IB CHEM- REACTIVITY 1

Transfer of energy terms

Conduction

Convection

Radiation

Conduction

transfer of energy by direct contact

Convention

transfer of energy by movement of fluids

Radiation

transfer of energy by electromagnetic waves

Law of conservation of energy

energy cannot be created or destroyed in a closed system, it can only be transferred or transformed from one form to another

• ex: turning on light bulb- energy is transformed from electrical energy into light, thermal, and some sound

chemical reaction

a process that forms new substances by rearranging atoms of reactants to form new products

energy

the ability to do work or cause a change

• measured in Joules (J)

Types:

• potential

  • chemical- in food and fuels

  • nuclear- in centre of particles

  • gravitational- in object when above Earth’s surface

• experience

  • sound

  • light

• kinetic

  • thermal- moving particles, heat

  • electrical- charged particles moving through wire

  • mechanical- moving objects

Transfer of energy when holding an ice cube and forming an ice cube

Thermal energy transfer

Holding an ice cube

• thermal energy from hand→ ice cube

Forming an ice cube

• thermal energy from water→ atmosphere/surroundings/freezer

temperature

• quantitative measurement of the average kinetic energy of particles measured in K

NOT ENERGY

heat

a form of energy that is transferred from a region of high temperature to a region of low temperature

• measured in J

Similarity between heat and temperature

both involve movement of particles

influence the state of matter

Energy transfer in Chemical reactions

All chemical reactions involve a transfer of energy between the system and the surroundings, while total energy is conserved

• system- components of reaction

• surroundings- everything outside the system

exothermic

reactions (system) that release energy to surroundings

• surrounding temp. will increase (feels hot)

• freezing, condensation, deposition

endothermic

reactions (systems) that absorb energy from the surroundings

• surrounding temperature will decrease (feels cold)

• melting, evaporation, sublimation

Chemical potential energy

stored in bonds

• more potential energy=more likely to react (so less stable)

• less potential energy= more stable

Products are more stable (less potential energy)—> EXOTHERMIC

• more likely to happen b/c reactants are less stable

Reactants are more stable (more potential energy)—> ENDOTHERMIC

• less likely to happen because the reactants are more stable

enthalpy

the amount of heat a system has

• system is NOT easily measured

• changes in enthalpy are measured when heat is transferred from system to surroundings

  • units: KJ mol^-1

measuring enthalpy

DIFFICULT

because you can’t measure the total energy inside a system

• instead we measure CHANGES in enthalpy (delta H) by tracking the heat absorbed or released during a reaction ( w/ calorimeter)

reactions with negative enthalpy change vs reactions with positive enthalpy change

endothermic reactions(absorption of heat) → positive enthalpy change

exothermic reactions(releasing heat)→ negative enthalpy change

standard enthalpy change of reaction (delta H)

heat transferred @ constant pressure and under standard conditons

can be included with providing information about heat transfer for that process

Thermochemical equations

a balanced equation that includes the entalpy change of the reaction

enthalpy change diagrams

negative enthalpy change:

• reactants higher than products

positive enthalpy change:

• reactants are lower than products

Heat vs. Temperature

HEAT: form of ENERGY that is TRANSFERRED between objects of different temperatures (J)

TEMPERATURE: measure of average KINETIC ENERGY of particles in a system (K)

**Both involve the movement of particles and influence state of matter

Specific Heat Capacity

amount of energy required to raise 1 gram of a substance by 1 degree C or 1 K

• is unique to the identity of a substance

High Specific Heat

takes A LOT of energy to increase the temperature and takes awhile to cool down

Low Specific Heat

takes LESS energy to increase temperature and will cool down quickly

thermal energy equation

Q=mcat (MCdeltaT)

• Q= thermal energy (J)

• m= mass (g) of substance you are measuring change in temperature of

• Delta T= change in temperature

  • C= specific heat

Calorimetry-to determine enthalpy change of reaction

technique used to measure the heat transfer during a physical or chemical process

calorimeter

apparatus used to measure temperature changes to calculate enthalpy for a reaction

Bomb Calorimeter

used to measure extremely accurate thermal energy transferred from combustion of foods and fuels

• pricey equipment

Metal Calorimeter

used to easily measure thermal energy transferred from combustion of solids, liquids, and mixtures

• significant heat loss to surroundings

Coffee Cup

used to easily measure thermal energy transferred from reactions involving aqueous solutions

• heat loss to surroundings

Why is water used in all calometers?

