Physical Chemistry

Define bond enthalpy.

The energy required to break a bond between two atoms.

Define enthalpy change of combustion.

The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions, all reactants and products in their standard state.

Define enthalpy of formation.

Enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Define enthalpy change of reaction.

Enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions with all reactants and products in their standard states.

Define enthalpy change.

The heat energy transferred in a reaction at constant pressure.

Define enthalpy of solution.

Enthalpy change per mole of solute dissolved. Divide heat energy change for the reaction by the number of moles of solute at the start.

Define specific heat capacity.

The amount of heat energy it takes to raise the temperature of one gram of a solution by 1 K.

What are the standard conditions for enthalpy?

100 kilopascals pressure and the stated temperature, usually 298 K.

What is an exothermic reaction?

These reactions give out heat energy as they proceed and the temperature usually increases.

What is an endothermic reaction?

The reactions take in heat from the surroundings, so temperature usually falls.

How can endothermic reactions be used by physiotherapists to treat sports injuries?

Cold packs use an endothermic reaction to absorb 26 kilojoules per mole of heat energy from the surroundings, so they produce a cold sensation on the patient's muscle.

Define the standard enthalpy change of neutralisation.

Enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions.

What is bond making and bond breaking?

Bond making is always exothermic.

Bond breaking is always endothermic.

State the equation Q = mcΔT and what each symbol represents.

Q = mcΔT

Q = energy

m = mass

c = specific heat capacity

ΔT = change in temperature

State the equation for enthalpy change using energy and number of moles.

🔺H = Q/n

If the reaction is exothermic, what should the final answer be?

The answer should be negative.

How do you calculate energy density?

Energy density = energy released ÷ mass of fuel burned

Energy density = Q/m

Units: kJ g⁻¹

Why may enthalpy change of combustion values obtained experimentally differ from those obtained by calculation?

Heat lost to the surroundings.

Incomplete combustion of the fuel.

Heat energy transferred to the container instead of the water.

Using a fuel's these are usually volatile so they escape by evaporation.

Why may enthalpy change of combustion values obtained from a calorimeter differ from data book values?

The data book values are based on calculations made under standard conditions.

Suggest improvements to the calorimeter experiment.

Add a lid.

Reduce the distance between the flame and the beaker.

Put an insulating sleeve around the flame to protect it from draughts and reduce heat loss to the environment.

How can the accuracy of a flame calorimeter be improved?

Spiral chimney made of copper.

Flame enclosed.

Fuel burns in pure oxygen rather than the air.

What should you never do when drawing a graph for enthalpy change?

Never extrapolate the lines.

Give two disadvantages of using a glass beaker and a tripod and gauze.

Glass is a poorer conductor than copper.

Tripod and gauze would reduce heat transfer.

Suggest two reasons why the value of enthalpy of combustion is less exothermic than the data book value.

Heat lost to surroundings through the copper.

Incomplete combustion.

What is Hess's law?

Hess's law states that enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products.

How do you calculate enthalpy change using enthalpies of formation and combustion?

Enthalpy of formation is products minus reactants.

Enthalpy of combustion is reactants minus products.

What is the enthalpy change of formation of an element in its standard state?

Elements in their standard state should always be zero.

Why couldn't the enthalpy change of reaction of an anhydrous solid with water be measured directly?

Difficult to measure the temperature rise of a solid.

Impossible to react the exact amount of water with the solid.

Difficult to prevent the solid from dissolving in the water.

What would happen if the concentration of hydrogen increases?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Equilibrium will shift to the right to oppose the addition of hydrogen. The yield of NH₃ will increase.

Define mean bond enthalpy.

The energy required to break one mole of a covalent bond, averaged over a range of compounds.

What does a more positive bond enthalpy indicate?

The more positive the bond enthalpy, the larger amount of energy needed to break the bond and so the stronger the bond is.

State the equation for calculating enthalpy change using bond enthalpies.

Bond breaking - Bond making

Bend and Mex

A claim is made that the value given for the O=O bond enthalpy is not a mean value. Explain why.

Oxygen is the only substance that has the O double O bond.

Write the complete combustion equation for a hydrocarbon and explain why the enthalpy change would be difficult to determine directly.

Hydrocarbon + oxygen → carbon dioxide + water

Difficult to prevent C reacting with the O₂ to form some CO₂.

Activation energy

The minimum amount of kinetic energy that the particles need to have in order to react when they collide.

Catalyst

A substance that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy.

