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AS Chemistry

Unit 2

Introduction

Unit Modules

1. Basic concepts in organic chemistry2. Alcohols, halogenoalkanes and

analysis.3. Energy4. Resources

Plus assessed practicals.

AS ChemistryUnit 2, Module 3,

Energy

Enthalpy Introduction

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;

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.

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.

Definitions

NOW write definitions of:

• Exothermic reaction

• Endothermic reaction

• Enthalpy

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.

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.

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.

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

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

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

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?

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.

Self heating cans use exothermic reactions

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.

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?

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.

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

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

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?

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.

Enthalpy Profile Diagram• A simple enthalpy profile diagram:

Exothermic

Enthalpy Profile Diagram• A simple enthalpy profile diagram:

Endothermic

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.

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.

Activation Energy• An

endothermic reaction has

activation energy too.

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?

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;