Chemical Reactions & Equations

Cheatsheet Content

### Introduction Chemical reactions are processes where substances react to form new substances with different properties. ### Key Definitions - **Reactant:** A substance that takes part in a chemical reaction. - **Product:** A substance that is formed as a result of a chemical reaction. ### Q&A: Magnesium Burning - **Q:** Why is magnesium rubbed with sandpaper before burning? - **A:** To remove the oxide layer from its surface, allowing the ribbon to burn effectively and achieve the desired result. When a magnesium ribbon burns in air, it reacts with oxygen to form magnesium oxide: $$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$$ - **Magnesium ribbon:** Silver-white metal, burns with dazzling white flames. - **Magnesium oxide:** White powder. ### Characteristics of Chemical Reactions These characteristics help identify if a reaction has occurred: #### Change in State - **Example:** $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$ - **Explanation:** Reactants (hydrogen and oxygen) are gases, product (water) is a liquid. #### Change in Colour - **Example:** $2KI(aq) + Pb(NO_3)_2(aq) \rightarrow 2KNO_3(aq) + PbI_2(s)$ (Lead iodide is yellow) - **Explanation:** Colourless solutions react to form a yellow precipitate. #### Evolution of Gas - **Example:** $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$ - **Explanation:** Hydrogen gas is evolved. #### Change in Temperature - **Example:** $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g) + \text{Heat}$ - **Explanation:** Heat is released when zinc reacts with hydrochloric acid. #### Formation of Precipitate - **Definition:** A solid substance that separates from a liquid in a chemical process. Denoted by $(\downarrow)$ in reactions. - **Example:** $2KI(aq) + Pb(NO_3)_2(aq) \rightarrow 2KNO_3(aq) + PbI_2(s)$ - **Explanation:** A yellow precipitate of lead iodide is formed. ### Chemical Equations The method of representing a chemical reaction using symbols and formulae of the substances involved. - **Example:** Zinc metal reacts with dilute sulfuric acid to form zinc sulfate and hydrogen gas. - Zinc + Sulfuric acid $\rightarrow$ Zinc sulfate + Hydrogen - $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$ #### Balanced Chemical Equation - An equation with an equal number of atoms of each element on both reactant and product sides. - Balanced to satisfy the law of conservation of mass. #### Balancing Chemical Equations The process of making the number of different types of atoms equal on both sides of an equation. - **Example 1:** $Fe + H_2O \rightarrow Fe_3O_4 + H_2$ (Unbalanced) - **Balanced:** $3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$ #### Making Chemical Equations More Informative 1. **Indicating Physical States:** $(s)$ solid, $(l)$ liquid, $(g)$ gas, $(aq)$ aqueous solution. - **Example:** $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$ 2. **Indicating Heat Change:** - **Example (Exothermic):** $C(s) + O_2(g) \rightarrow CO_2(g) + \text{Heat}$ 3. **Indicating Conditions:** Temperature, pressure, catalyst, light. - **Example (Pressure):** $CO(g) + 2H_2(g) \xrightarrow{340 \text{ atm}} CH_3OH(l)$ - **Example (Light/Catalyst):** $6CO_2(aq) + 12H_2O(l) \xrightarrow{\text{Sunlight/Chlorophyll}} C_6H_{12}O_6(aq) + 6O_2(aq) + 6H_2O(l) + \text{energy}$ ### Types of Chemical Reactions #### Combination Reaction - Two or more substances (elements or compounds) combine to form a single product. - **Examples:** 1. $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$ (Quicklime to Slaked lime) 2. Burning of coal: $C(s) + O_2(g) \rightarrow CO_2(g) + \text{Heat}$ 3. Formation of water: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l) + \text{Heat}$ - **Important Application (B.K.Q):** - $CaO + H_2O \rightarrow Ca(OH)_2$ (Calcium oxide/Quicklime to Calcium hydroxide/Slaked