Review: What is the law of conservation of matter? What is stoichiometry? How many mL are in a L? How many grams are in a kg?

EnergyEnergy is the ability to do work or produce heat. Two forms of energy exist, potential and kinetic. Potential energy is due to composition or position. Kinetic energy is energy of motion. The law of conservation of energy says in any chemical reaction or physical process, energy can be converted from one form to another, it is neither created nor destroyedlaw of conservation of energy In Chemistry we call this the first law of thermodynamics. Energy

Chemical potential energyChemical potential energy is energy stored in a substance because of the substance composition. HeatHeat is energy that is flowing from a warmer object to a cooler object. q is used to symbolize heat. A calorie (cal) is the amount of energy required to raise the temperature of 1 gram of water 1 C.calorie Food is measured in Calories (kcal), or 1000 calories A joule is the SI unit of heat and energy, equivalent to calories. In Chemistry we use both joules (J) and kilojoules (kJ)joule Energy in Chemistry

Converting Energy Units

The specific heat of any substance is the amount of heat required to raise one gram of that substance to 1 C.specific heat Some objects require more heat than others to raise their temperature. (These have larger specific heats) Notice: These are dependant on state of matter Specific Heat

We use the specific heat when calculating heat absorbed and released In order to do this we use the following equation: q = m c ΔT Where: q = heat absorbed or released c = specific heat of substance (Heat Capacity) m = mass of substance in grams ΔT = change in temperature in Celsius Notice: In this instance, Δ stands for change (Final – Initial)

Practice Problems Example 1: How many Joules would it take to raise the temperature of 250 g of ice from -20 °C to -5 °C? (specific heat of ice is J/g*°C) Example 2: What is the specific heat of ethyl alcohol if 100 grams of ethyl alcohol was heated from 30 °C to 50 °C when 1160 calories of heat was applied?

Molar heat of fusion (∆H fus ) : is the amount of heat required to melt one mole of a solid substance. The energy is used only to change the state of matter. Example: H 2 O (s)  H 2 O (l) -Heat of fusion is usually greater for ionic solids than molecular solids. For ice to water … ∆H fus = 6.01 kJ/mol Molar heat of vaporization (∆H vap ): the heat (energy) required to vaporize one mole of a liquid substance. Example: H 2 O (l)  H 2 O (g) For water to steam … ∆H vap = kJ/mol Heat of Fusion & Heat of Vaporization

Heating Curve of Water

How do we use a Heating Curve? What do we do if we are given a substance that changes temperature through its boiling point or melting point? We will use the ∆H fus or ∆H vap in the heat equation. When we are at the melting point we will use : q= m ∆H fus And at the boiling point we will use: q= m ∆H vap NOTE: ON the previous slide, we talked about “MOLAR” ∆H fus or ∆H vap. Make sure that your units for m match your units for the constant!

Practice Problems 1)How much energy is required to raise 75 grams of water from -20 °C to 20 °C? (∆H fus = 6.01 kJ/mol and ∆H vap = kJ/mol The specific heat of ice is Joule/gram °C, and the specific heat of water is joule/gram °C ) q = mc∆T q= m ∆H vap q= m ∆H fus Big picture: Solid water heats up, melts, and then liquid water heats up.

Calorimetry A calorimeter is an insulated device used for measuring the amount of heat absorbed or released in a chemical reaction or physical process.calorimeter It measures changes in water temperature after a reaction is performed.

Chemical Energy ThermochemistryThermochemistry is the study of heat changes that accompany chemical reactions and phase changes. The system is the specific part of the universe that contains the reaction or process you wish to study.system The surroundings are everything else other than the system in the universe.surroundings The universe is defined as the system plus the surroundings.universe

Chemical Energy EnthalpyEnthalpy is the heat content of a system at constant pressure. In an exothermic reaction, energy is released. Enthalpy changes for exothermic reactions are always negative. In an endothermic reaction, energy is absorbed (this makes the reaction feel cold) Enthalpy changes for endothermic reactions are always positive.

Calculating ΔH Enthalpy (heat) of reactionEnthalpy (heat) of reaction is the change in enthalpy during a reaction symbolized as ΔH rxn. ΔH rxn = H final – H initial ΔH rxn = Σ H ̊ f products – Σ H ̊ f reactants Enthalpy can be calculated with the heats of formation (ΔH ̊ f ) or with the bond enthalpies. ∆H = Σ Energy bonds broken – Σ Energy bonds formed

The different types of charts ∆H ̊ f (kJ/mol)BOND ENTHALPIES (kJ/mol) CH 4 (g)-75H-H436 CO 2 (g)-394N-H391 C 2 H 2 (g)227N-N161 H 2 O (g)-242N=N418 H 2 O (l)-286N=NN=N945 NH 3 (g)-46O-H463 HCl (g)-92C-O351 HCl (aq)-167C=O728 CaCO 3 (s)-1208C=OC=O1072 CaO (s)-635C-H413

Practice Problems 1. Using standard heats of formation, calculate the standard enthalpy change for the following reaction. CO 2 (g) + 4H 2 (g) -----> CH 4 (g) + 2H 2 O(g)

2. Using average bond enthalpies estimate the standard enthalpy change for the same reaction. CO 2 (g) + 4H 2 (g) -----> CH 4 (g) + 2H 2 O(g)

Energy in Reactions Energy can also be expressed in rxns. We call this a thermochemical reaction. If the energy is on the left (reactant) side, energy is being put into the system. This means it is endothermic. If the energy is on the right (product) side, energy is released from the system. The rxn is exothermic. Is the following exo or endothermic? 2S + 3O 2  2SO kJ

Stoichiometry and Heat Energy in a chemical equation can be used as part of a mole fraction. For example: 2S + 3O 2  2SO kJ How much energy is produced if 45 grams of sulfur is reacted?