Measuring Energy Changes

Define Energy and its important parts

• ability to do ‘work’

  • move an object against an opposing force

• potential or kinetic

• ex. heat, light, sound, electricity and chemical energy

Tell me about Heat

• energy transfer due to temperature difference

• produces an increase in disorder in how particles behave

• increases the average kinetic energy of the molecules

What is the Chemistry Universe? What are its components?

• the universe in chemistry that is split into two systems

• Chemical System - area of interest/made of reactants and products

• Surroundings - everything else in the universe

  • typically air or water

What are the two types of Systems?

1. Open

   1. can exchange energy and matter with the surroundings

2. closed

   1. can exchange energy, but not matter. eg. glow sticks

What is the Law of Conservation of Energy?

Energy cannot be created or destroyed only converted from one form into another

What does the Law of Conservation of Energy tell us about chemical reactions?

1. energy can be exchanged between system and surroundings, but total energy cannot change during the process

2. Energy from the system is gained by surroundings and vice versa

3. eg. bbq (chemical to thermal & light)

What is Enthalpy? tell me about it.

Total Internal Heat Content (at constant pressure)(sum of kinetic and potential energy)

it is impossible to measure the enthalpy of a system

Tell me about Enthalpy Change (∆H)

• measured in kJ/mol

• measures amount of heat energy contained in a substance/system

• result from bonds and intermolecular forces breaking and forming

• measured from difference in enthalpy between reactants and products

Tell me about Endothermic Reactions

• net absorption of energy from surroundings

  • (energy absorbed to break bonds is GREATER than energy released when products form)

• △H is positive

• temperature of surroundings decreases

Tell me about Exothermic Reactions

• net release of energy to surroundings

• (energy absorbed to break bonds is LESS than energy released when products form)

• △H is negative

• temperature of surroundings increases

What 3 things affect Heat Change Calculations?

1. mass of object (g)

2. heat added (K)

3. Specific Heat Capacity

Define Specific Heat Capacity. What does it depend on? What is its equation?

amount of heat needed to raise 1g of a substance by 1°C

• depends on # of particles

when a change occurs in a system (__), the chemical potential energy change is ___________ _____ to the ____ (_) transferred to the surroundings.

1. ∆H

2. numerically equal

3. heat (q)

Equation for heat transfer between System and Surrounding

What do Enthalpy Changes Depend on?

the conditions of the reaction

What are standard conditions?

• 100 kPa (pressure)

• 1 mol dm⁻³ (concentration)

• 298 K (temp.)

How can we represent Enthalpy Change?

1. thermochemical equation

2. potential energy diagram

3. molar enthalpy change

   1. (energy change that occurs when 1 mol of a substance undergoes a physical, chemical, or nuclear change)

What does a Thermochemical Equation look like?

What does a Molar Enthalpy Change look like?

• represented by ∆H_^r

• units are J/mol

• *the molecule you are getting the ΔHr MUST have a coefficient of 1

Draw a potential energy diagram for: CO₂(s) + 20.3 kJ → CO₂ (g)

Energy is on the reactant side, which means its endothermic. Product will have greater potential energy than reactants.

Tell me about Calorimetry

• process of measuring energy changes during a chemical or physical change

• use a device called a calorimeter

• we assume the calorimeter is a closed system

  • surroundings is water

EXAMPLE (p.397)

A coffee-cup calorimeter was used to measure the temperature change for the reaction between zinc powder and a 1.0 mol/dm⁻³ solution of copper (II) sulphate. The following results were recorded:

Determine the amount of heat released and the enthalpy change for this reaction.

1. Determine Heat Change

2. Determine limiting reactant

3. Calculate molar enthalpy