EnergyA Give and Take

10.1 The Nature of Energy Energy: the ability to do

work or produce heat

Potential energy (store energy): energy due to position or composition

Kinetic energy (motion energy): energy due to motion of the object and depends on the mass of the object and its velocityKE = ½ (mv2)

Law of conservation of energy: that energy can be converted from one from to another but can be neither created or destroyed.Energy of the universe is constant

The nature of energyWork: force

acting over a distancew = F x d

State function: property of the system that changes independently of its pathway

Temperature and HeatTemperature: is a measure of the

random motions of the components of a substanceE.g H2O molecules move rapidly in hot

water than in cold water

Heat: a flow of energy due to a temperature differenceTfinal = average temp from mixing (hot

& cold temp)

Exothermic and Endothermic Process

System – everything we focus on in experiment

Surroundings – everything other the system

exothermic (energy flows out of system to surrounding (via heat)

endothermic ( energy flows into system from surrounding (via heat)

ExamplesIdentify whether these process are

exothermic or endothermicYour hand gets cold when you touch

iceThe ice melts when you touch itPropane is burning in a propane

torchTwo chemicals mixing in a beaker

give off heat

ThermodynamicsIs the study of energy.

First law of thermodynamics: the energy of the universe is constant

Internal energy – energy of the system ∆E = q + w∆ => change in the functionq => represents heatw => represents work

10.5 Measuring Energy changes

calorie: the amount of energy (heat) required to raise the temperature of one gram of water by 1oC1Calorie = 1000 calories

Joule (J) – SI unit1 calories = 4.184 joules

Converting Calories to JoulesExpress 60.1 cal of energy in units

of Joules

How many calories of energy corresponds to 28.4 J?

60.1 cal 4.184 J = 251 J

1 cal

28.4 J 1 cal = 6.79 cal

4.184 J

Calculating Internal Energy

Calculate ΔE for q = 34 J, w = -22 J

ΔE = q + w

ΔE = 34 J + (-22 J) = 12 J

Is this exothermic or endothermic?ΔE > 0, therefore it is endothermic

Specific heatThe amount of energy

required to change the temperature of one gram of a substance by 1oCDenoted as s

Heat required = specific heat x mass x change in temp

Q = s x m x ∆T

Calculating Energy Requirements

725 J = Tf – 27.5 °C

48.95J/°C 14.8 °C = Tf – 27.5 °C

Tf = 42.3 °C

A sample of gold requires 3.1 J of energy to change its temperature from 19oC to 27oC. What is the mass of this sample of gold?

Q = s x m x ΔT

sgold = 0.13 J/g oC