THERMOCHEMISTRY ENERGETICS/ENTHALPY INTRODUCTION All reactions require energy to break bonds in the reactants, and all reactions give off energy when new bonds form to make products The difference in energy required to break the bonds and to make the new bonds can tell you whether the reaction
is endothermic or exothermic. PRIOR KNOWLEDGE FROM GCSE Before you start remind yourself of what you already should know: Atoms bond together to fill their outer shells of electrons Atoms may gain or lose electrons to get full outer shells of electrons Chemical reactions may give out heat exothermic
reactions. Chemical reactions may take in heat energy from their surroundings endothermic reactions 4.1 Endothermic and exothermic reactions Thermochemistry is the study of heat changes during chemical reactions. When a chemical reaction takes place bonds must be broken and new bonds must be formed.
Breaking bonds uses energy from the surroundings; forming bonds gives off heat energy to the surroundings. During a reaction there is always an overall change in energy. The energy may be in different forms light, electrical or, most usually, heat. Define the terms endothermic and exothermic.
Endothermic reactions are reactions that take in heat from the surroundings by the end of the reaction. Exothermic reactions are reactions that have given out heat by the end of the reaction. It is always the case that a reaction that is endothermic in one direction is exothermic in the reverse direction. ENERGY PROFILE DIAGRAM OF EXOTHERMIC REACTIONS
The diagram below shows an energy profile diagram for an exothermic reaction In an exothermic reaction the products have less energy than the reactants, so energy is given out Also in an exothermic reaction more bonds are made than are broken, so overall energy is given
out Activation energy (Ea) is the minimal amount of energy needed to start the reaction (i.e. minimum amount of energy to break bonds in the reactants). ENERGY PROFILE DIAGRAM OF ENDOTHERMIC REACTIONS The diagram below shows an energy profile diagram for an endothermic
reaction In an endothermic reaction the products have more energy than the reactants, so energy is taken in Also in an endothermic reaction more bonds are broken than are made, so overall energy is taken in
EXAMPLES OF EXO AND ENDOTHERMIC REACTIONS DEMO 1 THERMAL DECOMPOSITION OF LIMESTONE DEMO 2 HEATING HYDRATED COPPER II SULPHATE CRYSTALS QUANTITIES Energy is always measured in kJmol-1 in Chemistry In general we always talk about 1 mole of a fuel burning So when writing balanced equations such as:
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) DH = -890kJmol-1 This means that when ONE mole of methane reacts with 2 MOLES of oxygen then 890kJ of energy is given out OR C2H6(g) + 3O2(g) 2CO2(g) + 3H2O(l) DH = -86kJmol-1 (NOT 2C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(l)) So in this case the value of -86kJmol-1 is the energy given off by 1 MOLE of ethane NOT for two moles of ethane
REMEMBER FROM GCSE THAT If DH is negative in value then the reaction is exothermic If DH is positive in value then the reaction is endothermic So, C(s) + O2(g) CO2(g) DH = -394kJmol-1 (This means that 394kJ of energy is given out for 1 mole of carbon burnt NH4NO3(s) + (aq) NH4NO3(aq) DH = +26kJmol-1 (This means that 26kJ of energy is absorbed by 1 mole of NH 4NO3
solid) So why do we bother to work out the energy given out for different fuels: ANSWER: It allow us to compare the efficiency of different fuels. WORKING OUT ENERGY OF COMBUSTION OF FUELS IN kJg-1 Chemists generally refer to the energy given out when a fuel burns in kJmol-1
because this compares the same number of molecules of each fuel. For use as fuels it is sometimes better to convert the units from kJmol-1 to kJg-1 (OR the energy density) of a fuel NOTICE: That petrol stores significantly more energy than either ethanol or methanol. This is important when considering vehicles fuelled by either of these alcohols. Also at first sight we might think the hydrogen energy density
is amazing, but thats not the only thing to consider. All the other three fuels are liquids whereas hydrogen is a gas. Although hydrogen stores more energy per gram, a gram of hydrogen takes a lot of space because of the low density of gases. How to store the hydrogen efficiently is a challenge for designers. QUESTIONS
Basic Ideas behind RDF RDF uses Web identifiers (URIs) to identify resources RDF describes resources with properties and property values Everything can be represented as triples The essence of RDF is the (s,p,o) triple Resource (subject) Value (object) Property (predicate)...
uoft Other titles: Arial MS Pゴシック Calibri Wingdings Times New Roman Office Theme Reversing the Trends Towards Greater Active School Travel Presentation Overview Setting the Scene Why AST? Why AST? Why AST? Declining Trends in AST Why has AST declined?...
In this poem we, once again, see Bishop's photographic eye for detail. The theme of this poem is the unique beauty of everyday life. As the curious poet probes beneath the surface of the greasy, grimy, filling station, she discovers...
Refractory Stage of Shock. Cell destruction severe. Condition refractory to treatment. Multiple organ failure. Refractory hypotension. Refractory hypoxemia. Renal shutdown. GI failure (no absorption of nutrients) Neurological failure (no response)
Understanding verb tense What are the verb tenses? Present and present perfect ... By haytham kilani What are the verb tenses? Verbs do a lot of work in sentences. They show actions and states of being. They even take different...
for modifiability and reuse, factor out common services and modify them in a single location or small number of locations. for scalability and availability, centralize the control of these resources and services while distributing the resources themselves across multiple physical...
Linnaeus' taxonomy. 20C. Quantum mechanics. Newtonian mechanics "Today, there is an ever-widening gap between folk knowledge and scientific and technological advances and an accompanying need to acquire novel academic competencies"
Logical . Access - Controls provide reasonable assurance that logical access to system resources is reasonable and restricted to properly authorized individuals. 8. Network . Security - Controls provide reasonable assurance that the network is protected from unauthorized access and...
Ready to download the document? Go ahead and hit continue!