energetics

• what is an enthalpy change? (triangle H)

  the heat energy change measured under constant pressure
• in a chemical reactions, what does the heat energy change occur between?

  between the system and the surroundings.
• in a chemical reaction, what is the system?

  contains the reactants and products.
• in a chemical reaction, what is the surroundings?

  the environment around the system.
• what's an example of a surrounding for a chemical reaction?

  a container in which the reaction takes place.
• what are the 2 options that a chemical reaction can be?

  endothermic and exothermic.
• describe the energy profile diagram from an endothermic reaction.


  the reactants is lower than the products.
• what occurs in an endothermic reaction?

  the system takes in heat energy from the surroundings, so the temperature of the surroundings decreases. The energy of the reactants is lower than the energy of the products. The enthalpy of the system increases during an endothermic reaction and the enthalpy change values produced are always positive.
• look at this and learn. ENDOTHERMIC.

  
• describe the energy profile diagram for an exothermic reaction?

  the products are lower than the reactants.
• what occurs in an exothermic reaction?

  the system gives out heat energy to the surroundings, so the temperature of the surroundings increases. The energy of the reactants is higher than the energy of the products. The enthalpy of the system decreases during an exothermic reaction and the enthalpy change values produced are always negative.
• look at this and learn. EXOTHERMIC

  
• what are standard enthalpy changes?

  those which are measured under standard conditions, eg: pressure 100kPa, temperature 298 K.
• how is standard enthalpy changes represented?

  x represent the enthalpy change symbol and the theta symbol shows that standard conditions have been used,

  
• what is the definition for standard enthalpy of combustion?

  the enthalpy change that occurs when 1 mole of a compound undergoes complete combustion in oxygen under standard conditions with all substances being in their standard states.

  When writing the equation to represent standard enthalpy of combustion changes we must make sure the number of moles of the compound undergoing combustion doesn't change. State symbols should be included.
• what is the definition for standard enthalpy of formation?

  the enthalpy change that occurs when 1 mole of a compound is formed from its elements under standard conditions, with all substances being in their standard states.