UNit 3 Chem- Definitions

Law of conservation of energy

• Energy cannot be created or destroyed; it can only be transferred or converted.

Kinetic energy

Energy of motion. A moving ball, running person, or moving molecule has kinetic energy.

Potential energy

Stored energy due to position or arrangement. A ball held above the ground has gravitational potential energy.

System

• The part you're studying, such as a chemical reaction

Surroundings:

Everything outside the system, such as the water, container, or air around the reaction.

Heat is energy transferred because of a…..

temperature difference.

-Temperature tells you how hot or cold something is; heat is energy moving from one place to another.

Enthalpy

is the heat transferred by a process at constant pressure.

The symbol is ΔH, pronounced “delta H.”

Delta means change

H Means enthalpy

Endothermic vs. exothermic

Endothermic

ΔH > 0

The system absorbs heat from its surroundings.

Example: ice melting.

___________________

Exothermic

ΔH < 0

The system releases heat into its surroundings.

Example: burning fuel.

ENDO

enters the system, so \(\Delta H\) is positive.

EXO

exits the system, so \(\Delta H\) is negative.

Formula 1: Heat and temperature

q = mc Δ T

q= Heat gained or lost usually J

m= mass in GRAMS!!!

c= specific heat capicty in J

Delta T= Final temp - initial temp

q= mc Delta T example: Suppose you heat 50.0 g of water from 20.0°C to 30.0°C.

Step 1: Identify the values.

m= 50.0 g

c= 4.184 J/ (g x Celcius)

Delta T= 30.0 - 20.0 = 10.0 C

Step 2: Substitute into the formula.

q= (50.0) (4.184)(10.0)

Step 3: Calculate

q= 2092 J = 2.092 kJ

The heat is positive because the water gained energy.

A balanced chemical equation can include an enthalpy change.

Hess's law

Hess's law says that the total enthalpy change depends only on the starting substances and final products, not on the pathway taken.

In simple terms, you can add chemical equations together to calculate an unknown \(\Delta H\).

Hess Law Equation

• Write the target equation.

• Find the given equations that can combine to produce it.

• Reverse equations when needed, changing the sign of their \(\Delta H\).

• Multiply equations when needed, multiplying their \(\Delta H\) values too.

• Add the equations and cancel substances appearing on both sides.

• Add the adjusted enthalpy values.

q will be positive if heat

is being absobed by the system

q will be negeative if

heat is being released by the system

Standard heats of reaction

ΔH rxn = Sigma n ΔH f (products) - Sigma n ΔH f (reactants)