Thermochemistry

A look at the new unit

Chapter 11: Thermochemistry– Endothermic – Exothermic – Changes in states of water

Chapter 19.3 and 19.4: Spontaniety of a reactionUnder what conditions will a reaction occur?

-Heat/energy changes that occur during chemical reactions; it is either released or absorbed (it is NOT the same as temp.)-Symbol = q-Unit = Joules or KJ or calories -Heat energy that transfers from one object to another -This is witnessed as a change in temperature

Thermochemistry: pgs. 293-294

Types of Heat Transfers: pgs.294-295

Endothermic Reaction- demo: ammonium nitrate and water-the system/chemical reaction absorbs heat from the surroundings (usually water): system increases in q and surroundings decrease in q-q is positive-Energy is a reactant

- Exothermic Reaction- demo: calcium chloride and water- the system releases heat into the

surroudings; the system decreases in q and the surroundings increase in q

- q is negative- Energy is a product

How is this energy measured? pgs. 295-296

Calories or Joules

What is a calorie?

- the quantity of heat needed to raise 1 gram of water 1 degree Celsius (ERE rears its ugly head)

So...What is the difference between a food Calorie and a heat calorie?

1 Calorie (food) = 1 kilocalorie = 1000 calories (heat energy)

Joule (J) - SI unit for heat and energy

That’s Prescott Joule

Conversion Factors: 1 J = 0.239 cal.

4.18 J = 1 cal

So…how can we measure an energy change??

• Although the temperature change can be measured, this is not the same as the ENERGY change

• The energy change will be calculated by how much energy is absorbed or released by the surroundings

• In order to do this, you must know the specific heat capacity of the surroundings (this will usually be water for class)

Heat Capacity: pg.297

- the amount of heat energy needed to increase the temp of ANY object by 1 degree CelsiusSpecific Heat Capacity / Specific Heat

- the amount of heat energy needed to raise 1 gram of substance 1 degree Celsius

- represented by C

q

(m) (T)C =

heat (joules or calories)

mass (g) * change in Temp (oC)

=

Water has a HIGH heat capacity

Calculating q

• You will usually be given the C (specific heat) for your surroundings and will be asked to find q.

• Rearrange this equation to solve for q.

How do we measure heat changes? Pages 300-307

Calorimetry - measurement for heat energy

Calorimeter - instrument used to measure heat absorbed or released

Measures Enthalpy (H), which is the same as heat at constant pressure; H can be positive (endothermic) or negative (exothermic)

Endothermic = change in enthalpy is positive

H > 0

Exothermic = change in enthalpy is negative

H < 0

Formula for Enthalpy or Heat:

q = H = (m) (C) ( T)

heat = change in enthalpy = mass * specific heat * change in temp (oC)

Enthalpy and Equations...

CaO + H2O Ca(OH)2 + 65.2 kJ

CaO + H2O Ca(OH)2

OR...

H = -65.2 kJ

Exothermic Reactions:

Endothermic Reactions:

2NaHCO3 Na2CO3 + H2O + CO2

OR...

H = 129 kJ

2NaHCO3 + 129 kJ Na2CO3 + H2O + CO2

Homework

• p. 299 #4-9; p. 322 #37, 39, 40, 41ab, 42, 44, 45

• FYI: test is Friday 3/14