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CHEMISTRY: TOPIC 5 ENERGETICS


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CHEMISTRY: TOPIC 5 ENERGETICS


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What is heat?
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heat is a energy form that flows from something at a higher temp to something at a lower temp

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33 questions
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What is heat?
Heat is a energy form that flows from something at a higher temp to something at a lower temp
What is temperature?
A measure of the average kinetic energy of particles
What is internal energy?
Total amount of energy (kinetic and potential) in a substance
What will happen if a 100 g block of iron at 100 degrees is placed in contact with a 50g iron block at 50 degrees?
Heat will flow from the hotter block to the colder one till = same temperature When they are the same temp the 100g will have a higher internal energy bec there is more of it than the 50g block But the average kinetic energy will be the same bec same temp
What is "system" and "surroundings"?
System = chemical reaction surroundings = everything else
What is an exothermic reaction (EXit)
Heat energy is transferred from the system to surroundings = surroundings gets hotter because of a decrease in internal energy by creating bonds system gets hot Reactants have more energy enthalpy change is negative = heat is going out
What is an endothermic reaction? ENDO = INTO
Gets cold heat energy is taken into the system from the surroundings = surroundings gets cooler energy taken in is converted into internal energy by breaking bonds (heat = chemical energy) products have more energy takes a lot more activation energy enthalpy change is positive = heat is coming in
What is enthalpy change?
Amount of heat energy taken in / given out in a chemical reaction to the surroundings at a constant pressure (H) not possible to measure the enthalpy of a system (total energy) only change
What is the calculation for enthalpy change?
Product enthalpy - reactants enthalpy
What does stability in a reaction depend upon?
If the products has less energy than the reactants it is more stable
What is a standard enthalpy change?
Enthalpy changes has diff values depending on the conditions so we put them under the same set of condition = standard conditions it has a symbol of triangle H o with dash o with dash = under standard conditions
What is standard enthalpy change of reaction?
When molar amounts of reactants react under standard conditions to make products triangle H r circle dash can be plus or minus
What is standard enthalpy change of combustion?
Triangle H c circle dash enthalpy change when 1 mole of a substance is completely burnt in O2 under standard conditions ALWAYS NEGATIVE BEC combustion is an exothermic process
What is the definition of specific heat capacity? (SHC)
Energy needed to raise the temp of 1g of a substance by 1 degree shows how much energy is needed to heat up a substance substances with higher SHC is more difficult to heat up than lower SHC also applies when a substance cools for the amount of energy given out
How is enthalpy change of combustion measured?
With a calorimetry heat given out in a combustion reaction is used to heat another substance with a known SHC (water) use the equation q=mc(t change)
Steps to working out SHC
You would first do SHC with the water g, temp change and 4.18 bec the water is being heated change in ethanol for g and then work out moles with m/mr then you would do the answer from first step divided by the moles divide by 1000 for J=KJ Negative because combustion is always exothermic
Problems with innaccuracy with the calorimeter?
Heat lost to the surroundings = could have insulated the copper evapouration of water and alcohol incomplete combustion so some ethanol is burned and produces CO and C = gives out less heat bec not enough O2 for the flame
What is a more accurate measurement of enthalpy change of combustion?
Bomb calorimeter = heavily insulated piece in which the substance is ignited electronically with a lot of oxygen
What is enthalpy change of neutralisation? (3)
Enthalpy change when 1 mol of H20 molecules are formed when an acid (H+) reacts with an alkali (OH-) under standard conditions H+ + OH- = H2O exothermic
What is enthalpy change of solution (3)
Enthalpy change when 1 mol of solute is dissolved in excess solvent (H2O) to form a solution of "infinite dilution" under standard conditions Infinite dilution = further dilution would lead to no more enthalpy change exo / endo
What is standard enthalpy change of formation? (4)
Enthalpy change when 1mole of substance i formed from its elements in their standard states under standard condition can be exothermic or endothermic for any element = 0 products - reactants
What is bond enthalpy? (3)
Enthalpy change when 1 mole of covalent bonds, in a gaseous molecule is broken under standard condition breaking bonds = needs energy = exothermic = + bond making = releases energy = exothermic = - for reactions happening in gaseous state
Is bond breaking endo or exo?
Needs energy so it is endothermic = +
Is bond making endo or exo?
Releases energy so it is exothermic = -
Why are the bond energies in the table not accurate?
Bec they are average values , averaged over the values of that bond in many diff compounds will be slightly different in every compound can cause some innacuracies into calculations so bond energy calcs are more accurate when they involve few bonds
What is average bond enthalpy? (2)
Average amount of energy to break 1 mole of covalent bonds in gaseous molecules under standard conditions Average = bond enthalpy is diff in diff molecules so the value is an average amount of energy to break a bond in a range of molecules
Is bond enthalpy or enthalpy change of formation more accurate?
Enthalpy changes of formation is more reliable bec it is specific to the particular substance whereas bond enthalpies are average value
What does Hess's law state? (3)
That the enthalpy change with a chemical reaction is independent of the pathway bw the initial and final states The enthalpy change for the direct conversion of A to B is the same as the enthalpy change for the indirect route between A and B delta Hr = delta H1 + delta H2
What does standard state mean?
The pure substance at 100 kPa and a specified temperature or if not 298 K for any element the standard enthalpy change of formation would be 0 because no heat energy is taken in or given out
Why can bond energies only be used for reactions in the gas phase?
Because for solids and liquids, other interactions like intermolecular forces must be taken into account
How is UV absorbed by O2 and O3?
The molecules undergo dissociation which breaks the bonds between oxygen atoms UV-C breaks the double bond in O2 UV-B (lower energy) breaks the single and double bond in O3