Energetics of Reactions

Exothermic

Endothermic

Neither

It depends on the wind

Only the chemicals in the flask

Everything outside the reacting chemicals (like the water, flask, or air)

The atoms inside the molecules

The catalyst used

Exothermic

Endothermic

A physical change

Freezing

Downwards

Upwards

Sideways

It does not exist

$\Delta H = H_{\text{reactants}} - H_{\text{products}}$

$\Delta H = H_{\text{products}} - H_{\text{reactants}}$

$\Delta H$ is always positive for combustion.

$\Delta H$ is always negative for melting.

An increase in enthalpy.

An endothermic reaction.

An exothermic reaction.

A reaction with zero activation energy.

Thermal decomposition

Neutralization

Precipitation

Oxidation

The beaker and the thermometer used in the experiment.

The chemical reactants and products themselves.

The solvent (usually water) in which the reaction occurs.

The entire laboratory room.

Hand warmers containing iron powder that rusts.

Instant cold packs used for treating sports injuries.

Burning natural gas in a boiler to heat a house.

The reaction in a car engine cylinder.

$890\text{ kJ}$

$1780\text{ kJ}$

$445\text{ kJ}$

$222.5\text{ kJ}$

Mass

Volume

Heat content

Pressure

The heat released to the air

The difference in energy between reactants and products

The energy of the sparks used

The temperature of the chemicals

The energy of either reactants or products

The speed of the reaction

The time taken

The change in temperature

Cooling the mixture

Heating the mixture

Removing a catalyst

Adding more product

Energy is needed to form the products.

Energy is needed to break the bonds in the reactants first.

Energy is needed to cool down the surroundings.

Energy is needed to make the products less stable.

Highly endothermic.

Exothermic.

A reaction with zero activation energy.

A reaction that does not involve bond breaking.

Lower than the energy of the reactants.

Higher than the energy of the reactants.

Lower than the energy of the products.

Equal to the energy of the products.

The profile shows energy over time; the distribution shows the number of particles with a certain energy

The profile only applies to gases; the distribution applies to all states

The profile shows $E_a$; the distribution does not show $E_a$

The profile changes with temperature; the distribution does not

$E_{a,forward} > E_{a,reverse}$

$E_{a,forward} < E_{a,reverse}$

$E_{a,forward} = E_{a,reverse}$

They cannot be compared without knowing $\Delta H$

$\Delta H = +40\text{ kJ}$, $E_a = 30\text{ kJ}$

$\Delta H = -40\text{ kJ}$, $E_a = 30\text{ kJ}$

$\Delta H = -40\text{ kJ}$, $E_a = 70\text{ kJ}$

$\Delta H = +40\text{ kJ}$, $E_a = 80\text{ kJ}$

Lower total bond enthalpy

Higher total bond enthalpy

A $\Delta H$ of zero

A very small bond enthalpy

$+500\ kJ$

$-500\ kJ$

$+5500\ kJ$

$-5500\ kJ$

Low bond enthalpy

High bond enthalpy

The bond breaks easily at room temperature

The bond does not contain energy

$996\ kJ/mol$

$498\ kJ/mol$

$249\ kJ/mol$

$1992\ kJ/mol$

$655\text{ kJ/mol}$

$1894\text{ kJ/mol}$

$2294\text{ kJ/mol}$

$413\text{ kJ/mol}$

$+1000\text{ kJ}$

$-1000\text{ kJ}$

$0\text{ kJ}$

$+500\text{ kJ}$

$C-H$ and $O=O$

$C=O$ and $O-H$

$C-O$ and $H-H$

$C-C$ and $O-O$

To eliminate the influence of intermolecular forces.

Because all reactions occur in the gas phase.

Gases have no kinetic energy.

Bonds cannot break in liquids or solids.

-206 kJ/mol

+206 kJ/mol

-103 kJ/mol

+103 kJ/mol

391 kJ/mol

1173 kJ/mol

586.5 kJ/mol

293 kJ/mol

Water is a fuel

Water conducts electricity and could cause a shock

Water provides oxygen to the fire

Water makes the fire hotter

Kerosene

Biodiesel

Petrol

Heavy fuel oil

It slows down the reaction

It increases the amount of fuel used

It speeds up the reaction without being consumed

It removes the need for oxygen

Uranium

LPG (Liquefied Petroleum Gas)

Crude oil

Coal dust

It is highly toxic if touched.

It is extremely flammable and can leak easily through small gaps.

It produces large amounts of $CO_2$ even without burning.

It is too heavy to be moved easily.

Methane will release more energy per mole because it is smaller.

Octane will release more energy per mole because it has more bonds to break and form.

Both will release the same amount of energy because they are both hydrocarbons.

Neither releases energy; they both require energy to burn.

$CH_4$

$C_2H_6$

$C_3H_8$

$C_4H_{10}$

$143.5\text{ kg}$

$239.2\text{ kg}$

$115.6\text{ kg}$

$191.4\text{ kg}$

Wood is endothermic.

Natural gas has a higher activation energy.

The heterogeneous nature of solid wood limits the contact between fuel and oxygen compared to gaseous methane.

Wood contains more hydrogen than natural gas.

$CH_4$

$C_2H_6$

$C_3H_8$

$CO_2$

The container feels cold to the touch.

Heat is taken in from the surroundings.

The products are more stable than the reactants.

The enthalpy change is positive.

The pressure lowers the activation energy.

The higher pressure increases the temperature of the boiling water.

The pressure acts as a catalyst.

The surface area of the food increases.

It decreases.

It increases.

It remains the same.

It stops completely.

$+60\ kJ$

$-60\ kJ$

$+140\ kJ$

$-140\ kJ$

The reaction is endothermic and the products are more stable than the reactants.

The reaction is exothermic and the products are more stable than the reactants.

The reaction is endothermic and the reactants are more stable than the products.

The reaction is exothermic and the reactants are more stable than the products.

The reaction is very slow

The reaction has a low activation energy and is likely to be fast

The reaction is highly endothermic

The reaction cannot occur without a catalyst

As temperature increases, particles move slower and collide less often

As temperature increases, particles move faster and the frequency of collisions increases

As temperature increases, the frequency of collisions remains constant but the energy per collision increases

As temperature increases, the activation energy decreases, making collisions more likely

-0.4 kJ

-400 kJ

+0.4 kJ

-4.0 kJ

+46 kJ

-46 kJ

+92 kJ

-92 kJ

Large marble chips at 20 °C

Small marble chips at 20 °C

Powdered marble at 40 °C

Large marble chips at 40 °C