Thermochemistry

What is energy?

Energy is just the ability to do something:

• move

• make heat

• make things happen

Example:

• Food gives you energy to walk

• Gas gives a car energy to move

• Fire gives energy as heat

Law of Conservation of Energy

Big rule:
Energy cannot be created ❌
Energy cannot be destroyed ❌
Energy can only change form ✅

Example:

• Gas → heat → movement

• Food → body heat + motion

👉 The total energy in the universe stays the same.

Ways Energy Moves (Transfers)

🔥 “Heat = energy moving because of temperature difference”

This just means:

👉 Heat is energy that moves ONLY because one thing is hotter than another.

If two things are the same temperature, NO heat moves.

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🌡 “Hot → cold (ALWAYS)”

This means:

👉 Heat always moves from the hot thing to the cold thing.

Examples:

• Hot coffee sitting on a table → coffee cools down, table/air warms up

• Hot shower → bathroom warms up

• Your hand on ice → your hand loses heat, ice gains heat

This happens automatically. No effort needed.

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❌ “Never cold → hot by itself”

This means:

👉 Cold things cannot heat up hotter things on their own.

Example:

• Ice will NEVER make your hand hotter

• A cold room will not heat your body

For cold → hot to happen, you need extra energy (like a heater, stove, microwave).

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🏃 “Temperature tells us how fast particles are moving”

This is the KEY idea 👶:

• Hot = particles moving fast

• Cold = particles moving slow

So:

• Hot coffee → molecules zooming around

• Cold water → molecules moving slowly

When hot touches cold:
👉 fast particles bump into slow ones
👉 fast ones slow down
👉 slow ones speed up
👉 eventually they’re the same speed (same temperature)

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🧠 ONE-LINE MEMORY TRICK (EXAM GOLD)

• Heat = energy in motion

• Temperature = how fast particles move

• Heat flows hot → cold, NEVER the other way

🏋 Work

Work = force moving something

• Pushing a box

• Expanding a gas

• Lifting a weight

Pathway

🛣 “Pathway = HOW energy moves”

Pathway just means:
👉 the route or method energy takes to move

Not where it starts or ends — HOW it gets there

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❗ “Total energy change does NOT depend on the path”

This means:

👉 Only the starting point and ending point matter
👉 It does NOT matter how you got there

Real-life example (this is the key):

You start:

• At the bottom of a hill

You end:

• At the top of the hill

No matter what:

• You climbed

• You drove

• You zig-zagged

👉 You end at the same height
👉 Same total energy change

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🔥 “Heat and work DO depend on the path”

This is the part that trips people.

It means:

👉 How tired you get depends on HOW you go up
👉 How much heat or work happens depends on the path

Same hill example:

🚶‍♂ Walk up:

• You sweat a lot (more heat)

• You use more effort (more work)

🚗 Drive up:

• Less effort from YOU

• Engine does the work instead

Same start
Same end
Different heat & work

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🧠 Chem Translation (VERY IMPORTANT)

In chemistry:

• Total energy change (ΔE) → does NOT care about path

• Heat (q) and Work (w) → CARE about path

That’s why:

ΔE=q+w

Different paths → different q and w
But same ΔE

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🧠 ONE-LINE BABY SUMMARY (EXAM SAFE)

• Start + End decide total energy

• Path decides heat & work

• Same place → same ΔE

• Different routes → different q and w

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🚨 EXAM TRAP TO WATCH FOR

If they ask:

“Does energy change depend on pathway?”

Answer:
❌ NO (for total energy)

If they ask:

“Does heat depend on pathway?”

Answer:
✅ YES

State Function

✅ OFFICIAL DEFINITION (EXAM-SAFE)

A state function is a property whose value depends only on the current state of the system and not on the path taken to reach that state.

👉 That sentence is what you memorize.

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🧠 NOW TRANSLATED INTO NORMAL ENGLISH

• Property = something you can measure (energy, temperature, etc.)

• Current state = what the system looks like right now

• Not on the path = doesn’t care how it happened

So in human words:

A state function only depends on where the system starts and ends, not how it got there.

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🧪 CHEM EXAMPLE

If a reaction starts with:

• 100 J of energy

and ends with:

• 50 J of energy

Then:

• ΔE = −50 J

No matter:

• fast reaction

• slow reaction

• one step

• many steps

👉 SAME ΔE every time

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🧠 ONE-LINE EXAM MEMORY

State function = depends only on initial and final state

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🚨 VERY COMMON EXAM QUESTION

Which is a state function?

✔ Energy
✔ Enthalpy
✔ Temperature

❌ Heat
❌ Work

Heat Flow

Heat ALWAYS flows:
👉 Hot → Cold

Why?

