Energetics
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What is an exothermic reaction?
Release heat to surroundings. Enthalpy of products less than enthalpy of reactants.
What are some examples of exothermic reactions?
Combustion Respiration
What is an endothermic reaction?
Take in heat from surroundings. Enthalpy of products greater than Enthalpy of reactants.
What are some examples of endothermic reactions?
Decomposition Photosynthesis
What is the equation for the overall energy change, ∆H, of a reaction
∆H = bonds broken - bonds formed
What is average bond enthalpy?
The energy needed to break one mole of a particular type of bond averaged over many compounds.
What is the standard enthalpy of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions.
What is the enthalpy of formation of an element in its standard state?
Zero
What is the standard enthalpy of combustion?
The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.
What are enthalpies of combustion always?
Exothermic reactions.
What products are always formed from the standard enthalpy of combustion of organic compounds?
Carbon dioxide Water
What is the conservation of energy?
Energy cannot be created or destroyed but it can change from one form to another.
What is Hess’s Law?
The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
What are the steps for using Hess’s Law in a standard enthalpy of combustion calculation?
• Write equation for formation of compound. • Construct energy cycle using enthalpies of combustion. • Apply Hess’s Law to cycle.
What is the procedure for calculating the enthalpy of combustion of a reaction?
• Using measuring cylinder, measure 25cm^3 of water and transfer into boiling tube. Clamp at angle over spirit burner. • Measure temp of water using thermometer. • Weigh spirit burner containing alcohol. • Place burner under boiling tube and light fuel. • Heat water until temp rise by 20oC. • Stir and measure final temp of water. • Reweigh spirit burner. • Repeat steps 1-7. • Take average.
What equation can be used to calculate the heat energy change released in a reaction?
q = m c ∆T
What equation can be used to calculate the enthalpy change of an enthalpy of combustion reaction?
∆H = -q / n
How can accuracy be improved when calculating the enthalpy of combustion of a reaction during a practical?
Insulting beaker of liquid. Shielding flame from droughts. Stir liquid so heat distributed evenly throughout it.
Why would the experimental value for the enthalpy change be lower than the data book value when calculating the enthalpy of combustion of a reaction during a practical?
Energy lost due to surrounding so temp increase lower than under ideal conditions. May be incomplete combustion. May not be under standard conditions.
What is the standard enthalpy of neutralisation?
Enthalpy change when one mole of water is produced by neutralisation reaction under standard conditions.
What is the procedure for calculating the enthalpy of neutralisation of a reaction?
• Use measuring cylinder to measure 50cm^3 of base and transfer to polyester cup. • Record temperature. • Measure 25cm^3 of acid and add to base. • Stir and record highest temp reached. • Use q = m c ΔT. • Use q and moles of water to calculate ΔH.