Enthalpy Changes

The amount of heat energy transferred during a chemical reaction at constant pressure. It has the units of kJ mol-1

in an exothermic reaction energy is transferred from the system to the surrounding. The enthalpy change will always be negative. Some examples are combustion, neutralisation and respiration.

The products are at a lower energy than the reactants and the activation energy is from the reactants to the peak of the graph

in an endothermic reaction energy is transferred from the surroundings to the system. The enthalpy change will always be positive and some examples are thermal decomposition and photosynthesis.

In an endothermic profile diagram, the products are a higher energy than the reactants and the activation energy is from the reactants to the peak of the graph

The minimum energy required for a reaction to take place

Temp= 298 K Pressure= 100kPa All solutions at 1.00 moldm-3

The enthalpy change for a given equation and it can be exothermic or endothermic

The enthalpy change when 1 mol of a substance completely combust at 298K and 100 KPa and it’s always exothermic

The enthalpy change when 1 mol of a compound is formed from its elements at 298K and 100KPa it can be either exothermic or endothermic.

It’s zero by definition because there is no change involved

The enthalpy change when 1 mol of H2O is formed from the neutralisation reaction of H+ an OH- at 298K and 100 KPa It is always exothermic and approximately -57 kJ per more

Every 1 gram of water requires 4.18J of energy to increase in temperature by 1°C so 4.18 is known as specific heat capacity

• q=mc🔺t q= energy change (J) m= mass of solution (g) c= specific heat capacity (Jg-1, k-1) 🔺t= change in temp (K or °C) • 🔺H = -+q/n 🔺H= enthalpy change (kJmol-1) q= energy change (J) n= moles of reactant

🔺H =( -+mc🔺t/ 1000)/ n

Since change in enthalpy is per one mol of the reaction you must divide the moles you calculated by the coefficient of that species in the balance equation

Heat loss to surroundings incomplete combustion of fuel evaporation of fuel from wick evaporation of water from combustion

Ensure a plentiful supply of oxygen for complete combustion add a lid to the beaker to reduce heat loss use a copper can instead of a beaker cover wick when not burning

Heat lost or gained to or from surroundings water evaporates from the beaker incomplete reaction The ‘c’ of the solution is not the same as the water

Using polyester cup use a lid to prevent heat loss or gain ensure standard conditions

The changing enthalpy stays the same, but it can change the q or n or m or the change in temperature

The enthalpy change for the breaking of 1 mol of bonds in gaseous molecules. The bond enthalpy value indicates the strength of a covalent bond , the more positive the bond enthalpy the larger the amount of energy needed to break the bond and so the stronger the bond

Because bond breaking requires energy so it’s endothermic and bondmaking releases energy so it is exothermic

You do the sum of the bond enthalpies of the bonds broken take away the sum of the bond enthalpies of the bonds formed