• Hot particles move faster

• Cold particles move slower

• They share energy until equal

When equal → same temperature

System vs Surroundings

🧪 System

The thing you care about

• The reaction

• Chemicals reacting

🌍 Surroundings

Everything else

• Air

• Table

• Room

• You

Types of Reactions

🔥 Exothermic

• Heat leaves the system

• System gets colder

• Surroundings get warmer

Example:

• Fire

• Hand warmers

• Combustion

ΔH is negative (−)

Endothermic

• Heat enters the system

• System gets warmer

• Surroundings get colder

Example:

• Ice melting

• Cold packs

ΔH is positive (+)

Thermodynamics

Thermodynamics = study of energy and how it changes

First Law:
👉 Energy is conserved (same rule again)

Internal Energy (E)

🧠 WHAT “INTERNAL ENERGY (E)” ACTUALLY MEANS Internal energy = ALL the energy INSIDE the system

That’s it.

If the system is:

• a beaker

• a reaction

• a gas in a container

👉 Internal energy is the energy stored in that thing

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🔹 WHAT MAKES UP INTERNAL ENERGY? 1⃣ Particle motion

This is just:

• particles moving

• vibrating

• zooming around

👉 This is kinetic energy

Hotter = faster particles
Colder = slower particles

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2⃣ Attractions between particles

This is:

• particles pulling on each other

• bonds

• intermolecular forces

👉 This is potential energy

Strong attractions = more stored energy

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🔹 HOW CAN INTERNAL ENERGY CHANGE?

Only two ways:

🔥 Heat (q)

• Energy moves because of temperature difference

• Hot ↔ cold

If heat enters system:

• q is positive

• internal energy increases

If heat leaves system:

• q is negative

• internal energy decreases

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🏋 Work (w)

• Energy moves because something is pushed, pulled, or expanded

Example:

• Gas expands → does work → loses energy

• Gas is compressed → gains energy

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🔹 THE MONEY FORMULA 💰 ΔE=q+w

Translation:

Change in internal energy = heat + work

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🧠 ONE-LINE BABY SUMMARY

• Internal energy = energy stored inside

• Comes from particle motion + attractions

• Can ONLY change by:

  • heat

  • work

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🚨 EXAM TRAP TO WATCH

They might ask:

“Which can change internal energy?”

Answer:
✔ Heat
✔ Work

NOT:
❌ temperature by itself
❌ mass
❌ pathway

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🧠 CAVEMAN MODE 🦴

System has energy
Heat in → energy up
Work out → energy down

ΔE Equation

ΔE=q+w

• q = heat

• w = work

Signs (+ / −) — VERY IMPORTANT

From the SYSTEM’S view:

q positive (+)
→ heat enters system (endothermic)

q negative (−)
→ heat leaves system (exothermic)

Enthalpy (H)

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🧠 WTF IS ENTHALPY (H) — BABY MODE Enthalpy is just a special way of talking about energy

👉 when pressure stays constant (which is almost always in lab).

That’s it.

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🔹 “Energy at constant pressure” — WHAT THAT MEANS

In most chemistry labs:

• reactions happen in open containers

• pressure = atmospheric pressure

• pressure does NOT change

So chemists said:

“Let’s define a type of energy that works nicely when pressure is constant.”

That energy is called enthalpy (H).

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🔹 WHY DO WE CARE ABOUT ENTHALPY?

Because in constant-pressure conditions:

ΔH=heat gained or lost\Delta H = \text{heat gained or lost}ΔH=heat gained or lost

Meaning:
👉 ΔH tells you how much heat the reaction releases or absorbs

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🔹 WHY YOU DON’T MEASURE H DIRECTLY

Because:

• H is like a bank balance from birth

• You don’t know the starting value

But you CAN measure:
👉 how much it changes

That’s ΔH.

Same as:

• You don’t know how much money you had at age 3

• But you know how much you spent today

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🔹 WHAT ΔH TELLS YOU 🔥 Exothermic

• Heat leaves system

• ΔH negative

• Products have LESS energy

❄ Endothermic

• Heat enters system

• ΔH positive

• Products have MORE energy

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🧠 ONE-LINE EXAM DEFINITION

Enthalpy is a state function that represents the heat content of a system at constant pressure.

Memorize that 👆

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🧠 CAVEMAN MODE 🦍

Pressure same
Reaction happens
Heat measured = ΔH

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🚨 EXAM TRAP

If they ask:

“When is ΔH equal to heat?”

Answer:
✔ At constant pressur

ΔH at Constant Pressure

At constant pressure:

ΔH= q ^p

Meaning:
👉 Heat measured = enthalpy change

Comparing Enthalpy

• Products HIGHER → ΔH positive → endothermic

• Products LOWER → ΔH negative → exothermic

Calorimetry

🧠 WTF IS CALORIMETRY (BABY MODE) Calorimetry = measuring heat

That’s it.
Nothing mystical.

