Enthalpy Changes

• If the enthalpy value of the reactants is lower than the enthalpy value of the products, then energy must have been gained from the surroundings during the chemical reaction. This is an endothermic reaction.
• Endothermic - heat is taken in from the surroundings (the reactants gain energy).
• What would you expect to happen to the temperature of the surroundings in an endothermic reaction?

  The temperature of the surroundings will fall.
• For a chemical reaction to take place, strong bonds must be broken in the reactants and new ones formed in the products.
• To break bonds, you need to put energy in, so this process is endothermic.
• When forming bonds, energy is released, so this process is exothermic.
• If more energy is needed to break bonds than is released in forming bonds, then the process is endothermic overall.
• If less energy is needed to break bonds than is released in forming bonds, then the process is exothermic overall.
• Regardless of whether the reaction is exothermic or endothermic, all chemical reactions need an initial input of energy to start the chemical reaction by the breaking of bonds. This is the activation energy.
• Activation energy is the minimum energy required to start a reaction by the breaking of bonds.
• The difference in enthalpy between reactants and products and the activation energy can be qualitatively illustrated using an 'Enthalpy Profile' diagram.
• On an exothermic Enthalpy Diagram, the enthalpy of the products is lower than that of the reactants.
• On an endothermic Enthalpy Diagram, the enthalpy of the products is higher than that of the reactants.
• Arrows on an Enthalpy Profile diagram should never be double-headed.
• Enthalpy Profile diagrams should always have labelled axes, show the reactants and products, have a labelled Ea, and show the change in enthalpy.
• In an exothermic reaction the products have a lower enthalpy value than the reactants so ΔH is negative as energy is lost.
• In an endothermic reaction the products have a higher enthalpy value than the reactants so ΔH is positive as energy is taken in.
• Enthalpy changes for different chemical reactions can only be compared if they are measured under the same set of conditions. These are known as standard conditions.
• Standard conditions are a pressure of 100kPa, a temperature of 298K, and a concentration of 1.0moldm-3.
• Any enthalpy change measured under standard conditions is called a standard enthalpy change.
• What is the definition of (Standard ) Enthalpy Change of Reaction?

  The enthalpy change associated with a stated equation with all reactants and products in their states and under standard conditions.
• State and explain in terms of bond breaking and bond forming, why the combustion of methane is exothermic.

  More energy is released from the forming of bonds than is required to break the bonds.
• State and explain whether the combustion of methane is an exothermic or endothermic process.

  It is exothermic as energy is transferred from the system to the surroundings.
• Explain what is meant by standard conditions.

  A pressure of 100kPa, temperature of 298K, and a concentration of 1.0moldm-3.
• Explain in terms of bond breaking and bond forming, why the reaction of sodium bicarbonate with hydrochloric acid is endothermic.

  More energy is required to break the bonds than is released when they form.
• Write an equation, including state symbols, to represent the enthalpy change of vaporisation of bromine.

  Br2(l) ----> Br2(g)
• The instrument used to measure a change in temperature during a chemical reaction is known as a calorimeter.
• Enthalpies of reaction can be found by using a polystyrene cup calorimeter to determine temperature change of a known mass solution.
• Q = m x c x ΔT
• In the equation Q = m x c x ΔT, what does Q stand for?

  The heat energy gained by solution in Joules.
• In the equation Q = m x c x ΔT, what does m stand for?

  The mass of solution being heated.
• In the equation Q = m x c x ΔT, what does c stand for?

  The specific heat capacity of solution (4.18).
• In the equation Q = m x c x ΔT, what does ΔT stand for?

  The change in temperature of the solution, in degrees celsius.
• The specific heat capacity is the amount of energy needed to raise the temperature of 1kg of a substance by 1 degree.
• The SHC of water is 4.18 as it requires 4.18J of energy to raise the temperature of 1kg of water by 1 degree.
• The equation for enthalpy change is ΔH = q/n.
• In the equation ΔH = q/n, what does ΔH stand for?

  The enthalpy change in kJ mol-1.
• In the equation ΔH = q/n, what does q stand for?

  The energy lost/gained, kJ.
• In the equation ΔH = q/n, what does n represent?

  The moles of the limiting reagent in mol.
• If the solution gets hotter the reaction must be exothermic and the ΔH is negative.