lime) - $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$ (Calcium carbonate/limestone/marble) - Slaked lime is used for whitewashing. $CaCO_3$ gives a shiny finish to walls after 2-3 days. #### Decomposition Reaction - A compound splits into two or more simpler substances. - **Types of Decomposition:** - **Thermal Decomposition (Heat):** $2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$ - $CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)$ (Used in cement manufacturing) - $2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g)$ (Brown fumes of $NO_2$) - **Electrolytic Decomposition (Electricity):** $2H_2O(l) \xrightarrow{\text{Electricity}} 2H_2(g) + O_2(g)$ - **Photolytic Decomposition (Sunlight):** - $2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g)$ - $2AgBr(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Br_2(g)$ (Used in black and white photography) - **Observations:** - Ferrous sulfate $(FeSO_4)$ - Green colour - Ferric oxide $(Fe_2O_3)$ - Brown colour - Ferrous sulfate crystal $(FeSO_4 \cdot 7H_2O)$ #### Q&A: Electrolysis of Water - **Q(i):** Is the volume of gas collected the same in both test tubes during water electrolysis? - **A(i):** No, because water is composed of 2 parts hydrogen $(H_2)$ and 1 part oxygen $(O_2)$. - **Q(ii):** What happens when a burning candle is brought near the gases collected, and which gas is present in each tube? - **A(ii):** - **Less gas volume test tube (Oxygen):** Burns rapidly with a yellow flame. - **More gas volume test tube (Hydrogen):** Burns with a blue flame and makes a pop sound. This is due to saturated compounds burning with a clean flame. #### Displacement Reaction - A chemical reaction where a highly reactive element displaces a less reactive element from its solution. - **Reactivity Series:** (Higher element displaces lower element) - Most reactive: Potassium, Sodium, Calcium, Magnesium, Aluminium, Carbon, Zinc, Iron, Tin, Lead, Hydrogen, Copper, Mercury, Silver, Gold: Least reactive. - **Examples:** - $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$ - $Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$ - $Pb(s) + CuCl_2(aq) \rightarrow PbCl_2(aq) + Cu(s)$ - **Observations:** - $CuSO_4$: Blue solution - $FeSO_4$: Brownish green solution #### Double Displacement Reaction - A type of chemical reaction where two compounds react, and the positive ions (cations) and negative ions (anions) of the two reactants switch places, forming two new compounds or products. - **Examples:** - $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$ - $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$ - $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl_2(s) + NaNO_3(aq)$ (Silver chloride is a white precipitate) #### Reactions Based on Heat Released or Absorbed ##### Exothermic Reaction - Reactions that release heat/energy. - **Examples:** - Burning of natural gas: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$ - Cellular respiration: $C_6H_{12}O_6(aq) + 6O_2(aq) \rightarrow 6CO_2(aq) + 6H_2O(l) + \text{energy}$ - Decomposition of vegetable matter into compost. ##### Endothermic Reaction - Reactions that absorb heat/energy. - **Examples:** - $N_2(g) + O_2(g) + \text{Heat} \rightarrow 2NO(g)$ - All decomposition reactions are endothermic. ### Oxidation and Reduction (Redox) - This is a very important topic. #### Oxidation - A process involving the addition of oxygen or any electronegative element, or the removal of hydrogen or any electropositive element. #### Reduction - A process involving the addition of hydrogen or any electropositive element, or the removal of oxygen or any electronegative element. #### Redox Example 1 - **Reaction:** $CuO + H_2 \rightarrow Cu + H_2O$ - **Oxidation:** $H_2 \rightarrow H_2O$ (Hydrogen is oxidized) - **Reduction:** $CuO \rightarrow Cu$ (Copper oxide is reduced) - **Substance Oxidized:** $H_2$ - **Substance Reduced:** $CuO$ - **Oxidizing