You can’t see heat directly, so instead you:

👉 watch temperature change

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🔥 WHY TEMPERATURE?

Because:

• Heat going in → temperature goes up

• Heat going out → temperature goes down

So temperature is like a thermometer spy telling you about heat.

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☕ REAL EXAMPLE (COFFEE CUP)

• You mix two chemicals in a cup

• Reaction happens

• Thermometer moves

If temp:

• goes UP → reaction released heat (exothermic)

• goes DOWN → reaction absorbed heat (endothermic)

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🧠 CHEM TRANSLATION

Calorimetry works because:

q=mcΔTq = m c \Delta Tq=mcΔT

You don’t need to panic — this just means:

• mass

• heat capacity

• temperature change

tell you how much heat moved.

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🧠 ONE-LINE EXAM DEFINITION (MEMORIZE)

Calorimetry is the experimental measurement of heat associated with a physical or chemical process by observing temperature change.

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🚨 EXAM TRAP

If they ask:

“How is heat measured in calorimetry?”

Answer:
✔ By measuring temperature change

NOT:
❌ directly weighing heat
❌ measuring pressure
❌ watching color change

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🧠 CAVEMAN MODE 🦴

Reaction happens
Temperature moves
Heat happened

Heat Capacity (C)

Heat capacity = how hard it is to heat something up

Specific heat

• Per gram

Molar heat

• Per mole

Water has HIGH heat capacity
→ heats slowly
→ cools slowly

Coffee Cup Calorimeter ☕

• Styrofoam cups

• Measures heat at constant pressure

• Used for reactions in solution

If temperature:

• Goes UP → exothermic

• Goes DOWN → endothermic

Heat Is Extensive

More substance = more heat

Double reaction → double heat

Bomb Calorimeter 💣

ONE-SENTENCE MEMORY (THIS IS THE GOAL)

In a bomb calorimeter, the container cannot expand, so no work is done and all the energy change appears as heat.

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CAVEMAN MODE 🦴

Box locked
Fire inside
No pushing
Only heat

Mixing Temperatures

🧠 WHAT “MIXING TEMPERATURES” MEANS

You mix:

• one hot thing

• one cold thing

Heat flows:
👉 hot → cold

They keep exchanging heat until:
👉 same temperature

That temperature is the final temperature.

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🔹 WHY FINAL TEMP IS BETWEEN THE TWO

Example:

• Hot water: 80°C

• Cold water: 20°C

Final temperature:
👉 cannot be 100°C (no extra heat)
👉 cannot be 0°C (no freezer)

So it must be:
👉 between 20°C and 80°C

This is ALWAYS true.

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🔹 WHY SOMETIMES IT’S NOT THE MIDDLE

Now the important part 👇

Heat capacity matters

If one thing:

• has higher heat capacity

• or more mass

it controls the final temperature more.

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🔥 EASY EXAMPLE Case 1: Same stuff, same amount

• 50 g hot water at 80°C

• 50 g cold water at 20°C

Final temp:
👉 around 50°C

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Case 2: Different heat capacity

• 50 g water at 20°C

• 50 g iron at 80°C

Water has higher heat capacity than iron.

So:

• water changes temp less

• final temp is closer to water’s temp

👉 NOT 50°C

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🧠 ONE-LINE RULE (EXAM GOLD)

The final temperature lies between the initial temperatures and is closer to the substance with the higher heat capacity.

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🧠 CAVEMAN MODE 🦴

Hot gives heat
Cold takes heat
Meet in middle

Big heat sponge decides where

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🚨 EXAM TRAP

If they ask:

“Which ends up with the smaller temperature change?”

Answer:
✔ the substance with higher heat capacity

Hess’s Law

Doesn’t matter HOW reaction happens
Only START and END matter

Steps don’t change ΔH

Rules for ΔH

• Reverse reaction → flip sign

• Multiply equation → multiply ΔH

• Scale reaction → scale ΔH

Standard Enthalpy of Formation (ΔH°f)

👉 1 mole of compound forms
👉 from elements in standard states

Standard States

• Gas → 1 atm

• Solid/Liquid → pure

• Solution → 1 M

• Elements → natural form

Example:

• O₂ (g)

• Na (s)

Important Rule

Elements in standard state:

ΔH^f= 0

Enthalpy Calculations Formula

ΔHrxn​=∑(ΔHf∘​ products)−∑(ΔHf∘​ reactants)

EXAM SURVIVAL SUMMARY

• Exothermic → ΔH negative

• Endothermic → ΔH positive

• Heat flows hot → cold

• System POV matters

• Elements in standard state = 0

• Hess’s Law = steps don’t matter