Thermochemistry & Kinetics Review

Thermochemistry & Kinetics Review

By Leslie Dunham

• Release heat

• Feel hot

• Energy of products is LESS than reactants

• ΔH (enthalpy) is negative

• Heat is a product
  

​

Exothermic reactions

• Absorb heat

• Feel cold

• Energy of reactants is LESS than products

• ΔH (enthalpy) is positive

• Heat is a reactant

​

Endothermic reactions

Determine how much heat was involved in a reaction.

Calculating heat lost or gained

Multiple Choice

$$6\ CO_2+\ 6\ H_2O\ +\ Energy\ \rightarrow\ C_6H_{12}O_6\ +\ 6\ O_2$$

• endothermic reaction

• exothermic reaction

Multiple Choice

A student mixed two chemicals to allow them to react. They feel the beaker and it feels cold. Which of the following is true?

• The specific heat of the compounds decreased

• It is an endothermic reaction

• It is an exothermic reaction

• Energy is released from the reaction into the surrounding.

Multiple Choice

 How many joules of heat are needed to raise the temperature of 10.0 g of aluminum from 22°C to 55°C, if the specific heat of aluminum is 0.90 J/gx°C?    

• 297 Joules
• 0.003 Joules
• 297 J/gx°C
• 0.003 J/gx°C

Multiple Choice

Specific heat is....

• The measure of kinetic energy of an objects particles
• A measure of the energy needed to increase the temperature of 1 g of a substance by 1 degree C

• The heating caused by the motion of flluid due to temperature difference
• The transfer of energy by electromagnetic radiation

Solids have the least energy - molecules held tightly together
Liquids have more energy and looser structure
Gases have highest energy, no structure

​

Phases of matter

Gases have more energy than solids
Endothermic processes add energy
Exothermic processes release energy

​

Phase change

When the line has a slope, one phase of matter is present
When the line is flat, more than one phase of matter is present and a phase change is occurring

​

Heat Curves

• KE increases when one phase of matter is present

• PE changes when a transition happens

• IMF are being broken/weakened during phase change
  

​

Change in energy

Multiple Choice

At which letter is the substance changing its state of matter?

• A only

• A, C, and E

• D and E

• B and D

Multiple Choice

A substance's heating curve is shown in the graph.  What is its boiling point?

• 100 C
• 60 C
• -60 C

• 20 C

Multiple Choice

Describe the substance at letter A.

Multiple Choice

What is happening during a plateau (flat line) on the graph?

• PE is increasing and Intermolecular Forces are strengthening

• PE is increasing and Intermolecular Forces are weakening

• KE is increasing and Intermolecular Forces are weakening

• KE is increasing and Intermolecular Forces are strengthening

Reactions occur in multistep processes
Overall heat of a reaction can be determined using the heat of the individual reactions

​

Hess's Law

For a reaction to occur:

1. Particles must collide
2. They must be in the correct orientation
3. They must overcome activation energy

​

Collision Theory

Minimum energy needed for a reaction to start

​

Activation Energy

Increase concentration
Increase temperature
Increase surface area
Increase pressure
Add a catalyst

​

Reaction Rate

Multiple Choice

Using the equations below:

C(s) + O₂(g) → CO₂(g) ∆H = –390 kJ

Mn(s) + O₂(g) → MnO₂(s) ∆H = –520 kJ

what is ∆H (in kJ) for the following reaction?

MnO₂(s) + C(s) → Mn_(s) + CO₂(g)

• 910

• 130

• -130

• -910

Multiple Choice

Consider the following equations.

Mg(s) + O₂(g) → MgO(s) ∆H = –602 kJ

H₂(g) + O₂(g) → H₂O(g) ∆H = –242 kJ

What is the ∆H value (in kJ) for the following reaction?

MgO(s) + H₂(g₎ → Mg(s) + H₂O(g)

• -844

• -360

• +360

• +844

Multiple Choice

When surface area is decreased the rate of reaction...

• Decreases, because there are LESS possible sites for correct collisions

• Increases, because there are LESS possible sites for correct collisions

• Decreases, because there are MORE possible sites for correct collisions

• Increases, because there are MORE possible sites for correct collisions

Multiple Choice

Which of the following increase the reaction rate?

• less surface area

• lower temperature

• increased concentration

Multiple Choice

What happens to the equilibrium if we add methane (CH₄)?

• It shifts left, towards the reactants

• It shifts right, towards the products

• It does not shift

Reactions can shift toward making more products or making more reactants based on stresses

​

Le Chatelier's Principle

Multiple Choice

What happens to the equilibrium if we remove water (H₂O)?

• It shifts left, towards the reactants

• It shifts right, towards the products

• It does not shift

Multiple Choice

What happens to the equilibrium if we remove heat?

• It shifts left, towards the reactants

• It shifts right, towards the products

• It does not shift

Multiple Choice

What happens when sodium ions (Na⁺) are added?

• The reaction shifts left, towards the reactants

• The reaction shifts right, towards the products

• There is no shift