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How would you sort these?
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AS Chemistry
Unit 2
Introduction
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Unit Modules
1. Basic concepts in organic chemistry2. Alcohols, halogenoalkanes and
analysis.3. Energy4. Resources
Plus assessed practicals.
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AS ChemistryUnit 2, Module 3,
Energy
Enthalpy Introduction
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Objectives - Enthalpy2.3.1 Enthalpy Changes
Candidates should be able to:• (a) explain that some chemical reactions are accompanied
by enthalpy changes, exothermic (ΔH, negative) or endothermic ΔH, positive);
• (d) construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products;
• (e) explain qualitatively, using enthalpy profile diagrams, the term activation energy;
• (f) define and use the terms:– (i) standard conditions,– (ii) enthalpy change of reaction,– (iii) enthalpy change of formation,– (iv) enthalpy change of combustion;
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PracticalWe will do two simple test-tube experiments, in each case we add a solid to a liquid:
1. Hydrochloric acid plus magnesium ribbon.
2. Water plus ammonium nitrate
For each case, write a prediction of what will happen.
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Practical
Practical 1:1. Put 5 cm depth of hydrochloric acid in a test tube.2. Add a piece of magnesium ribbon.3. Write down ALL your observations.4. Write an equation for what happened.
Practical 2:1. Put 5 cm depth of water in a test tube.2. Add a spatula of ammonium nitrate.3. Write down ALL your observations.4. Write an equation for what happened.
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Definitions
NOW write definitions of:
• Exothermic reaction
• Endothermic reaction
• Enthalpy
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EnthalpyDefinitions:
• Enthalpy, H, is the heat content that is stored in a system.
• Chemical system is the reactants and the products
• Surroundings is whatever is outside the chemical system.
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Enthalpy• It is impossible for us to actually measure
how much heat energy is in a system. • What can we measure?• We can measure the energy that is given
out or absorbed by a chemical system during a chemical change.
• We are actually measuring the heat exchange between the chemical system and its surroundings.
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The law of conservation of energy
The law of conservation of energy states…
…that energy cannot be created or destroyed, only moved from one place to another.
So if a chemical system releases energy, the surroundings must gain exactly the same amount of energy.
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Conservation of Energy
Heat loss in a chemical system
= heat gain in the surroundings
We see this as a _________ in temperature
Heat gain in a chemical system
= heat loss from the surroundings
We see this as a _________ in temperature
n increase
decrease
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Enthalpy Changes
• In a chemical reaction, either the products or the reactants will have more energy.
• An enthalpy change, ΔH is:– The heat exchange with the surroundings
during a reaction, at constant pressure.– The difference between enthalpy of the
products and enthalpy of the reactants:
ΔH = Hproducts - Hreactants
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Activity
• Cut out the boxes and stick in place on the diagram to explain exothermic and endothermic reactions.
• Don’t stick them down yet.
• Place them where you think they go
• We will cover more of the topic and continue to look back at this
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Exothermic & Endothermic
We now need better definitions of exothermic and endothermic reactions.
For each type of reaction:
1. Is heat lost to or gained from the surroundings?
2. Which has the smaller enthalpy, reactants or products?
3. Is ΔH negative or positive?
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Exothermic1. Heat is lost to the surroundings?
2. The enthalpy of the products is smaller than the enthalpy of the reactants.
3. H products < H reactants
4. ΔH is negative.
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Self heating cans use exothermic reactions
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Endothermic1. Heat is gained from the surroundings?
2. The enthalpy of the products is larger than the enthalpy of the reactants.
3. H products > H reactants
4. ΔH is positive.
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Exothermic ReactionsThe most exploited exothermic reaction in the world is…
…combustion of fuels
• Write an equation for burning methane (natural gas) in oxygen.
• Which have more enthalpy, products or reactants?
• Is ΔH positive or negative?
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Exothermic ReactionsCombustion of Fuels
Burning methane in oxygen:
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890kJ.mol-1
The units are kJ.mol-1 so it is the
amount of energy per mole.
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Exothermic ReactionsRespiration
Probably the most important reaction for living things, it gives out energy so is exothermic:
Glucose + Oxygen Carbon dioxide + WaterC6H12 O6 (aq) + 6O2(g) 6CO2(g) + 6H2O(l) ΔH = -2801kJ.mol-1
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Endothermic ReactionsThermal decomposition of limestone
Endothermic reactions require energy input for them to take place.
CaCO3 (s) CaO(s) + CO2(g) ΔH = +178 kJ.mol-1
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Endothermic ReactionsPhotosynthesis
Probably the most important endothermic reaction for living things, it requires energy:
6CO2(g) + 6H2O(l) C6H12 O6 (aq) + 6O2(g) ΔH = +2801kJ.mol-1
What type of energy does it take in?
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Enthalpy Profile Diagram• A simple enthalpy profile diagram shows:
– the symbols and enthalpy of products– the symbols and enthalpy of reactants– the enthalpy change
• You need to be able to draw them and include all the features.
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Enthalpy Profile Diagram• A simple enthalpy profile diagram:
Exothermic
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Enthalpy Profile Diagram• A simple enthalpy profile diagram:
Endothermic
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Activation Energy• Why doesn’t magnesium spontaneously
burn in air?• It’s products have less energy than it’s
reactants so there is no reason they can’t form?
• Yes there is – Activation energy!• We need an input of energy in the first
place to break the bonds and allow the reaction to start.
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Activation Energy• Once the
reaction has been started it is giving out energy so the activation energy can continue to be overcome and the reaction is self-sustaining.
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Activation Energy• An
endothermic reaction has
activation energy too.
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Now• Check you have all of the enthalpy mind map complete• Then complete questions 1 and 2 on page 189:• Draw an enthalpy profile for the following reactions
1) You are given the data for the following reaction
CO(g) + NO2 (g) CO2 (g) + NO (g)
ΔH = -226 kJ mol-1 Ea = + 134 kJ mol-1
2) You are given the data for the following reaction
H2 (g) + I2 (g) 2HI (g)
ΔH = +53 kJ mol-1 Ea = + 183 kJ mol-1
More questions
a) What does enthalpy mean?
b) What decides if a reaction is endothermic or exothermic?
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Objectives - Enthalpy2.3.1 Enthalpy Changes
Candidates should be able to:• (a) explain that some chemical reactions are accompanied
by enthalpy changes, exothermic (ΔH, negative) or endothermic ΔH, positive);
• (d) construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products;
• (e) explain qualitatively, using enthalpy profile diagrams, the term activation energy;
• (f) define and use the terms:– (i) standard conditions,– (ii) enthalpy change of reaction,– (iii) enthalpy change of formation,– (iv) enthalpy change of combustion;