Energy calculations + stoichiometry thingss

chapter 3 !

Energy efficiency (%)

useful energy/energy input x 100

Energy efficiency meaning

percentage of energy from a source that is converted into useful energy
- Energy can be lost during transformations

Calorimetry

measuring the heat energy released or absorbed by a chemical reaction such as heat loss

Calorimeter

designed to minimise heat loss by ensuring energy released can be transferred directly to or from measured volume of water, reaction takes place in a solution

Solution calorimeter used to determine enthalpy changes in:

• Glucose dissolving in water

• Acids reacting with bases

• Metals reacting with acids

• Solids dissolving in water

When temperature of water increases…

an exothermic reaction has occurred

When temperature of water decreases…

an endothermic reaction has occurred

How to prevent heat loss ?

• Using a lid around the beaker

• Stirring water to keep constant temperature of water

• Using draft shield around set up

• Using polystyrene insulator cup

Calibration factor

To make measurements more accurate for heat produced, how much energy required to change the temperature of the water by 1C for the calorimeter is used, needs to be determined

Calculation for Energy (J)

Volts x Amps x Seconds

Calibration factor (J C^-1)

energy / change in temperature

Enthalpy change, general formula (KJ mol)

energy/ mol

Specific heat capacity of water calculations (J)

q= mcAt
c = 4.18

Converting between mass and volume of water rules

1g = 1mL

Enthalpy change in food (KJ mol)

energy/mass (can be change in mass)

Heat loss

When energy is transformed from a burning fuel across an open space, heat loss to the surroundings such as the air around the burning fuel.

Limiting reagent

Reactant that is completely consumed

Excess reagent

Reactant remaining unreacted at the end of the chemical reaction

Steps in stoichiometry involving reagents, excess or limiting

1. Write balanced chemical equation

2. Calculate number of moles of each reactant

3. Using mole coefficient, determine excess or limiting

4. Using mole ratio, multiply with limiting reactant to calculate moles of product

Mass of substance (g)

mol = mass / Molar mass

Volume of gas at SLC

mol = volume / 24.8

Volume of known gas at non-standard conditions

PV=NRT
R= 8.31
T= is in kilopascals (kPa)

Gas volume-volume calculations

mole ratios in balanced equation become volume ratios at same temperature and pressure