Reaction Rates (cont.)Energy in a Reaction (p.11)

Experimental Measurement of Reaction Rates (p.11t)• Here is a typical plot of [reactant] vs. time:

• The rate of rxn can be found at any point by measuring the slope of the tangent line at that point.

• Look at the slope of the line at different points. Where is the rate the fastest? Slowest? Why?

Rate Calculation (p.11b)• Question 19a asks for the average rate from 0 to 60 s using

this data. How do you go about doing that?

Rates & Collision Theory (p.12t)• In order for molecules to react they have to collide.• This may seem obvious but make sure to always talk

about rate in terms of collisions.

Speeding Up A Reaction (p.12m)• How can we speed up this rxn (2 general ways)?

• Make more frequent collisions: increase temperature, concentration, pressure, surface area

• Make more energetic collisions: increase temperature

• Do #20-22 together

Proper name

Enthalpy Changes in Rxns (p.13t)

• Remember that bond breaking requires energy & bond forming gives off energy (Chem 11).

• In diagram, H is an unstable atom (high E) but forms a stable H2 molecule (lower E)

• PE is stored energy (ex. energy of e- in chemical bonds)

• KE is the energy of movement• Bond energy is amount of E required to break a

bond between 2 atoms.• Something like N2 is very stable b/c it has a high

bond energy.

Reaction Heats (p.14t)• Enthalpy is the total KE & PE in a system.• H is the change in enthalpy during a rxn.

• The heat of reactants and heat of products are never found, only the difference between them is important.

The Sign of H (p.14b)• Endothermic Reactions:

• Products have more E than reactants• H > 0• 2 alternate ways to show an endothermic rxn:

• 2 N2 + O2 + 164 kJ 2 N2O

• 2 N2 + O2 2 N2O ; H = +164 kJ

• Endothermic rxns feel cool b/c they take energy in from the surroundings (including your hand)

The Sign of H cont. (p.15m)• Exothermic Reactions:

• Products have less E than reactants• H < 0• 2 alternate ways to show an exothermic rxn:

• H2 + Cl2 2 HCl + 184 kJ

• H2 + Cl2 2 HCl ; H = -184 kJ

• Exothermic rxns feel hot b/c they give off E to the surroundings (including your hand)

Practice• Draw a potential energy diagram for the following reaction.

Will the surroundings feel warm or cold?R P + 10 kJ

Homework• Lab 11A prelab ready for next class.• Hebden #17-26, 28 for the class after next.