THERMOCHEMISTRY UNIT 9 STUDY GUIDE

Heat vs Temperature

Heat

Energy transferred between objects due to temperature differences

Temperature

Measure of average kinetic energy of particles

System vs Surroundings

System

The part of the reaction being studied

Surroundings

Everything outside the reaction

Endothermic Reaction

Endothermic

Reaction that absorbs heat from surroundings

Endothermic Sign of ΔH

Positive

Endothermic Energy Placement

Energy written on reactant side

Endothermic Energy Diagram

Products higher than reactants

Exothermic Reaction

Exothermic

Reaction that releases heat to surroundings

Exothermic Sign of ΔH

Negative

Exothermic Energy Placement

Energy written on product side

Exothermic Energy Diagram

Products lower than reactants

Enthalpy Definitions

Enthalpy of Reaction

Overall heat change during reaction

Enthalpy of Reactants

Energy stored in reactant bonds

Enthalpy of Products

Energy stored in product bonds

Specific Heat Capacity

Specific Heat Capacity

Amount of energy needed to raise 1 g of a substance by 1°C

Specific Heat Equation

q = mcΔT

q

Heat energy (J or kJ)

m

Mass (g)

c

Specific heat capacity (J/g°C)

ΔT

Change in temperature = final − initial

Thermochemistry Signs

Positive q or ΔH

Heat absorbed

Negative q or ΔH

Heat released

Potential Energy Diagram Labels

x-axis

Reaction progress

y-axis

Potential energy

Ea

Activation energy

ΔH

Difference between reactants and products

Exothermic Energy Diagram

Exothermic Diagram

Reactants higher than products and ΔH negative

Endothermic Energy Diagram

Endothermic Diagram

Products higher than reactants and ΔH positive

Bond Enthalpy

Breaking Bonds

Requires energy (endothermic)

Forming Bonds

Releases energy (exothermic)

Bond Energy Equation

ΔH = Bonds Broken − Bonds Formed

Thermochemical Equations

Exothermic Example

CH4 + 2O2 → CO2 + 2H2O + energy

Endothermic Example

energy + CaCO3 → CaO + CO2

ΔH Notation Examples

Exothermic ΔH Example

ΔH = −482.6 kJ

Endothermic ΔH Example

ΔH = +482.6 kJ

Dimensional Analysis with Thermochemistry

Use mole ratios from balanced equation to convert:

grams → moles → kJ

kJ → moles → grams

Important Conversion

1 mol = molar mass in grams

Calorimetry

Heat absorbed by surroundings = negative of heat released by reaction

qsurroundings = −qsystem

Common Mistakes

For ΔT always use final − initial

Exothermic reactions have negative ΔH

Endothermic reactions have positive ΔH

Always balance equation before calculations

Use coefficients for mole ratios

Hydrogen and Oxygen Reaction

2H2 + O2 → 2H2O(g) + 482.6 kJ

ΔH = −482.6 kJ

Reaction Type

Exothermic

Heat Transfer

System releases heat to surroundings

How to Tell if Reaction is Endothermic or Exothermic

Energy on products side

Exothermic

Energy on reactants side

Endothermic

Negative ΔH

Exothermic

Positive ΔH

Endothermic

Practice Questions

What happens to surroundings during an exothermic reaction?

Surroundings gain heat and warm up