Dynamic equilibrium

The concentration of the reactants and products remain constant; the rate of the forward reaction equals the rate of the reverse reaction.

Homogeneous equilibrium

The reactants and products are all in the same physical state.

Le Chatelier's principle

If a change is made to a system in dynamic equilibrium, the position of the equilibrium will move to counteract the change.

What would happen if the concentration of hydrogen decreases?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium will shift to the left to oppose the reduction of hydrogen. The yield of ammonia will decrease.

What would happen if the pressure increased?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

There are fewer moles on the right. The equilibrium shifts to the right to oppose the increase in pressure. The yield of ammonia increases.

What would happen if the pressure decreased?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

There are more moles on the left. The equilibrium shifts to the left to oppose the decrease in pressure. The yield of nitrogen and hydrogen increases.

What would happen if the temperature increased and the forward reaction is exothermic?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The backward reaction is endothermic. Equilibrium will shift to the left to oppose the increase in temperature. The yield of ammonia will decrease.

What would happen if the temperature decreased and the forward reaction is exothermic?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium will shift to the right to oppose the decrease in temperature. The yield of ammonia increases.

Why does a catalyst not affect the position of equilibrium?

A catalyst increases the rate of the forward and backward reaction equally.

Why are catalysts added to the Haber process, which is a reversible reaction that produces ammonia?

To reduce the time taken to reach dynamic equilibrium.

How does temperature affect the value of Kc?

The value of Kc is given at a fixed temperature. If the change means more product is formed, Kc increases. If the change means less product is formed, Kc decreases.

How do changes in concentration and pressure affect the value of Kc?

Changes in concentration and pressure only change the position of equilibrium. The value of Kc is fixed at a given temperature.

Does the presence of a catalyst affect the equilibrium constant Kc?

If all other conditions stay the same, the equilibrium constant Kc is not affected by the presence of a catalyst. A catalyst speeds up both the forward and reverse reactions at the same rate. The ratio of products to reactants remains unchanged.

How do you find the partial pressure of gas A?

Partial pressure of A = mole fraction of A × total pressure.

How do you calculate the mole fraction of gas A?

Mole fraction of A = number of moles of gas A in the mixture ÷ total number of moles of all gases.

What will all of the mole fractions added together always equal?

1

What does Kp greater than 1 indicate?

Equilibrium shifts to the Right hand side

What does Kp less than 1 indicate?

Equilibrium is towards the left-hand side.

What does Kp exactly equal to 1 indicate?

Kp = 1. This means the ratio of the relevant partial-pressure terms is equal to 1; it does not necessarily mean equilibrium is in the middle.

What is the only condition that causes Kp to change?

A change in temperature.

What are the five steps of calculating Kp?

Initial moles, change, equilibrium moles, mole fraction, and partial pressure. Remember the ICE box for this.

How would an increase in pressure affect the position of equilibrium?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium will shift to the right. There are fewer gaseous moles on the right-hand side. Equilibrium shifts to oppose the increase in pressure.

How would a decrease in temperature affect the value of Kp?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The value of Kp will increase. The forward reaction is exothermic. The equilibrium will shift to oppose the decrease in temperature.

How would an increase in temperature affect the equilibrium yield of ammonia?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium yield of ammonia will decrease. The forward reaction is exothermic. The equilibrium will shift to oppose the increase in temperature.

Define activation energy.

The minimum energy that colliding particles must have for a reaction to occur.

Define a catalyst.

A substance that increases the rate of reaction without being used up by providing an alternative pathway with a lower activation energy.

Define a first-order reaction.

A reaction where the rate is directly proportional to the concentration of one reactant.

Define a second-order reaction.

A reaction where the rate is proportional to the square of the concentration of one reactant, or the product of two reactant concentrations.

Define a zero-order reaction.

A reaction where the rate is independent of the concentration of the reactant.

Define the rate-determining step.

The slowest step in the reaction mechanism, which controls the overall rate of reaction.

Define overall order.

The sum of the orders of reaction with respect to each reactant.

Define initial rate.

The rate of reaction measured at the start of the reaction before the concentration changes significantly.

Factors affecting rate: concentration of a solution.

The higher the concentration, the faster the reaction. Particles are closer together, so there are more frequent successful collisions.

Factors affecting rate: surface area of a solid.

The more pieces the solid is broken into, the greater the surface area and the faster the reaction. More particles are exposed at the surface, so there are more frequent collisions.

Factors affecting rate: pressure of gases.

