pdp chemistry energetics
DESCRIPTION
Topic 5: Energetics energy exothermic and endothermic reactions calorimetry enthalpy change bond enthalpy temperature and heat absorption and emission of radiation burning fuelsTRANSCRIPT
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PDP ChemistryEnergetics
Thermochemistry: energy changes in chemical reactions
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Topic 5: Energetics• energy• exothermic and endothermic
reactions• calorimetry• enthalpy change• bond enthalpy• temperature and heat• absorption and emission of
radiation• burning fuels
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PDP moles tests comments time was short… show working for quantitative questions 13.b) graph scales- we need to do more work with these… Chemical equations- balancing, subscripts and coefficients,
charges — 1. work out reactants and products (a word equation may help
here) — 2. write the formula for each compound (use the charge on ions
to find the formula, but the charges are not written in the formula) — 3. balance. change only the coefficients, NOT the formulas
13.d) it is hard to pick out the relevant information and ignore what you don’t need
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Energy
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Energy: the ‘capacity to do work’ the unit of energy is the Joule (J) forms of energy:
kinetic gravitational potential elastic potential chemical potential electric potential nuclear potential thermal (internal energy) radiant
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Law of conservation of energyThe total energy of an isolated system remains constant- it is said to be conserved over time.Energy can be neither created nor be destroyed, but it can change form.
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Exothermic and endothermic reactions
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Why are reactions endothermic or exothermic?
In chemical reactions bonds break and new bonds form.Different bonds have different amounts of chemical energy. Exothermic reaction: less energy in the bonds after
the reaction energy released as heat Endothermic reaction: energy is needed as there is
more energy in the bonds after the reaction energy taken in
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Graphs sharp pencil and ruler graph paper- use the whole sheet choose a linear scale for each axis plot each point with an x or a + (or error bars) titles and units on axes title for graph trendline (or maybe join points with straight
lines)- no snakes
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Description and explanation of results
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Conclusion Description and explanation of results from one
exothermic and reaction and upload to MB
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Temperature
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Temperature and energy Which gas is at the higher temperature? Which gas contains more energy?
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Temperature- average KE per particle
higher average speed higher temperature
more particles at same average speed same temperature
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Measuring temperatureAbsolute scale of temperature: zero Kelvin = zero kinetic energyzero K = -273°C
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Convert to Celcius1. melting point of ice 273K
2. room temperature 300K
3. temperature of deep space 4K
4. boiling point of oxygen at standard pressure 90K 5. surface of the sun 6000K
6. temperature of an exploding nuclear bomb 106K
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Distribution of speeds of particles
number of particles
speed
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Distribution of speeds
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Calorimetry
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How much energy does it take to heat water?
4.18Joules of energy are needed to heat up 1 gram of water (approx 1mL) by 1°C.(4.18 is the specific heat capacity of water, and aqueous solutions are almost the same)
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Calculating how much energy is released
1. Calculate how many Joules are released in each of your experiments from last week
2. Calculate how many kJ are released by each mole of reactant in the experiments.
4.18Joules of energy are needed to heat up 1 gram of water (approx 1mL) by 1°C.
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Exothermic and endothermic reactions
Ammonium nitrate and water Iron and oxygen
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Hot pack (more than one is true)
A. The temperature of the hot pack decreases during the reactionB. The temperature of the hot pack increases during the reactionC. The hot pack transfers heat to the person touching itD. Heat is taken in from the person to the hot packE. The hot pack has less energy stored in chemical bonds after it is
usedF. The hot pack has more energy stored in chemical bonds after it
is used
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Energy changes
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Exothermic reactionsStored chemical energy, J
time
reactants
products
∆H
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Endothermic reactionsStored chemical energy, J
time
reactants
products
∆H
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Enthalpy change
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Enthalpy, H Energy stored in chemical bonds of reactants (in
Joules) PE and KE of particles + energy to make space for
substance
Image: https://en.wikipedia.org/wiki/File:Ammonium_Nitrate.jpg
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Burning fuels
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Fuels combine with oxygen in combustion
most chemical fuels are organic compounds, often hydrocarbons (contain C, H) or carbohydrates (contain C, H, O) carbon oxidized to CO2 hydrogen oxidized to H2O
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Calculate a theoretical ∆H in kJ/mol for the reactions from yesterday’s lab
C2H6O + 3O2 2CO2 + 3H2O
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Energy density
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Energy density used to compare fuels
Fuel Energy density, kJ/g
hydrogen 142natural gas (methane) 56
ethanol 26coal 24wood 16
energy released by when 1g of fuel is burned
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Standard enthalpy
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Standard enthalpy change of a reaction
∆H⊖ to compare energy changes in reactionsin kJ/mol(measured at STP 298K and 1atm)
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Calculating standard enthalpy0.2g of magnesium reacts with 10mL excess hydrochloric acid to increase the temperature by 50°C. Calculate the standard enthalpy of reaction.
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Bond enthalpy
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Bond enthalpy: the enthalpy change when one mole of bonds is formed in the gaseous state
X (g) + Y(g) X-Y(g)
Forming bonds is exothermic (negative ∆H)Breaking bonds is endothermic (positive ∆H)
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Exothermic reactioncombustion of methane
CH4 + 2 O2 CO2 + 2H2O + heat
Reactants ProductsDH = (Products) – (Reactants) negative value
Exothermic reactions: DH < OEndothermic reactions DH > O
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Enthalpy of combustionBond Average bond
enthalpy kJ/molC-C 347C=O 746C-H 413O=O 498O-H 464C-O 358
Image: https://en.wikipedia.org/wiki/Burning
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Calculating standard enthalpy change
What is the enthalpy change in kJ per mole if 45kJ are given out when 0.8g of methane is burned?What is the enthalpy change in kJ per mole if 1.6g of methanol is used to heat 200mL water from 20C to 38C?
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Using temperature to calculate ∆Hᶱ
Heat energy = mass x specific heat capacity x temperature changeQ = mc∆T
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Pollution from combustion
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Acid rain
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Greenhouse gases
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Carbon monoxide
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Particulate pollution
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Image: http://en.wikipedia.org/wiki/File:Periodic_table.svg