Enthalpy Changes
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What does the triangle symbol mean in chemistry?
Change or difference
What does H mean in chemistry?
Heat enthalpy - units = kJmol^-1
What is the enthalpy change for exothermic reactions?
Negative - energy transferred from system (chemicals) to surroundings
What is the enthalpy change for endothermic?
Positive - energy transferred from the surroundings to the system
What does the symbol of a circle with a line through it mean in chemistry?
When enthalpy change is measured at standard conditions
Give the definition of standard enthalpy change of a reaction
The enthalpy change for a given reaction in the molar quantities shown in the equation
Give the definition of standard enthalpy change of combustion
The enthalpy change when 1 mol of a substance completely combusts at 298K and 100kPa
Give the definition of standard enthalpy change of formation
The enthalpy change when 1 mol of a compound is formed from its elements at 298K and 100kPa
Give the definition of standard enthalpy change of neutralisation
The enthalpy change when 1 mol of water is formed from reaction of H+ and OH- at 298K and 100kPa
What is the value of enthalpy change for formation of an element?
Zero
What is the equation for energy change (J) and state what each value means
q =mc^T (^ is the triangle symbol) q = energy change (J) m = mass of solution (g) - 1g is 1cm^3 of water c = specific heat capacity - always 4.18 for water ^T = change in temp of solution (celcius or K)
What is the equation for enthalpy change and state what each value means
^H = -q/n ^H = enthalpy change (kJmol^-1) q = energy change (J) n = moles of reactant - if there are two uses moles of limiting reactant
What is the combined equation of enthalpy change and energy change?
^H = -(mc^T)/1000/n
How many different methods of calorimetry is there?
2
What should you do when there is more than 1 mole of the product you are forming when calculating enthalpy of formation?
Divide by the number of moles of product formed
Describe combustion calorimetry
Known mass of substance (fuel) burned. Heats a known mass of water. Measure ^T of water. The water acts as the surrounding and the fuel burning is the chemical system
What are reasons for the value of enthalpy change being different than the textbook value?
Heat losses. Incomplete combustion. Date book uses standard values. Evaporation from wick. Evaporation of water from beaker
How would you minimise errors during combustion calorimetry?
Ensure a plentiful supply of oxygen, add a lid to beaker, use a copper can instead of a beaker, cover wick when not burning
Describe calorimetry with a reaction in solution
Known quantity of reactant (limiting reactant used in calculation), reaction occurs in a known volume of water, measure ^T of the solution, the solution acts as the surroundings, the chemical system is the chemical(s) reacting or dissolving
What are the sources of error in calorimetry with a reaction in solution?
Heat loss/gain to from surroundings, water evaporates from the polysterine cup, specific heat capacity of solution not the same as pure water
How would you minimise errors during calorimetry with a reaction in solution?
Use a polysterine cup, use a lid to prevent heat loss/gain, ensure standard conditions
How would you predict the temperature change in calorimetry when the quantities have been changes of each reactant?
Use the equation q = n x ^H. ^H will always be the same, have the moles (n) halved or doubled, if so has the energy change (q) halved or doubled. Then use the equation q = mc^T. Has volume (m) halved or doubled or stayed the same, then you can work out if ^T has halved or doubled or stayed the same
Why are bond enthalpies always endothermic (have positive values)?
Bond breaking always requires energy - endothermic. Bond making always releases energy - exothermic
What does average bond enthalpy mean?
The enthalpy change for the breaking of 1 mol of bonds in gaseous molecules averaged (actual bond enthalpy in a given compound will vary from the average value) over a range of compounds