Energetics

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  • Created by: Emily
  • Created on: 25-05-14 10:54

Endothermic vs Exothermic

Energy must be put in to break bonds, and given out when bonds are formed.

Exothermic - reaction in which heat is given out to its surroundings as the reactants change to products

Endothermic - reaction in which heat is taken in from its surroundings as the reactants change to products

Enthalpy - A measure of heat energy given out or taken in when a chemical/physical change occurs at constant pressure

Enthalpy change = change in temperature of a reaction

ALWAYS NEED TO STATE CONDITIONS OF A REACTION AS REACTION WILL VARY WITH TEMPERATURE, PRESSURE AND CONCENTRATION

Pressure = 100kPa

Temperature = 298K

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Exothermic

(http://www.kentchemistry.com/images/links/Kinetics/exothermic_plain.gif)

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Endothermic

(http://www.kentchemistry.com/images/links/Kinetics/endothermic_plain.gif)

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Enthalpy Changes - Definitions

Standard enthalpy of formation 

The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products in their standard states - 298K and 100kPa

Standard enthalpy of combustion 

The enthalpy change when one mole of substance is completely burned in oxygen under standard conditions, all reactants and products in their standard states -  298K and 100kPa

Specific Heat Capacity

The amount of heat needed to raise the temperature of 1g of substance by 1K

Rate of Reaction

Change in concentration per unit of time

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Measuring Enthalpy Changes

Q=mc∆T

Q = enthalpy change = J, Joules

m = mass of substance heated = g, grams

c = specific heat capacity = Jg/K-1

∆T = change in temperature = K, Kelvin

To work out enthalpy change of combustion, first work out Q, change J to kJ and then divide by no of moles 

Example

q=mc∆T = 100x4.18x6 = 2508J

2.508kJ / 0.017 = 147.53kJmol-1

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Hess's Law & Enthalpy Change of Formation

The enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products

(http://www.drbateman.net/asa2sums/sum2.1/sum2.12.gif)
Left hand side would be negative as going in opposite direction to the arrow

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Enthalpy Change of Combustion

The enthalpies of all elements in their standard states are taken as zero

(http://www.4college.co.uk/as/df/hess/image001.GIF)
Right hand side would be negative because it goes in the opposite direction to the arrow.

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Bond Enthalpies

Bond Dissociation Enthalpy

The enthalpy change required to break a covalent bond with all species in the gaseous state

Mean Bond Enthalpy

The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds.

∆Hf = Bonds Broken - Bonds Made

There will be differences between ∆Hf values and mean bond enthalpies as ∆Hf values use specific compounds values rather than averages

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