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What are the thermodynamic standard conditions?

298K, 1 atm pressure, concentration of 1mol per dm3

What is enthalpy change?

The heat change measured at a constant pressure and temperature.

Define standard enthalpy of formation

The enthalpy change when a single mole of compound is formed from its elements under standard conditions of 298K and 1atm pressure with all the reactants in their standard states.

Why is enthalpy change for exothermic reactions negative?

Because energy has been released to the surroundings. The products therefore have a lower enthalpy than the reactants and the final enthalpy is less than the initial enthalpy.

Classify as endothermic or exothermic

• NaOH dissolves in water & temp. of solution rises - Exothermic
• Ammonium Chloride dissolves in water and temp. of surroundings drop - Endothermic
• H and O combine explosively into water - Exothermic
• Liquid water condenses into ice at 0 C - Exothermic
• Liquid Nitrogen boils spontaneously at room temp.- Endothermic

Why is standard enthalpy changes of combustion always negative?

Because heat is always evolved, the reaction is exothermic and therefore heat is lost to the surroundings. The energy required to break bonds is always less than the energy released when bonds are formed.

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