Look carefully at the table – what are the problems?

1. Look carefully at the table – what are the problems?

2. Objectives • To be know what calorimetry is. • To safely complete a calorimetry experiment. • To be able to evaluate a calorimetry experiment.

3. Calorimetry • What is your independent variable? • What is your dependent variable? • What are your control variables? • How could you display your results in a scientific way?

4. Using your results Energy released (J) = mass of water heated (kg) x specific heating capacity of water (J/KgxK) x temperature rise (K) E = mC∆T Where C = 4200J/KgxK But how can you compare each fuel?

5. Plenary • Think about your experiment. What is the biggest source of uncertainty (error)? Is it systematic or random? Are your results reliable? Are your results accurate? How do you know?

6. Why do reactions need energy to start them?

7. Objectives • To be know the definitions and some examples of exothermic and endothermic reactions. • To be able draw energy level diagrams and bond energies to explain a chemical reaction.

8. Exothermic and endothermic reactions What are exothermic and endothermic reactions? exothermic reactions release energy – they get hot • ex = out (as in ‘exit’) • thermic = relating to heat endothermic reactions absorb energy – they get cold • en = in (as in ‘entrance’) Most chemical reactions are exothermic.

9. Exothermic reactions Exothermic reactionsrelease thermal energy (heat) into their surroundings. Exothermic reactions can occur spontaneously and some are explosive. • What are some examples? • combustion • respiration • neutralization of acids with alkalis • reactions of metals with acids • the Thermit Process.

10. Exothermic reaction

11. Exothermic reaction What happens to energy in the reaction between magnesium and hydrochloric acid? • No external heat source is used so the heat released during the reaction must come from the reactants. • During the reaction, chemical energy in the reactants is converted to thermal energy (heat). This causes the temperature of the reaction mixture to rise. • This thermal energy is eventually lost to the surroundings and the temperature of the reaction mixture returns to normal.

12. Endothermic reaction

13. Exothermic or endothermic?

14. Making and breaking chemical bonds energy absorbed energy released Most chemicals will break up (decompose) if they are heated strongly enough. This means that energy is needed to break chemical bonds – an endothermic process. Because bond-breaking is endothermic, bond-making must therefore be exothermic. This means that energy is released when chemical bonds are made.

15. Ea: exothermic reactions

16. Ea: endothermic reactions

17. Calculating bond energies + hydrogen chlorine  hydrogen chloride + H2 Cl2  2HCl What are the energy changes in the reaction between hydrogen and chlorine? energy for bond-breaking energy from bond-making = H–H + Cl–Cl = H–Cl + H–Cl = 432kJ + 240kJ = 428kJ + 428kJ = 672kJ = 856kJ total energy change = energy out – energy in = 856kJ – 672kJ = 184kJ

18. Multiple-choice quiz