Unit 10: Thermochemistry

bond enthalpy

energy required to break a bond

Reactants - Products

(The sum of )Σ (bonds broken) - Σ (bonds formed) =

△ H (change in heat)

Exothermic

- △ H

Energy released

Temperature increases

(surroundings) feels hot

Endothermic

+ △ H

Energy absorbed

Temperature decreases

(surroundings) feels cold

Writing thermochemical equations

1. Write the chemical equations from its words

2. Add the amount of energy on to the product or the reactant side

If energy is released, then it is a product, it is an exothermic reaction

If the energy is absorbed, then it is a reactant, it is an endothermic reaction

△ H is in KJ/mole rxn

Thermochemical equations and Stoichiometry

1. Using the energy attached to the balanced equation, solve for either the heat (kilojules) or the grams

Grams to amount of energy conversion

Grams --> Moles --> Kj

To get from Grams to Moles use molar mass

To get from Moles to Kj, use the △H provided in the thermochemical equation

Amount of Energy to grams conversion

Kj --> Moles --> Grams

To get from Kj to Moles use the △H provided in the thermochemical equation

To get from Moles to Grams, use the molar mass

Energy diagrams

Visual representation of energy changes in reactions.

Activated complex in energy diagrams

The highest point of energy that a reaction goes through/to

Activation Energy in energy diagrams

It is the amount of energy needed to start a reaction

to calculate this on the graph, activation complex - reactants

△ H in energy diagrams

To calculate this in energy diagrams, it is the products - the reactants

Exothermic vs. Endothermic reactions in energy diagrams

Endothermic reactions: the products are above the reactants and △ H > 0 and is positive

Exothermic reactions: the products are below the reactants and the △ H < 0 and is negative

Specific heat

the heat required to raise the temperature of one gram of a substance one degree celcius

Specific heat calculations

Q = m x C x ΔT

Q: heat/energy in joules (J)

m: mass (grams or kilograms)

C: specific heat (J/kilogram or g °C)

ΔT: change in temp; final - initial

Hess's Law

the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process

Hess's Law steps

1. Use the 3 given balanced equations to make up your goal equation

1a. Modify given balanced equations (multiply, divide, flip) as well as do the same with the △ H that comes with it

1b. Add the newly created given equations together (cancelling out if opposite sides and bringing it down if the same side) to form the goal equatiion

Equation multiplied △ H multiplied

Equation flipped △ H negative

1c. As you add up the newly created given equations, add up the △ H