Agent:** $CuO$ (causes oxidation, gets reduced) - **Reducing Agent:** $H_2$ (causes reduction, gets oxidized) #### Redox Example 2 - **Reaction:** $ZnO + C \rightarrow Zn + CO$ - **Substance Oxidized:** $C$ - **Substance Reduced:** $ZnO$ - **Oxidizing Agent:** $ZnO$ - **Reducing Agent:** $C$ #### Redox Example 3 - **Reaction:** $MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2$ - **Substance Oxidized:** $HCl$ - **Substance Reduced:** $MnO_2$ - **Oxidizing Agent:** $MnO_2$ - **Reducing Agent:** $HCl$ #### Redox Example 4 - **Reaction:** $2Cu + O_2 \rightarrow 2CuO$ - **Substance Oxidized:** $Cu$ - **Important Note:** All displacement reactions are redox reactions, but the reverse is not true. #### Q&A: Copper Powder Heating - **Q:** When copper powder is heated, its surface turns black. Why? What is the black substance? Write the chemical equation. - **A:** When heated, copper powder reacts with oxygen in the air to form copper (II) oxide, which is black. - **Equation:** $2Cu(s) + O_2(g) \rightarrow 2CuO(g)$ ### Redox Questions #### Question 1 - **Reaction:** $H_2S(g) + Cl_2(l) \rightarrow 2HCl(l) + S(g)$ - **Q:** Which reactant acts as the oxidizing agent? - **A:** $Cl_2$ (Chlorine is reduced, so it's the oxidizing agent). #### Question 2 - **Reaction:** $2NH_3(g) + 3Br_2(l) \rightarrow 6HBr(g) + N_2(g)$ - **Q:** Here $NH_3$ gets oxidized to $N_2$ by losing hydrogen. What can you say about $NH_3$? - **A:** It acts as a reducing agent (it gets oxidized). #### Q&A: Heating Copper (II) Nitrate - **Q:** On heating blue copper (II) nitrate, copper oxide (black), oxygen gas, and a brown gas X are formed. - (a) Write a balanced chemical equation. - (b) Identify the brown gas X. - (c) Identify the type of reaction. - (d) What could be the pH range of an aqueous solution of gas X? - **A:** - (a) $2Cu(NO_3)_2(s) \xrightarrow{\text{heat}} 2CuO(s) + O_2(g) + 4NO_2(g)$ - (b) The brown gas X is nitrogen dioxide $(NO_2)$. - (c) This is a decomposition reaction. - (d) Nitrogen dioxide is an oxide of a non-metal, so it dissolves in water to form an acidic solution. Thus, the pH of this solution is less than 7. ### Characteristic Tests of Gases #### (a) Carbon Monoxide (CO) - No simple characteristic test provided in the original text. #### (b) Sulfur Dioxide (SO2) - Turns acidic potassium permanganate solution (purple) colourless because $SO_2$ is a strong reducing agent. - $2KMnO_4 + 2H_2O + 5SO_2 \rightarrow K_2SO_4 + 2MnSO_4 + 2H_2SO_4$ #### (c) Oxygen (O2) - Brings a glowing splint to the mouth of the test tube; the splint relights or burns brighter because oxygen supports combustion. #### (d) Hydrogen (H2) - Burns with a "pop" sound when a burning splint is brought near it. #### Balanced Chemical Equations and Reaction Types - (a) Nitrogen gas treated with hydrogen gas (Haber process): - $N_2(g) + 3H_2(g) \xrightarrow{773K} 2NH_3(g)$ - **Type:** Combination reaction - (b) Sodium hydroxide solution treated with acetic acid: - $NaOH(aq) + CH_3COOH(aq) \rightarrow CH_3COONa(aq) + H_2O(l)$ - **Type:** Double Displacement reaction / Neutralisation reaction - (c) Ethanol warmed with ethanoic acid in presence of concentrated $H_2SO_4$: - $C_2H_5OH + CH_3COOH \xrightarrow{\text{conc. } H_2SO_4} CH_3COOCH_3 + H_2O$ - **Type:** Double Displacement reaction / Esterification reaction - (d) Ethene burnt in the presence of oxygen: - $C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(g) + \text{Heat} + \text{Light}$ - **Type:** Exothermic reaction ### Q&A: Grapes Fermentation - **Q:** Grapes hanging on the plant do not ferment, but after being plucked, they can. Under what conditions do they ferment? Is it a chemical or physical change? - **A:** Grapes attached to the plant are living and have an immune system that prevents fermentation. Once plucked, microbes can grow on them, and under anaerobic conditions (lack of oxygen), they can ferment. This is a chemical change.