energy changes

• Exothermic reaction


  Reaction that transfers energy from the reacting molecules to the surroundings, causing the temperature of the surroundings to increase
• Endothermic reaction


  Reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease
• Energy profile diagram


  1. Shows the energy changes that take place in a reaction

  2. Reactants have more energy than products in exothermic reactions

  3. Products have more energy than reactants in endothermic reactions

  4. Difference in energy between reactants and products is the energy released/absorbed
• Uses of exothermic reactions


  • Hand warmers
  • Self-heating cans
• Activation energy


  Minimum amount of energy particles must have in order to react
• Representing activation energy on energy profile diagram

  Activation energy is the energy from reactants to the peak of the curve
• Exothermic reaction


  Energy is transferred from the chemical to the surroundings
• Endothermic reaction


  Chemicals gain energy from the surroundings
• Calculating energy changes


  1. Look at formed energy when breaking chemical bonds

  2. Determine energy required to break bonds

  3. Determine energy released when making bonds

  4. Calculate final energy change
• Bond energy


  Energy required to break that bond
• Calculating energy change for reaction

  1. Energy required to break H-H and Cl-Cl bonds = 678 kJ

  2. Energy released when making 2 H-Cl bonds = 862 kJ

  3. Final energy change = -184 kJ
• The reaction is exothermic as the final energy change is negative
• Calculating energy change for a chemical reaction using bond energies

  1. Break bonds on left side

  2. Make bonds on right side

  3. Calculate total energy input

  4. Calculate total energy output

  5. Subtract energy output from energy input to get overall energy change
• Bond energy


  Energy required to break or make a chemical bond
• Bond breaking is endothermic (positive energy change), bond making is exothermic (negative energy change)
• The overall energy change for a reaction is calculated by subtracting the total energy of bond making from the total energy of bond breaking
• Exothermic reactions release energy, endothermic reactions absorb energy
• Investigating the variables that affect temperature changes and chemical reactions

  1. Measure the maximum temperature reached in an exothermic neutralization reaction between hydrochloric acid and sodium hydroxide

  2. Use a measuring cylinder to measure 30 cm3 of dilute hydrochloric acid

  3. Transfer the acid into a polystyrene cup

  4. Stand the polystyrene cup inside a beaker

  5. Measure the temperature of the acid using a thermometer

  6. Measure 5 cm3 of sodium hydroxide solution using a measuring cylinder

  7. Transfer the sodium hydroxide solution to the polystyrene cup

  8. Fit a plastic lid to the cup and place the thermometer through the hole

  9. Gently stir the solution

  10. Record the highest temperature reached

  11. Repeat the experiment using 10 cm3, 15 cm3, 20 cm3, 25 cm3, 30 cm3, 35 cm3, and 40 cm3 of sodium hydroxide solution

  12. Calculate the mean value for the maximum temperature reached for each volume of sodium hydroxide solution

  13. Plot a graph of the results
• Independent variable


  • Volume of sodium hydroxide solution
• Dependent variable


  • Maximum temperature reached
• Control variables


  • Volume of hydrochloric acid
  • Concentrations of hydrochloric acid and sodium hydroxide solution
• Increasing the volume of sodium hydroxide solution

  Increases the maximum temperature reached
• Adding large volumes of sodium hydroxide solution

  Decreases the maximum temperature reached
• Reason for using a polystyrene cup with a lid

  To reduce heat loss through the sides, bottom, and air
• Polystyrene is a good thermal insulator
• Cell


  Two different metals placed in an electrolyte to produce electricity
• Producing electricity in a cell

  1. Chemical reaction on the surface of the two metals

  2. Potential difference and electric current generated
• A simple cell does not generate electrical current for long
• Cells only produce electricity if metals with different reactivity are used
• Difference in reactivity between metals

  Determines the size of the potential difference produced by the cell
• The electrolyte also affects the potential difference produced by the cell
• Battery


  Two or more cells connected in series to produce a greater voltage
• Alkaline batteries are non-rechargeable as the reactions cannot be reversed
• Rechargeable batteries can have their chemical reactions reversed by applying an electrical current
• Fuel cell


  A cell that generates electricity through a chemical reaction, using a fuel such as hydrogen
• Hydrogen fuel cell operation


  1. Hydrogen molecules split into hydrogen ions and electrons at the negative electrode

  2. Oxygen molecules combine with the electrons and hydrogen ions at the positive electrode to form water
• Hydrogen fuel cells


  Use hydrogen and oxygen to produce electricity, with water as the only waste product
• Advantages of hydrogen fuel cells compared to rechargeable batteries

  • Produce electricity as long as fuel is provided
  • Do not become less efficient over time
  • Can produce drinking water
• Disadvantages of hydrogen fuel cells compared to rechargeable batteries

  • Hydrogen is an explosive gas and difficult to store safely
  • Produce a relatively low voltage