The higher the pressure, the faster the reaction. Particles are closer together, so there are more frequent successful collisions, provided the rate depends on the gaseous reactant's concentration.

Factors affecting rate: temperature.

The higher the temperature, the faster the rate of reaction. Particles have more kinetic energy, and more particles have energy greater than or equal to the activation energy. There are more frequent successful collisions.

Factors affecting rate: catalyst.

A catalyst provides an alternative mechanism with a lower activation energy. A greater proportion of collisions are successful, increasing the rate of reaction.

How can a colorimeter be used to monitor a reaction?

Measure how the absorbance of the reaction solution changes at regular intervals as the reaction takes place. The concentration of the coloured substance can be determined from the absorbance value using a calibration curve.

How do you determine reaction orders from initial rates data?

Compare experiments 1 and 2 to find the order with respect to one reactant. Compare experiments 2 and 3 to find the order with respect to the second reactant. Use both orders to write the overall rate equation.

What happens to the rate when the temperature increases by 10 K?

The rate approximately doubles. This is a general approximation, not a universal rule.

What must the species in the rate-determining step match?

The rate equation must be consistent with the species involved in the rate-determining step, taking into account any preceding fast equilibria.

What are the units of rate of reaction?

mol dm⁻³ s⁻¹

What are the units of the rate constant, k, for a first-order reaction?

s⁻¹

Why are samples cooled before titration in a rate experiment?

Samples are cooled to reduce the temperature and therefore the kinetic energy of the particles. This slows the reaction so it does not continue significantly during titration, giving a more accurate result.

Describe the continuous monitoring method.

Follow the course of a single reaction and take measurements at regular intervals. Measure the concentration using a suitable method, such as a colorimeter, and use the results to obtain a rate-time graph.

Describe the initial rates method.

Use multiple experiments, vary the initial concentration of a reactant and measure the initial rate. Use the results to find the order of reaction with respect to each reactant.

How does doubling the concentration affect a first-order reaction?

The rate doubles because the rate is directly proportional to the concentration.

How does doubling the concentration affect a second-order reaction?

The rate increases by a factor of four because the rate is proportional to the square of the concentration.

What happens when a reactant is in large excess?

Its concentration remains effectively constant during the reaction. Its concentration can be incorporated into the observed rate constant, giving a pseudo-order rate equation.

Why must the total volume remain constant in initial rates experiments?

When the total volume is constant, the initial concentration is proportional to the volume of solution used. This allows concentrations and rates to be compared fairly between experiments.

Where can intermediates appear?

Intermediates do not appear in the overall equation, but they can appear in the rate equation derived from the reaction mechanism.

Why can doubling the temperature have a greater effect on rate than doubling the concentration of a first-order reactant E?

Doubling the concentration of E doubles the rate. Increasing the temperature increases the proportion of particles with energy greater than or equal to the activation energy, potentially increasing the rate by a greater factor.

The rate equation is rate = k[H⁺][NO]².

The reaction mechanism has two steps. Identify the rate-determining step and explain your answer.

Step 2 is the rate-determining step because it is consistent with the rate equation, containing one H⁺ ion and two NO molecules.

Arhenious Equations

Define oxidation and and where so the electrons go

Oxidation is the loss of electrons.

Oxidation state increases.

Electrons are on the right-hand side.

Mg → Mg²⁺ + 2e⁻

Define reduction and where do the electrons go

Reduction is the gain of electrons.

Oxidation state decreases.

Electrons are on the left-hand side.

Cu²⁺ + 2e⁻ → Cu

State the common positive ions of Ag, H, Na, K, Li, Mg, Ca and Zn.

Ag⁺ — silver

H⁺ — hydrogen

Na⁺ — sodium

K⁺ — potassium

Li⁺ — lithium

Mg²⁺ — magnesium

Ca²⁺ — calcium

Zn²⁺ — zinc

How can oxidation and reduction be identified using electron transfer and oxidation states?

Oxidation: loss of electrons; oxidation state increases.

Reduction: gain of electrons; oxidation state decreases.

Define an oxidising agent and a reducing agent.

Oxidising agent: accepts electrons and is reduced.

Reducing agent: donates electrons and is oxidised.

State the basic rules for assigning oxidation states.

An uncombined element has oxidation state 0.

A simple ion's oxidation state equals its charge.

The sum of oxidation states equals the overall charge.

Define a disproportionation reaction.

A reaction in which the same element is both oxidised and reduced in the same reaction.