1.4 - Energetics Flashcards
What is enthalpy change?
Delta H, is the heat energy transferred in a reaction at a constant pressure. KJ mol^-1
What are standard conditions?
100 kPa pressure and 298 K
What are exothermic reactions?
They give out energy. Delta H is negative. The temperature will go up. Oxidation reactions are usually exothermic, such as the combustion of a fuel or the oxidation of carbohydrates, such as glucose in respiration.
What are endothermic reactions?
They absorb energy and delta H is positive. E.g. thermal decomposition of calcium carbonate or photosynthesis. Temperature falls.
What are mean bond enthalpies not exact?
The energy needed to break a bond depends on the environment it’s in. The mean bond enthalpy is not exact and is averaged over a range of compounds.
How do you find the enthalpy change of a reaction with bond enthalpies?
Reactants - products
State the definitions of standard enthalpy of formation and combustion.
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
The enthalpy change when 1 mole of a substance is completely burned in oxygen in their standard states under standard conditions.
How do you use calorimetry?
You can find out how much heat is given out by measuring a temperature change. To find the enthalpy of combustion of a flammable liquid, you burn it inside a calorimeter. It has a fuel, air, combustion chamber, thermometer, stirrer and water. You can work out the heat energy that has been absorbed by the water if you know the mass of the water, temperature change, and specific heat capacity of water (4.18 J g^-1k^-1)
You will always lose heat to the surroundings, and some of the combustion may be incomplete. You might also lose some fuel due to evaporation because they are volatile.
How can you use calorimetry to measure enthalpy changes in solution?
Such as neutralisation, dissolution and displacement. To find the enthalpy change for a neutralisation reaction, add a known volume of acid to an insulated container (e.g a polystyrene cup) and measure the temperature. Then add a known volume of alkali (or known mass of solid) and record the temperature of the mixture at regular intervals over a period of time and stir the solution to make sure it’s evenly heated. Find the temperature change for the experiment and use it to calculate the enthalpy change of the reaction. You’ll need to know the mass of the solutions you’ve used in order to calculate the enthalpy change of the reaction.
How do you find an accurate temperature change from a graph?
Draw two lines of best fit - one for the points before the reaction and one one after it started. Extend both lines so they both pass the time when the reaction started. Calculate the temperature change.
How do you calculate enthalpy change?
Q = mc delta T
Q is energy lost or gained. This is the same as enthalpy change if pressure is constant.
M is mass of water or other solution in the calorimeter in grams.
C is the specific heat capacity of water, which is the amount of heat energy it takes to rise the temperature of 1g of water by 1 K. Change in temp works for K or C.
What is hess’s law
The total enthalpy change of a reaction is independent of the route taken.
How do you calculate enthalpy with Hess cycles?
Formation arrows go up.
Combustion arrows go down.
Enthalpy of formation of elements are zero.
Describe an experiment with improvements on enthalpy change in a beaker.
Insulate beaker
Use burette/pipette instead of measuring cylinder
Reading watch glass after adding the solid
Use powdered solid
Measure initial temperature few minutes before addition
Measure the temperatures after addition at regular interval l for 8 mins + or until a trend is observed.
Plot graph of temperature against time. Extrapolate to point of addition. Determine change in T at point of addition.
Give two disadvantages of using a glass beaker on a tripod and gauze instead of a copper calorimeter
Glass not as good of a conductor as copper
Tripod and gauze would reduce heat transfer
