Title: Revision of Enthalpy Learning Objectives: By the end of the lesson I will be able…….. To understand and explain enthalpy To describe the difference between endo and exothermic reactions Be able to draw and understand Activation Energy and enthalpy profile diagrams And be able to do enthalpy calculations

http://youtu.be/XgiCn1IpvzM Endo Exo thermic youtube song Endo exothermic Song   http://youtu.be/XgiCn1IpvzM Endo Exo thermic youtube song

Pub quiz answers An exothermic reaction is when a chemical reaction releases or gives out heat. TRUE Breaking a bond needs energy. TRUE In enthalpy calculations which Greek letter symbolises standard states and measurements under standard conditions. Θ, Δ, or Ω The standard conditions in a reaction are 100kPa (1 atm) pressure and a temp of 273K (0οC) False Enthalpy change ΔH is measured in KJ mol-1 TRUE All combustion reactions are endothermic. FALSE Is this a balanced equation for combustion of methane? CH4(g) + 2O2(g) - CO2(g) + 2H20(l) TRUE

Write down an example of bond enthalpy using H­2 Answer H2(g) 2H (g) If heat is lost by reacting chemicals is less than the amount and gained by the surroundings then ΔH is negative. TRUE The quantity of energy needed to break a particular bond in a molecule is called bond enthalpy. TRUE Write down an example of bond enthalpy using H­2 Answer H2(g) 2H (g) If you need more energy to break the bonds than is released when bonds are formed then ΔH is negative ? FALSE This is an enthalpy profile diagram for an endothermic reaction. FALSE The activation energy is the largest amount of energy needed to break bonds and start a chemical reaction. FALSE it’s the smallest Standard enthalpy change of formation ΔHf Θ is the enthalpy change when 1 mole of an element is formed from its compounds in their standard states under standard conditions. False

Demo Endothermic and Exothermic Endo: requires heat energy Exo: releases heat energy

Activity 1: Ice melting: What is causing this ice cube to melt Activity 1: Ice melting: What is causing this ice cube to melt? Is this an endothermic or exothermic reaction? Is heat energy being absorbed from the surroundings (endo) causing the ice to melt? Or is heat energy given out from the ice cube (exo) to the surroundings causing the ice to melt?

So to break a bond…….

Mini whiteboard activity To break a bond…………. We need energy to break a bond! So is this an endo or exothermic reaction? …………………………………………………………….

When a bond is made…………. Energy is released or given out! 1 Nitrogen molecule Energy is released or given out! Use your data book to calculate the bond energies of the above; (H-H), (N-N), (N-H) So is this an endo or exothermic reaction? …………………………………………………………….

What is enthalpy (H)? Heat content stored in a chemical system

What do these symbols mean? Δ means …………… H means …………… Ѳ means ………….. Enthalpy is measured in …………..

AFL: To break a bond…………. So is (∆H) POSITIVE OR NEGATIVE? …………………………………………………………….

Bond energies =The energy needed to break a chemical bond Different chemical bonds have different bond energies Do activity 1 15

Activity 1 Work out the enthalpy change (∆H) for this reaction 2H2 + O2 = 2H2O Hint: Enthalpy change of reaction = products - reactants Is it endothermic or exothermic reaction? See if you can sketch an energy profile diagram

Examples of Exothermic reactions Oxidation and neutralisation reactions are all exothermic reactions. -they give out heat energy so ∆H is positive or negative?

AFL: When a bond is formed in a reaction…………. 1 Nitrogen molecule Energy is released or given out! So is (∆H) POSITIVE OR NEGATIVE? …………………………………. activity 2

Activity 2 REACTANTS -> PRODUCTS HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) Energy of the PRODUCTS = +153 kJ Energy of the REACTANTS = + 500 kJ What is the enthalpy change ΔH of this reaction? Give the units….. Is this reaction endothermic or exothermic? Draw an energy profile diagram Hint: Enthalpy change of reaction = products - reactants

Look at the figures on the Y-axis to explain why this is Energy Profile Diagram Look at the figures on the Y-axis to explain why this is an endothermic reaction. Write down your explanation using words: bond breaking, bond forming, energy

Endo or Exo

Endo or Exo activity 3

Activity 3: Fill in the following words on the diagram Enthalpy reactants, products, activation energy arrow, ΔH arrow +ΔH/ – ΔH, correct units for enthalpy

What’s the difference between these two diagrams?

Activity 4 Sketch an enthalpy profile diagram for this reaction CO (g) + NO2 -> CO2(g) + NO(g) ΔH = -226kJ mol-1 Ea = 134kJ mol-1 Show activation energy arrow and ΔH arrow State whether it’s an endo or exothermic reaction.

2H2 + O2 = 2H2O Activity 1 answers Work out the enthalpy change (∆H) for this reaction 2H2 + O2 = 2H2O 2X (436) + 498  2 X (464) ∆H = +442 kj mol -1 Is it an endothermic reaction

REACTANTS -> PRODUCTS Activity 2 answers REACTANTS -> PRODUCTS HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) Energy of the PRODUCTS = +153 kJ Energy of the REACTANTS = + 500 kJ What is the enthalpy change ΔH of this reaction? -347 kJ mol -1 It is an exothermic reaction energy profile diagram

Activity 3 answer: Fill in the following words on the diagram Enthalpy kJ mol -1 reactants – ΔH products reactants, products, activation energy arrow, ΔH arrow +ΔH/ – ΔH, correct units for enthalpy

Activity 4 answers Sketch an enthalpy profile diagram for this reaction CO (g) + NO2 -> CO2(g) + NO(g) ΔH = -226kJ mol-1 Ea = 134kJ mol-1 Show activation energy arrow and ΔH arrow State whether it’s an endo or exothermic reaction.

AFL: thumbs up/thumbs down How confident do I feel about……………. I can explain enthalpy to the person beside me I understand the difference between endo and exothermic reactions I can draw Activation Energy arrows and ∆H arrows on energy profile diagrams

Enthalpy Change of Combustion ΔHcѳ Fill in the blanks CH4(g) + …….. (g) -> …… (g) + …… (l) Do you think combustion reactions are endothermic or exothermic? …………… ΔHcѳ - the enthalpy change of combustion when one mole of substance is completely combusted. CH4(g) + 2O2 (g) -> …CO2… (g) + 2H2O (l) ΔHcѳ is exothermic -890kJ mol-1 31

Standard Enthalpy Change of Reaction ΔHѳr Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s) ΔHѳr is the enthalpy change when one mole of solid zinc reacts with 1 mole of copper sulphate (of concentration 1 mol dm-3) at 298K and 100kPa (1atm)

Standard Enthalpy Change of Combustion ΔHѳc Is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.