
Enthalpy vs energy changes
Enthalpy vs energy changes
- 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 reactions
Endothermic vs exothermic reactions
- An exothermic reaction is one which gives out heat energy.
- An exothermic reaction has a negative enthalpy change.
- Energy is transferred from the reaction system to the surroundings.
- An endothermic reaction is one which takes in heat energy.
- An endothermic reaction has a positive enthalpy change.
- Energy is transferred from the surroundings to the reaction system.
Examples
Examples
- Combustion is an exothermic process as it gives out heat!
- E.g. Burning methane:
- CH4 + 2O2 → CO2 + 2H2O
- ΔH = −882.00kJmol-1
- E.g. Burning methane:
- Thermal decomposition is an endothermic process.
- E.g. The thermal decomposition of calcium carbonate:
- CaCO3 → CaO + CO2
- ΔH = +178.30kJmol-1
- E.g. The thermal decomposition of calcium carbonate:
Forming solutions
Forming solutions
- The formation of a solution may be viewed as a stepwise process:
- Energy is consumed to overcome solute-solute and solvent-solvent attractions (endothermic processes).
- Energy is released when solute-solvent attractions are established (an exothermic process called solvation).
- The relative magnitudes of the energy changes determine whether the dissolution process overall will release or absorb energy.
- Solutions may not form if the energy required to separate solute and solvent species is much greater than the energy released by solvation.
Bond Breaking and Making
Bond Breaking and Making
The basis of chemical reactions is a series of bond breaking and making.
Bond enthalpies
Bond enthalpies
- During a reaction, some chemical bonds must be broken and made.
- The energy to break or make a bond is known as the bond enthalpy.
- Energy is needed to break a bond so it is an endothermic process.
- Energy is given off when a bond is made so it is an exothermic process.
- The enthalpy change of a reaction is a sum of the individual bond enthalpies being broken and made during the reaction.
The dominating term
The dominating term
- During a chemical reaction, we must determine whether the reaction requires more energy to break bonds or to make bonds.
- This will give an overall reaction enthalpy that is either positive or negative.
Endothermic vs exothermic
Endothermic vs exothermic
- Since bond breaking is endothermic and bond making is exothermic:
- More energy required to break bonds in a reaction will lead to an overall endothermic reaction.
- More energy released making bonds in a reaction will lead to an overall exothermic reaction.
Symbols
Symbols
- Enthalpy is given the symbol H.
- Enthalpy changes are given the symbol ΔH.
- A negative enthalpy change (pictured) is one which gives out heat.
- A positive enthalpy change is one which takes in heat.
Standard conditions
Standard conditions
- You'll hear the term 'standard conditions' a lot in chemistry. These conditions refer to:
- A pressure of 1 bar.
- A temperature of 298K.
- A substance's most stable state at 298K and 1 bar pressure.
- E.g. For water, it is liquid.
- E.g. For carbon, it is graphite.
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 Base
6.9.2Titration Experimental Detail
6.9.3Extended Response - Titration
6.9.4Titration Calculations
6.9.5Titration Curves
6.9.6Titration Calculation Strong Acid & Weak Base
6.9.7IB Multiple Choice - Titrations
6.9.8Polyprotic Acids
6.9.9Titration Calculations Strong Acid & Strong Base
6.9.10Titrations 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 Base
6.9.2Titration Experimental Detail
6.9.3Extended Response - Titration
6.9.4Titration Calculations
6.9.5Titration Curves
6.9.6Titration Calculation Strong Acid & Weak Base
6.9.7IB Multiple Choice - Titrations
6.9.8Polyprotic Acids
6.9.9Titration Calculations Strong Acid & Strong Base
6.9.10Titrations Curves 2
7Measurement, Data Processing & Analysis
7.1Uncertainties & Errors in Measurements & Results
7.2Graphical Techniques
7.3Spectroscopic Identification of Organic Compounds
7.4Infrared Spectroscpy
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