Termokimia Bag.1 - Pengertian, Sistem & Lingkungan, Eksoterm & Endoterm, Persamaan Reaksi Termokimia

Main Ideas and Lessons (Thermochemistry – Part 1)

What Thermochemistry Covers

Thermochemistry (KD 3.2, Chemistry Class 11) explains:

• The concept of enthalpy change of reaction at constant pressure
• How to write thermochemical reaction equations (chemical equations that include heat/enthalpy data)

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Definition of Thermochemistry

Thermochemistry is a branch of chemistry that studies the relationship between chemical reactions and the heat released/absorbed.

• Heat is energy that transfers from higher temperature to lower temperature objects.

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Key Terms: System vs. Environment

• System: the part of the reaction that is the focus/center of attention.
• Environment: everything outside the system (often including the container and surrounding air).

Example: Magnesium ribbon + HCl solution

• System: Mg mixed with HCl solution
• Environment: the test tube/container, air, and other surroundings outside the system

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Types of Thermodynamic Systems

• Open system: exchanges both matter and energy with the environment.
• Closed system: exchanges energy only, not matter.
• Isolated system: exchanges neither matter nor energy with the environment.

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Enthalpy (H) and Enthalpy Change (ΔH)

• Enthalpy, H: denoted by the symbol H.
• In thermochemistry (constant pressure context):
  • Enthalpy = heat energy contained in a system at constant pressure
• Absolute enthalpy cannot be measured directly.
  • What can be measured is enthalpy change (ΔH).

Enthalpy change definition (ΔH): [ \Delta H = (\text{sum of enthalpies of products}) - (\text{sum of enthalpies of reactants}) ] This requires knowing enthalpies of substances before and after the reaction (not just the temperature).

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Exothermic vs. Endothermic Reactions (Core Methodology)

Exothermic Reaction

• Heat flow: heat is released by the system to the environment
  • so the environment absorbs heat
• Sign of enthalpy change: ΔH is negative
• Observable effects:
  • the container/surroundings feel hot
  • the temperature of the environment increases
• Energy diagram logic:
  • energy of reactants > energy of products
  • therefore ΔH < 0
• Examples mentioned:
  • Combustion (e.g., campfire): hydrocarbon + O₂ → CO₂ + H₂O, releases heat
  • Examples like condensation (gas → liquid) and freezing were used to illustrate “releases heat”

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Endothermic Reaction

• Heat flow: heat is absorbed by the system from the environment
  • so the system absorbs heat
• Sign of enthalpy change: ΔH is positive
• Observable effects:
  • the container feels cold
  • the temperature of the environment decreases
  • reactions often involve phase changes that require heat
• Energy diagram logic:
  • energy of products > energy of reactants
  • therefore ΔH > 0
• Examples mentioned:
  • Photosynthesis (absorbs heat from sunlight)
  • Dissolving ammonium salt in water (container feels cold)
  • Melting (solid → liquid) and evaporation/boiling to show “requires heat first”

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Thermochemical Reaction Equations (Instructions / Rules Emphasized)

What a Thermochemical Equation Includes

A thermochemical reaction equation is:

• a chemical reaction equation plus enthalpy/heat data

Core idea: include ΔH (enthalpy change / heat released or absorbed) in the equation.

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State Symbols Must Be Included

Substances should be written with their physical states, such as:

• (g) gas
• (l) liquid
• (s) solid
• (aq) dissolved in water/aqueous solution (implied from dissolution discussion)

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Coefficient Scaling Rule

If you multiply the balanced reaction coefficients by a factor, then:

• ΔH must be multiplied by the same factor
• because heat is proportional to reaction stoichiometry

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Sign Interpretation

• Reaction releases heat (exothermic) → ΔH is negative
• Reaction absorbs heat (endothermic) → ΔH is positive

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Example Described: Combustion of Methane

• For 1 mol methane, write the reaction along with ΔH (shown as negative).
• Then scale to 2 mol methane, showing ΔH becomes 2× the original magnitude.

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Practice / Question Reasoning Included (Conceptual Steps)

• System identification

  • Example practice: lime chips + water in a test tube becomes hot
  • System: the reacting mixture (lime chips + water)
  • Environment: the test tube/hand/air outside the mixture

• Determining reaction type

  • If the container feels hot → exothermic → ΔH negative
  • If the container feels cold → endothermic → ΔH positive

• Using energy diagrams

  • Products lower than reactants → exothermic
  • Products higher than reactants → endothermic

• Stoichiometry in thermochemical equations

  • balance moles/coefficients
  • ensure states (g, l, s, aq, etc.) are correct
  • write ΔH with correct sign and magnitude
  • scale ΔH when coefficients change (e.g., 1 mol → 2 mol)

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Speakers / Sources Featured

• The video includes an instructor/lecturer (not clearly named in the subtitles).
• The name “Faisal” appears once in the subtitle text, but it is unclear whether this refers to a speaker or a transcription issue.
• No external sources/citations are clearly identifiable.