• water a high heat capacity- large amounts of heat produced will result in relatively small temperature changes in water compared to other substances that have low heat capacities

  • water = practical and safe substance b/c it will not change state easily or reach temperatures beyond range of thermometer

Q vs Delta H

Q- thermal energy

• involves measuring temperature of surroundings

Delta H- change in enthalpy

• requires involvement of number of moles involved in chemical reactions

**will be in opposite sign to each other

Equation that relates Q and Delta H

Delta H= - Q/n

• Delta H= change in enthalpy (kJ mol^-1)

• Q= thermal energy (J)

• n= number of moles

biggest source of error when performing calorimetry

• system may be closed but is never isolated from surroundings so heat loss results are in smaller changes in temperature than we expect

How actual. theoretical measurements differ

• recording a lower final temperature than theoretically possible for exothermic reactions

• higher final temperature for endothermic reactions

How to reduce experimental error

• insulation of calorimeter to reduce heat loss by surrounding calorimeter with insulating materials, increasing thickness of materials and minimizing contact with air

How to reduce error - data processing

use linear rate of cooling to estimate theoretical maximum temperature

Bond breaking

REQUIRES ENERGY

• endothermic

• +delta H (change in enthalpy)

• energy is absorbed

Bond forming

• exothermic

• -delta h (negative change in enthalpy)

• energy is released

bond enthalpy (H)

the energy required to break one mole of chemical bonds in the gaseous state

• also called bond dissociation energy (E)

average bond enthalpies (BE)

the same bond could be present in different molecules which would result in slightly different bond enthalpy values so the values are reported as the average of them

calculating the enthalpy of a reaction

Sum of [(BE of bonds broken)- (BE of bonds formed)]

and/or Sum of [(BE of reactant)- (BE of product)]

Hess’s Law

states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same

• hiking up a mountain: there are many ways to get to the top but you will end up with the same elevation gain no matter what path you take

• Route 1- straight from reactants to products

• Route 2- include intermediate products

Delta H₁= Delta H₂ + Delta H₃

** ARROWS MUST BE GOING THE SAME DIRECTION IF YOU NEED TO FLIP THEM CHANGE THE SIGN TO
MUST MAKE A CONTINUOUS CYCLE

biofuels

a fuel derived directly from living matter

fuel cells

a device that uses an external source of fuel and oxygen (or air) to transfer energy from a chemical store by electricity

combustion

the chemical reaction of a substance with oxygen to produce carbon dioxide and water

• combustion reaction involved

  • fuel ( a combustible substance)

  • oxygen gas

fuel

any substance that can be made to react with other substance. so that it releases energy

combustion of metals

• when reactive metals combine with oxygen they can produce tremendous amounts of energy, both in the form of heat and light

• less reactive metals also combine with oxygen

  • rusting (corrosion of iron when it reacts with oxygen and water) or tarnishing (when a metal losses lustre, usually as a result of exposure to oxygen gas an moisture

Combustion of non-metals

many non-metals react with oxygen to form non-metallic oxides

• oxides do not have fixed ratios, so the combustion reactions can produce several different product oxides for the same non-metal fuels

combustion of organic compounds

all organic compounds, especially hydrocarbons, easily undergo complete combustion reactions that produce only carbon dioxide and water

• products are always more energetically stable than the reactants→ overall complete combustion reaction is always endothermic

** organic compounds are flammable no matter the state

incomplete combustion

a fuel burns a restricted supply of oxygen and carbon monoxide and/or soot are produced

• lack of oxygen is usually caused by the combustion reaction occurring too quickly

only carbon monoxide is formed as a product instead of carbon dioxide