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 reaction Under 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

Types of Heat Transfers: pgs 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 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 = cal 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 the amount of heat energy needed to increase the temp of ANY object by 1 degree Celsius Specific 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 ( o C) = 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 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 ( o C)

Enthalpy and Equations... CaO + H 2 O Ca(OH) kJ CaO + H 2 O Ca(OH) 2 OR...  H = kJ Exothermic Reactions: Endothermic Reactions: 2NaHCO 3 Na 2 CO 3 + H 2 O + CO 2 OR...  H = 129 kJ 2NaHCO kJ Na 2 CO 3 + H 2 O + CO 2

Homework p. 299 #4-9; p. 322 #37, 39, 40, 41ab, 42, 44, 45 FYI: test is Friday 3/14