International Baccalaureate IB Chemistry

Chemical reactions involve a transfer of energy between the system and the surroundings, while total energy is conserved.

Understand the difference between heat and temperature.

Reactions are described as endothermic or exothermic, depending on the direction of energy transfer between the system and the surroundings.

Understand the temperature change (decrease or increase) that accompanies endothermic and exothermic reactions.

The relative stability of reactants and products determines whether reactions are endothermic or exothermic.

Sketch and interpret energy profiles for endothermic and exothermic reactions.

The standard enthalpy change for a chemical reaction, $\Delta H^\circ$, refers to the heat transferred at constant pressure under standard conditions and states.

It can be determined from the change in temperature of a pure substance. Apply the equations $Q = mc\Delta T$ and $\Delta H = -Q/n$ in the calculation of the enthalpy change of a reaction.

The units of $\Delta H^\circ$ are $\mathrm{kJ\,mol^{-1}}$.

Bond‑breaking absorbs and bond‑forming releases energy. Calculate the enthalpy change of a reaction from given average bond enthalpy data.

Hess’s law states that the enthalpy change for a reaction is independent of the pathway between the initial and final states.

Apply Hess’s law to calculate enthalpy changes in multistep reactions.

Standard enthalpy changes of combustion ($\Delta H_c^\circ$) and formation ($\Delta H_f^\circ$) data are used in thermodynamic calculations.

Deduce equations and solve problems involving these terms.

Calculate enthalpy changes of a reaction using $\Delta H_f^\circ$ data or $\Delta H_c^\circ$ data: $\Delta H^\circ = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants})$ or $\Delta H^\circ = \sum \Delta H_c^\circ(\text{reactants}) - \sum \Delta H_c^\circ(\text{products})$.

Reactive metals, non-metals and organic compounds undergo combustion reactions when heated in oxygen.

Deduce equations for reactions of combustion, including hydrocarbons and alcohols.

Incomplete combustion of organic compounds, especially hydrocarbons, leads to the production of carbon monoxide and carbon.

Deduce equations for the incomplete combustion of hydrocarbons and alcohols.

For a generic alkane $\mathrm{C_{n}H_{2n+2}}$ the incomplete combustion can be written as  

$$\mathrm{C_{n}H_{2n+2} + \frac{n}{2}\,O_{2} \rightarrow n\,CO + (n+1)\,H_{2}O}$$  

or, when carbon is formed,  

$$\mathrm{C_{n}H_{2n+2} + \frac{n}{2}\,O_{2} \rightarrow n\,C + (n+1)\,H_{2}O}$$  

For a generic alcohol $\mathrm{C_{n}H_{2n+1}OH}$ (i.e., $\mathrm{C_{n}H_{2n+2}O}$) the incomplete combustion can be expressed as  

$$\mathrm{C_{n}H_{2n+2}O + \frac{n}{2}\,O_{2} \rightarrow n\,CO + (n+1)\,H_{2}O}$$  

or  

$$\mathrm{C_{n}H_{2n+2}O + \frac{n}{2}\,O_{2} \rightarrow n\,C + (n+1)\,H_{2}O}$$

Fossil fuels include coal, crude oil and natural gas, which have different advantages and disadvantages.

Evaluate the amount of carbon dioxide added to the atmosphere when different fuels burn. 

Understand the link between carbon dioxide levels and the greenhouse effect.

Biofuels are produced from the biological fixation of carbon over a short period of time through photosynthesis.

Understand the difference between renewable and non‑renewable energy sources. 

Consider the advantages and disadvantages of biofuels. The reactants and products of photosynthesis should be known.

A fuel cell can be used to convert chemical energy from a fuel directly to electrical energy.

Deduce half‑equations for the electrode reactions in a fuel cell. Hydrogen and methanol should be covered as fuels.

$$\mathrm{H_2 \rightarrow 2H^+ + 2e^-}$$
$$\mathrm{\frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O}$$

$$\mathrm{CH_3OH + H_2O \rightarrow CO_2 + 6H^+ + 6e^-}$$
$$\mathrm{\frac{3}{2}O_2 + 6H^+ + 6e^- \rightarrow 3H_2O}$$

Entropy, $S$, is a measure of the dispersal or distribution of matter and/or energy in a system. The more ways the energy can be distributed, the higher the entropy. Under the same conditions, the entropy of a gas is greater than that of a liquid, which in turn is greater than that of a solid.

Predict whether a physical or chemical change will result in an increase or decrease in entropy of a system. 

Calculate standard entropy changes, $\Delta S^\circ$, from standard entropy values.

Change in Gibbs energy, $\Delta G$, relates the energy that can be obtained from a chemical reaction to the change in enthalpy, $\Delta H$, change in entropy, $\Delta S$, and absolute temperature, $T$.

Apply the equation $\Delta G^{\circ} = \Delta H^{\circ} - T\Delta S^{\circ}$ to calculate unknown values of these terms.

At constant pressure, a change is spontaneous if the change in Gibbs energy, $\Delta G$, is negative.

Interpret the sign of $\Delta G$ calculated from thermodynamic data. 

Determine the temperature at which a reaction becomes spontaneous.