5 3 Enthalpy Change And Exothermic And Endothermic Reactions 345u3f

Enthalpy Change and Exothermic and Endothermic Reactions OBJECTIVES •Demonstrate an understanding of the term enthalpy change, ΔH •Construct simple enthalpy level diagrams showing the enthalpy change •Recall the sign of ΔH for exothermic and endothermic reactions Edexcel AS Chemistry p32-35

STARTER All Hot and Bothered Miss Diffley has the energy values for different reactions. Take it in turns to guess the value, and she will respond “Higher” or “Lower”

Enthalpy Change in Exothermic and Endothermic Reactions Chemical reactions can either release energy to their surroundings, EXOTHERMIC or energy can be __________, transferred to them from the Video Clip ENDOTHERMIC surroundings, ____________. KEY IDEA Bonds contain energy. You need to put in energy to break bonds apart in the REACTANTS in a chemical reaction. Energy is released when new bonds form in the PRODUCTS in a chemical reaction. When bonds break, energy is absorbed (endothermic). When bonds form, energy is released (exothermic).

Energy Level Profile Diagrams 1

Ooh look – -ve

EXOTHERMIC The energy released on bond formation in the products is greater than that absorbed through breaking bonds in the reactants

Energy of reactants > energy of products These are the most common type of reaction eg. Combustion, neutralisation Example

Examples of Exothermic Reactions • • • • • •

Combustion Neutralisation Freezing Condensation Displacement Crystallising liquid salts

Energy Level Profile Diagrams 2 ENDOTHERMIC

Ooh look +ve

The energy needed to be absorbed to break bonds in the reactants is greater than the energy transferred to the surroundings as bonds are made in the products

Energy of reactants < energy of products These are less common chemical reactions eg. Photosynthesis Example

Examples of Endothermic Reactions Melting Boiling and Evaporation Photosynthesis Making an anhydrous salt from a hydrate • Forming a cation (+ve ion) from an atom in the gas phase • • • •

Which of the following is true when chemical bonds are broken? A.The process is exothermic B. Energy is given out C. A physical change is occurring D.The reaction will have a H that is positive(+).

Edexcel AS Chemistry p33 Answer Questions 1, 2 and 3 5 Minute Exercise Answers 1) Energy is released when bonds form. This increases the movement of particles of reacting substances and so causes particles in the surroundings to vibrate more. The temperature is seen to increase as energy is transferred as heat to the surroundings.

2) Endothermic reactions absorb energy from the surroundings. The energy needed to break the bonds in the reactants is greater than that released when new bonds form in the products, so the difference is absorbed from the surroundings. This is seen as a drop in temperature. Exothermic reactions release energy to the surroundings. Energy released by bond formation is greater than the energy needed to break the bonds in the reactants so energy is released to the surroundings. This is seen as a rise in temperature.

3a) Reaction A is endothermic, reaction B is exothermic. The final temperature is lower than the starting temperature for A, so A takes in energy overall. The final temperature is higher than the starting temperature for B, so B releases energy overall. b) Exothermic (A) = -20oC Endothermic (B) = +30oC

KEY IDEAS Enthalpy is the total energy content of the reacting materials. It is given the symbol, H. Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol ΔH. ΔH = ΔH products - ΔH reactants The units are kilojoules per mole (kJmol-1)

EXOTHERMIC An exothermic enthalpy change is always given a negative value, as energy is lost to the surroundings. ΔH = -kJmol-1 ΔH = ΔH products - ΔH reactants Small number

Bigger number

ENDOTHERMIC An endothermic enthalpy change is always given a positive value, as the energy is gained by the system from the surroundings. ΔH = + kJmol-1.

ΔH = ΔH products - ΔH reactants Big number Silly Video

Small number

How Science Works

Edexcel AS Chemistry p35 Heat Packs Video Answer Questions 1-4 1) Enthalpy is the total energy content of a system at constant pressure. Enthalpy change is the energy taken in or given out by a system during a chemical reaction at constant pressure. 2) An enthalpy level diagram represents the energy losses and gains as bonds are broken and formed during a reaction. It enables you to calculate whether a reaction is exothermic or endothermic.

3) Cool packs work by using endothermic reactions. When placed on injured tissues, the endothermic reaction in the cool pack takes heat away from the injured tissue. This cools the tissue and often helps reduce swelling

• The small studies have lasted a short period of time which reduces reliability • Data collected by medically/scientifically trained staff and based on clearly observed medical outcomes which increases reliability • Published in a reputable journal so results are well regarded by other professionals • Ethical issues for future experiments – if a process gives an advantage to heart attack patients, is it ethical to continue to deprive other patients of the cooling process?

Balance each of the equations below Name the compounds or elements present in each one Identify each one as exothermic or endothermic. CH4(s) + O2(g) → CO2(g) + H2O(l) ΔH = -890 kJmol-1 HCl(g) → H2(g) + Cl2(g)

ΔH = 185 kJmol-1

NH3(g) + O2(g) → NO(g) + H2O(l) ΔH = -1169 kJmol-1

ANSWERS CH4(s) + 2O2(g) → CO2(g) + 2H2O(l) Methane

Oxygen

Carbon dioxide

Water

2HCl(g) → H2(g) + Cl2(g) Hydrogen chloride

Hydrogen

EXOTHERMIC ENDOTHERMIC

Chlorine

4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l) EXOTHERMIC Ammonia

Oxygen

Nitrogen monoxide

Water

For each of the reactions above, construct a simple enthalpy level diagram showing the enthalpy change.

CH4(s) + 2O2(g)

ΔH =-890 kJmol-1

CO2(g) + 2H2O(l)

H2(g) + Cl2(g)

ΔH = +185 kJmol-1

HCl(g)

NH3(g) + O2(g)

ΔH =-1169 kJmol-1

NO(g) + H2O(l)