PDP ChemistryEnergetics

Thermochemistry: energy changes in chemical reactions

Topic 5: Energetics• energy• exothermic and endothermic

reactions• calorimetry• enthalpy change• bond enthalpy• temperature and heat• absorption and emission of

radiation• burning fuels

PDP moles tests comments time was short… show working for quantitative questions 13.b) graph scales- we need to do more work with these… Chemical equations- balancing, subscripts and coefficients,

charges — 1. work out reactants and products (a word equation may help

here) — 2. write the formula for each compound (use the charge on ions

to find the formula, but the charges are not written in the formula) — 3. balance. change only the coefficients, NOT the formulas

13.d) it is hard to pick out the relevant information and ignore what you don’t need

Energy

Energy: the ‘capacity to do work’ the unit of energy is the Joule (J) forms of energy:

kinetic gravitational potential elastic potential chemical potential electric potential nuclear potential thermal (internal energy) radiant

Law of conservation of energyThe total energy of an isolated system remains constant- it is said to be conserved over time.Energy can be neither created nor be destroyed, but it can change form.

Exothermic and endothermic reactions

Why are reactions endothermic or exothermic?

In chemical reactions bonds break and new bonds form.Different bonds have different amounts of chemical energy. Exothermic reaction: less energy in the bonds after

the reaction energy released as heat Endothermic reaction: energy is needed as there is

more energy in the bonds after the reaction energy taken in

Graphs sharp pencil and ruler graph paper- use the whole sheet choose a linear scale for each axis plot each point with an x or a + (or error bars) titles and units on axes title for graph trendline (or maybe join points with straight

lines)- no snakes

Description and explanation of results

Conclusion Description and explanation of results from one

exothermic and reaction and upload to MB

Temperature

Temperature and energy Which gas is at the higher temperature? Which gas contains more energy?

Temperature- average KE per particle

higher average speed higher temperature

more particles at same average speed same temperature

Measuring temperatureAbsolute scale of temperature: zero Kelvin = zero kinetic energyzero K = -273°C

Convert to Celcius1. melting point of ice 273K

2. room temperature 300K

3. temperature of deep space 4K

4. boiling point of oxygen at standard pressure 90K 5. surface of the sun 6000K

6. temperature of an exploding nuclear bomb 106K

Distribution of speeds of particles

number of particles

speed

Distribution of speeds

Calorimetry

How much energy does it take to heat water?

4.18Joules of energy are needed to heat up 1 gram of water (approx 1mL) by 1°C.(4.18 is the specific heat capacity of water, and aqueous solutions are almost the same)

Calculating how much energy is released

1. Calculate how many Joules are released in each of your experiments from last week

2. Calculate how many kJ are released by each mole of reactant in the experiments.

4.18Joules of energy are needed to heat up 1 gram of water (approx 1mL) by 1°C.

Exothermic and endothermic reactions

Ammonium nitrate and water Iron and oxygen

Hot pack (more than one is true)

A. The temperature of the hot pack decreases during the reactionB. The temperature of the hot pack increases during the reactionC. The hot pack transfers heat to the person touching itD. Heat is taken in from the person to the hot packE. The hot pack has less energy stored in chemical bonds after it is

usedF. The hot pack has more energy stored in chemical bonds after it

is used

Energy changes

Exothermic reactionsStored chemical energy, J

time

reactants

products

∆H

Endothermic reactionsStored chemical energy, J

time

reactants

products

∆H

Enthalpy change

Enthalpy, H Energy stored in chemical bonds of reactants (in

Joules) PE and KE of particles + energy to make space for

substance

Image: https://en.wikipedia.org/wiki/File:Ammonium_Nitrate.jpg

Burning fuels

Fuels combine with oxygen in combustion

most chemical fuels are organic compounds, often hydrocarbons (contain C, H) or carbohydrates (contain C, H, O) carbon oxidized to CO2 hydrogen oxidized to H2O

Calculate a theoretical ∆H in kJ/mol for the reactions from yesterday’s lab

C2H6O + 3O2 2CO2 + 3H2O

Energy density

Energy density used to compare fuels

Fuel Energy density, kJ/g

hydrogen 142natural gas (methane) 56

ethanol 26coal 24wood 16

energy released by when 1g of fuel is burned

Standard enthalpy

Standard enthalpy change of a reaction

∆H⊖ to compare energy changes in reactionsin kJ/mol(measured at STP 298K and 1atm)

Calculating standard enthalpy0.2g of magnesium reacts with 10mL excess hydrochloric acid to increase the temperature by 50°C. Calculate the standard enthalpy of reaction.

Bond enthalpy

Bond enthalpy: the enthalpy change when one mole of bonds is formed in the gaseous state

X (g) + Y(g) X-Y(g)

Forming bonds is exothermic (negative ∆H)Breaking bonds is endothermic (positive ∆H)

Exothermic reactioncombustion of methane

CH4 + 2 O2 CO2 + 2H2O + heat

Reactants ProductsDH = (Products) – (Reactants) negative value

Exothermic reactions: DH < OEndothermic reactions DH > O

Enthalpy of combustionBond Average bond

enthalpy kJ/molC-C 347C=O 746C-H 413O=O 498O-H 464C-O 358

Image: https://en.wikipedia.org/wiki/Burning

Calculating standard enthalpy change

What is the enthalpy change in kJ per mole if 45kJ are given out when 0.8g of methane is burned?What is the enthalpy change in kJ per mole if 1.6g of methanol is used to heat 200mL water from 20C to 38C?

Using temperature to calculate ∆Hᶱ

Heat energy = mass x specific heat capacity x temperature changeQ = mc∆T

Pollution from combustion

Acid rain

Greenhouse gases

Carbon monoxide

Particulate pollution

Image: http://en.wikipedia.org/wiki/File:Periodic_table.svg